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diff --git a/gas/doc/internals.texi b/gas/doc/internals.texi deleted file mode 100644 index 6dc9ac7c658..00000000000 --- a/gas/doc/internals.texi +++ /dev/null @@ -1,1694 +0,0 @@ -\input texinfo -@setfilename internals.info -@node Top -@top Assembler Internals -@raisesections -@cindex internals - -This chapter describes the internals of the assembler. It is incomplete, but -it may help a bit. - -This chapter was last modified on $Date$. It is not updated regularly, and it -may be out of date. - -@menu -* GAS versions:: GAS versions -* Data types:: Data types -* GAS processing:: What GAS does when it runs -* Porting GAS:: Porting GAS -* Relaxation:: Relaxation -* Broken words:: Broken words -* Internal functions:: Internal functions -* Test suite:: Test suite -@end menu - -@node GAS versions -@section GAS versions - -GAS has acquired layers of code over time. The original GAS only supported the -a.out object file format, with three sections. Support for multiple sections -has been added in two different ways. - -The preferred approach is to use the version of GAS created when the symbol -@code{BFD_ASSEMBLER} is defined. The other versions of GAS are documented for -historical purposes, and to help anybody who has to debug code written for -them. - -The type @code{segT} is used to represent a section in code which must work -with all versions of GAS. - -@menu -* Original GAS:: Original GAS version -* MANY_SEGMENTS:: MANY_SEGMENTS gas version -* BFD_ASSEMBLER:: BFD_ASSEMBLER gas version -@end menu - -@node Original GAS -@subsection Original GAS - -The original GAS only supported the a.out object file format with three -sections: @samp{.text}, @samp{.data}, and @samp{.bss}. This is the version of -GAS that is compiled if neither @code{BFD_ASSEMBLER} nor @code{MANY_SEGMENTS} -is defined. This version of GAS is still used for the m68k-aout target, and -perhaps others. - -This version of GAS should not be used for any new development. - -There is still code that is specific to this version of GAS, notably in -@file{write.c}. There is no way for this code to loop through all the -sections; it simply looks at global variables like @code{text_frag_root} and -@code{data_frag_root}. - -The type @code{segT} is an enum. - -@node MANY_SEGMENTS -@subsection MANY_SEGMENTS gas version -@cindex MANY_SEGMENTS - -The @code{MANY_SEGMENTS} version of gas is only used for COFF. It uses the BFD -library, but it writes out all the data itself using @code{bfd_write}. This -version of gas supports up to 40 normal sections. The section names are stored -in the @code{seg_name} array. Other information is stored in the -@code{segment_info} array. - -The type @code{segT} is an enum. Code that wants to examine all the sections -can use a @code{segT} variable as loop index from @code{SEG_E0} up to but not -including @code{SEG_UNKNOWN}. - -Most of the code specific to this version of GAS is in the file -@file{config/obj-coff.c}, in the portion of that file that is compiled when -@code{BFD_ASSEMBLER} is not defined. - -This version of GAS is still used for several COFF targets. - -@node BFD_ASSEMBLER -@subsection BFD_ASSEMBLER gas version -@cindex BFD_ASSEMBLER - -The preferred version of GAS is the @code{BFD_ASSEMBLER} version. In this -version of GAS, the output file is a normal BFD, and the BFD routines are used -to generate the output. - -@code{BFD_ASSEMBLER} will automatically be used for certain targets, including -those that use the ELF, ECOFF, and SOM object file formats, and also all Alpha, -MIPS, PowerPC, and SPARC targets. You can force the use of -@code{BFD_ASSEMBLER} for other targets with the configure option -@samp{--enable-bfd-assembler}; however, it has not been tested for many -targets, and can not be assumed to work. - -@node Data types -@section Data types -@cindex internals, data types - -This section describes some fundamental GAS data types. - -@menu -* Symbols:: The symbolS structure -* Expressions:: The expressionS structure -* Fixups:: The fixS structure -* Frags:: The fragS structure -@end menu - -@node Symbols -@subsection Symbols -@cindex internals, symbols -@cindex symbols, internal -@cindex symbolS structure - -The definition for the symbol structure, @code{symbolS}, is located in -@file{struc-symbol.h}. - -In general, the fields of this structure may not be referred to directly. -Instead, you must use one of the accessor functions defined in @file{symbol.h}. -These accessor functions should work for any GAS version. - -Symbol structures contain the following fields: - -@table @code -@item sy_value -This is an @code{expressionS} that describes the value of the symbol. It might -refer to one or more other symbols; if so, its true value may not be known -until @code{resolve_symbol_value} is called in @code{write_object_file}. - -The expression is often simply a constant. Before @code{resolve_symbol_value} -is called, the value is the offset from the frag (@pxref{Frags}). Afterward, -the frag address has been added in. - -@item sy_resolved -This field is non-zero if the symbol's value has been completely resolved. It -is used during the final pass over the symbol table. - -@item sy_resolving -This field is used to detect loops while resolving the symbol's value. - -@item sy_used_in_reloc -This field is non-zero if the symbol is used by a relocation entry. If a local -symbol is used in a relocation entry, it must be possible to redirect those -relocations to other symbols, or this symbol cannot be removed from the final -symbol list. - -@item sy_next -@itemx sy_previous -These pointers to other @code{symbolS} structures describe a singly or doubly -linked list. (If @code{SYMBOLS_NEED_BACKPOINTERS} is not defined, the -@code{sy_previous} field will be omitted; @code{SYMBOLS_NEED_BACKPOINTERS} is -always defined if @code{BFD_ASSEMBLER}.) These fields should be accessed with -the @code{symbol_next} and @code{symbol_previous} macros. - -@item sy_frag -This points to the frag (@pxref{Frags}) that this symbol is attached to. - -@item sy_used -Whether the symbol is used as an operand or in an expression. Note: Not all of -the backends keep this information accurate; backends which use this bit are -responsible for setting it when a symbol is used in backend routines. - -@item sy_mri_common -Whether the symbol is an MRI common symbol created by the @code{COMMON} -pseudo-op when assembling in MRI mode. - -@item bsym -If @code{BFD_ASSEMBLER} is defined, this points to the BFD @code{asymbol} that -will be used in writing the object file. - -@item sy_name_offset -(Only used if @code{BFD_ASSEMBLER} is not defined.) This is the position of -the symbol's name in the string table of the object file. On some formats, -this will start at position 4, with position 0 reserved for unnamed symbols. -This field is not used until @code{write_object_file} is called. - -@item sy_symbol -(Only used if @code{BFD_ASSEMBLER} is not defined.) This is the -format-specific symbol structure, as it would be written into the object file. - -@item sy_number -(Only used if @code{BFD_ASSEMBLER} is not defined.) This is a 24-bit symbol -number, for use in constructing relocation table entries. - -@item sy_obj -This format-specific data is of type @code{OBJ_SYMFIELD_TYPE}. If no macro by -that name is defined in @file{obj-format.h}, this field is not defined. - -@item sy_tc -This processor-specific data is of type @code{TC_SYMFIELD_TYPE}. If no macro -by that name is defined in @file{targ-cpu.h}, this field is not defined. - -@end table - -Here is a description of the accessor functions. These should be used rather -than referring to the fields of @code{symbolS} directly. - -@table @code -@item S_SET_VALUE -@cindex S_SET_VALUE -Set the symbol's value. - -@item S_GET_VALUE -@cindex S_GET_VALUE -Get the symbol's value. This will cause @code{resolve_symbol_value} to be -called if necessary, so @code{S_GET_VALUE} should only be called when it is -safe to resolve symbols (i.e., after the entire input file has been read and -all symbols have been defined). - -@item S_SET_SEGMENT -@cindex S_SET_SEGMENT -Set the section of the symbol. - -@item S_GET_SEGMENT -@cindex S_GET_SEGMENT -Get the symbol's section. - -@item S_GET_NAME -@cindex S_GET_NAME -Get the name of the symbol. - -@item S_SET_NAME -@cindex S_SET_NAME -Set the name of the symbol. - -@item S_IS_EXTERNAL -@cindex S_IS_EXTERNAL -Return non-zero if the symbol is externally visible. - -@item S_IS_EXTERN -@cindex S_IS_EXTERN -A synonym for @code{S_IS_EXTERNAL}. Don't use it. - -@item S_IS_WEAK -@cindex S_IS_WEAK -Return non-zero if the symbol is weak. - -@item S_IS_COMMON -@cindex S_IS_COMMON -Return non-zero if this is a common symbol. Common symbols are sometimes -represented as undefined symbols with a value, in which case this function will -not be reliable. - -@item S_IS_DEFINED -@cindex S_IS_DEFINED -Return non-zero if this symbol is defined. This function is not reliable when -called on a common symbol. - -@item S_IS_DEBUG -@cindex S_IS_DEBUG -Return non-zero if this is a debugging symbol. - -@item S_IS_LOCAL -@cindex S_IS_LOCAL -Return non-zero if this is a local assembler symbol which should not be -included in the final symbol table. Note that this is not the opposite of -@code{S_IS_EXTERNAL}. The @samp{-L} assembler option affects the return value -of this function. - -@item S_SET_EXTERNAL -@cindex S_SET_EXTERNAL -Mark the symbol as externally visible. - -@item S_CLEAR_EXTERNAL -@cindex S_CLEAR_EXTERNAL -Mark the symbol as not externally visible. - -@item S_SET_WEAK -@cindex S_SET_WEAK -Mark the symbol as weak. - -@item S_GET_TYPE -@item S_GET_DESC -@item S_GET_OTHER -@cindex S_GET_TYPE -@cindex S_GET_DESC -@cindex S_GET_OTHER -Get the @code{type}, @code{desc}, and @code{other} fields of the symbol. These -are only defined for object file formats for which they make sense (primarily -a.out). - -@item S_SET_TYPE -@item S_SET_DESC -@item S_SET_OTHER -@cindex S_SET_TYPE -@cindex S_SET_DESC -@cindex S_SET_OTHER -Set the @code{type}, @code{desc}, and @code{other} fields of the symbol. These -are only defined for object file formats for which they make sense (primarily -a.out). - -@item S_GET_SIZE -@cindex S_GET_SIZE -Get the size of a symbol. This is only defined for object file formats for -which it makes sense (primarily ELF). - -@item S_SET_SIZE -@cindex S_SET_SIZE -Set the size of a symbol. This is only defined for object file formats for -which it makes sense (primarily ELF). - -@item symbol_get_value_expression -@cindex symbol_get_value_expression -Get a pointer to an @code{expressionS} structure which represents the value of -the symbol as an expression. - -@item symbol_set_value_expression -@cindex symbol_set_value_expression -Set the value of a symbol to an expression. - -@item symbol_set_frag -@cindex symbol_set_frag -Set the frag where a symbol is defined. - -@item symbol_get_frag -@cindex symbol_get_frag -Get the frag where a symbol is defined. - -@item symbol_mark_used -@cindex symbol_mark_used -Mark a symbol as having been used in an expression. - -@item symbol_clear_used -@cindex symbol_clear_used -Clear the mark indicating that a symbol was used in an expression. - -@item symbol_used_p -@cindex symbol_used_p -Return whether a symbol was used in an expression. - -@item symbol_mark_used_in_reloc -@cindex symbol_mark_used_in_reloc -Mark a symbol as having been used by a relocation. - -@item symbol_clear_used_in_reloc -@cindex symbol_clear_used_in_reloc -Clear the mark indicating that a symbol was used in a relocation. - -@item symbol_used_in_reloc_p -@cindex symbol_used_in_reloc_p -Return whether a symbol was used in a relocation. - -@item symbol_mark_mri_common -@cindex symbol_mark_mri_common -Mark a symbol as an MRI common symbol. - -@item symbol_clear_mri_common -@cindex symbol_clear_mri_common -Clear the mark indicating that a symbol is an MRI common symbol. - -@item symbol_mri_common_p -@cindex symbol_mri_common_p -Return whether a symbol is an MRI common symbol. - -@item symbol_mark_written -@cindex symbol_mark_written -Mark a symbol as having been written. - -@item symbol_clear_written -@cindex symbol_clear_written -Clear the mark indicating that a symbol was written. - -@item symbol_written_p -@cindex symbol_written_p -Return whether a symbol was written. - -@item symbol_mark_resolved -@cindex symbol_mark_resolved -Mark a symbol as having been resolved. - -@item symbol_resolved_p -@cindex symbol_resolved_p -Return whether a symbol has been resolved. - -@item symbol_section_p -@cindex symbol_section_p -Return whether a symbol is a section symbol. - -@item symbol_equated_p -@cindex symbol_equated_p -Return whether a symbol is equated to another symbol. - -@item symbol_constant_p -@cindex symbol_constant_p -Return whether a symbol has a constant value, including being an offset within -some frag. - -@item symbol_get_bfdsym -@cindex symbol_get_bfdsym -Return the BFD symbol associated with a symbol. - -@item symbol_set_bfdsym -@cindex symbol_set_bfdsym -Set the BFD symbol associated with a symbol. - -@item symbol_get_obj -@cindex symbol_get_obj -Return a pointer to the @code{OBJ_SYMFIELD_TYPE} field of a symbol. - -@item symbol_set_obj -@cindex symbol_set_obj -Set the @code{OBJ_SYMFIELD_TYPE} field of a symbol. - -@item symbol_get_tc -@cindex symbol_get_tc -Return a pointer to the @code{TC_SYMFIELD_TYPE} field of a symbol. - -@item symbol_set_tc -@cindex symbol_set_tc -Set the @code{TC_SYMFIELD_TYPE} field of a symbol. - -@end table - -When @code{BFD_ASSEMBLER} is defined, GAS attempts to store local -symbols--symbols which will not be written to the output file--using a -different structure, @code{struct local_symbol}. This structure can only -represent symbols whose value is an offset within a frag. - -Code outside of the symbol handler will always deal with @code{symbolS} -structures and use the accessor functions. The accessor functions correctly -deal with local symbols. @code{struct local_symbol} is much smaller than -@code{symbolS} (which also automatically creates a bfd @code{asymbol} -structure), so this saves space when assembling large files. - -The first field of @code{symbolS} is @code{bsym}, the pointer to the BFD -symbol. The first field of @code{struct local_symbol} is a pointer which is -always set to NULL. This is how the symbol accessor functions can distinguish -local symbols from ordinary symbols. The symbol accessor functions -automatically convert a local symbol into an ordinary symbol when necessary. - -@node Expressions -@subsection Expressions -@cindex internals, expressions -@cindex expressions, internal -@cindex expressionS structure - -Expressions are stored in an @code{expressionS} structure. The structure is -defined in @file{expr.h}. - -@cindex expression -The macro @code{expression} will create an @code{expressionS} structure based -on the text found at the global variable @code{input_line_pointer}. - -@cindex make_expr_symbol -@cindex expr_symbol_where -A single @code{expressionS} structure can represent a single operation. -Complex expressions are formed by creating @dfn{expression symbols} and -combining them in @code{expressionS} structures. An expression symbol is -created by calling @code{make_expr_symbol}. An expression symbol should -naturally never appear in a symbol table, and the implementation of -@code{S_IS_LOCAL} (@pxref{Symbols}) reflects that. The function -@code{expr_symbol_where} returns non-zero if a symbol is an expression symbol, -and also returns the file and line for the expression which caused it to be -created. - -The @code{expressionS} structure has two symbol fields, a number field, an -operator field, and a field indicating whether the number is unsigned. - -The operator field is of type @code{operatorT}, and describes how to interpret -the other fields; see the definition in @file{expr.h} for the possibilities. - -An @code{operatorT} value of @code{O_big} indicates either a floating point -number, stored in the global variable @code{generic_floating_point_number}, or -an integer to large to store in an @code{offsetT} type, stored in the global -array @code{generic_bignum}. This rather inflexible approach makes it -impossible to use floating point numbers or large expressions in complex -expressions. - -@node Fixups -@subsection Fixups -@cindex internals, fixups -@cindex fixups -@cindex fixS structure - -A @dfn{fixup} is basically anything which can not be resolved in the first -pass. Sometimes a fixup can be resolved by the end of the assembly; if not, -the fixup becomes a relocation entry in the object file. - -@cindex fix_new -@cindex fix_new_exp -A fixup is created by a call to @code{fix_new} or @code{fix_new_exp}. Both -take a frag (@pxref{Frags}), a position within the frag, a size, an indication -of whether the fixup is PC relative, and a type. In a @code{BFD_ASSEMBLER} -GAS, the type is nominally a @code{bfd_reloc_code_real_type}, but several -targets use other type codes to represent fixups that can not be described as -relocations. - -The @code{fixS} structure has a number of fields, several of which are obsolete -or are only used by a particular target. The important fields are: - -@table @code -@item fx_frag -The frag (@pxref{Frags}) this fixup is in. - -@item fx_where -The location within the frag where the fixup occurs. - -@item fx_addsy -The symbol this fixup is against. Typically, the value of this symbol is added -into the object contents. This may be NULL. - -@item fx_subsy -The value of this symbol is subtracted from the object contents. This is -normally NULL. - -@item fx_offset -A number which is added into the fixup. - -@item fx_addnumber -Some CPU backends use this field to convey information between -@code{md_apply_fix} and @code{tc_gen_reloc}. The machine independent code does -not use it. - -@item fx_next -The next fixup in the section. - -@item fx_r_type -The type of the fixup. This field is only defined if @code{BFD_ASSEMBLER}, or -if the target defines @code{NEED_FX_R_TYPE}. - -@item fx_size -The size of the fixup. This is mostly used for error checking. - -@item fx_pcrel -Whether the fixup is PC relative. - -@item fx_done -Non-zero if the fixup has been applied, and no relocation entry needs to be -generated. - -@item fx_file -@itemx fx_line -The file and line where the fixup was created. - -@item tc_fix_data -This has the type @code{TC_FIX_TYPE}, and is only defined if the target defines -that macro. -@end table - -@node Frags -@subsection Frags -@cindex internals, frags -@cindex frags -@cindex fragS structure. - -The @code{fragS} structure is defined in @file{as.h}. Each frag represents a -portion of the final object file. As GAS reads the source file, it creates -frags to hold the data that it reads. At the end of the assembly the frags and -fixups are processed to produce the final contents. - -@table @code -@item fr_address -The address of the frag. This is not set until the assembler rescans the list -of all frags after the entire input file is parsed. The function -@code{relax_segment} fills in this field. - -@item fr_next -Pointer to the next frag in this (sub)section. - -@item fr_fix -Fixed number of characters we know we're going to emit to the output file. May -be zero. - -@item fr_var -Variable number of characters we may output, after the initial @code{fr_fix} -characters. May be zero. - -@item fr_offset -The interpretation of this field is controlled by @code{fr_type}. Generally, -if @code{fr_var} is non-zero, this is a repeat count: the @code{fr_var} -characters are output @code{fr_offset} times. - -@item line -Holds line number info when an assembler listing was requested. - -@item fr_type -Relaxation state. This field indicates the interpretation of @code{fr_offset}, -@code{fr_symbol} and the variable-length tail of the frag, as well as the -treatment it gets in various phases of processing. It does not affect the -initial @code{fr_fix} characters; they are always supposed to be output -verbatim (fixups aside). See below for specific values this field can have. - -@item fr_subtype -Relaxation substate. If the macro @code{md_relax_frag} isn't defined, this is -assumed to be an index into @code{TC_GENERIC_RELAX_TABLE} for the generic -relaxation code to process (@pxref{Relaxation}). If @code{md_relax_frag} is -defined, this field is available for any use by the CPU-specific code. - -@item fr_symbol -This normally indicates the symbol to use when relaxing the frag according to -@code{fr_type}. - -@item fr_opcode -Points to the lowest-addressed byte of the opcode, for use in relaxation. - -@item tc_frag_data -Target specific fragment data of type TC_FRAG_TYPE. -Only present if @code{TC_FRAG_TYPE} is defined. - -@item fr_file -@itemx fr_line -The file and line where this frag was last modified. - -@item fr_literal -Declared as a one-character array, this last field grows arbitrarily large to -hold the actual contents of the frag. -@end table - -These are the possible relaxation states, provided in the enumeration type -@code{relax_stateT}, and the interpretations they represent for the other -fields: - -@table @code -@item rs_align -@itemx rs_align_code -The start of the following frag should be aligned on some boundary. In this -frag, @code{fr_offset} is the logarithm (base 2) of the alignment in bytes. -(For example, if alignment on an 8-byte boundary were desired, @code{fr_offset} -would have a value of 3.) The variable characters indicate the fill pattern to -be used. The @code{fr_subtype} field holds the maximum number of bytes to skip -when doing this alignment. If more bytes are needed, the alignment is not -done. An @code{fr_subtype} value of 0 means no maximum, which is the normal -case. Target backends can use @code{rs_align_code} to handle certain types of -alignment differently. - -@item rs_broken_word -This indicates that ``broken word'' processing should be done (@pxref{Broken -words}). If broken word processing is not necessary on the target machine, -this enumerator value will not be defined. - -@item rs_cfa -This state is used to implement exception frame optimizations. The -@code{fr_symbol} is an expression symbol for the subtraction which may be -relaxed. The @code{fr_opcode} field holds the frag for the preceding command -byte. The @code{fr_offset} field holds the offset within that frag. The -@code{fr_subtype} field is used during relaxation to hold the current size of -the frag. - -@item rs_fill -The variable characters are to be repeated @code{fr_offset} times. If -@code{fr_offset} is 0, this frag has a length of @code{fr_fix}. Most frags -have this type. - -@item rs_leb128 -This state is used to implement the DWARF ``little endian base 128'' -variable length number format. The @code{fr_symbol} is always an expression -symbol, as constant expressions are emitted directly. The @code{fr_offset} -field is used during relaxation to hold the previous size of the number so -that we can determine if the fragment changed size. - -@item rs_machine_dependent -Displacement relaxation is to be done on this frag. The target is indicated by -@code{fr_symbol} and @code{fr_offset}, and @code{fr_subtype} indicates the -particular machine-specific addressing mode desired. @xref{Relaxation}. - -@item rs_org -The start of the following frag should be pushed back to some specific offset -within the section. (Some assemblers use the value as an absolute address; GAS -does not handle final absolute addresses, but rather requires that the linker -set them.) The offset is given by @code{fr_symbol} and @code{fr_offset}; one -character from the variable-length tail is used as the fill character. -@end table - -@cindex frchainS structure -A chain of frags is built up for each subsection. The data structure -describing a chain is called a @code{frchainS}, and contains the following -fields: - -@table @code -@item frch_root -Points to the first frag in the chain. May be NULL if there are no frags in -this chain. -@item frch_last -Points to the last frag in the chain, or NULL if there are none. -@item frch_next -Next in the list of @code{frchainS} structures. -@item frch_seg -Indicates the section this frag chain belongs to. -@item frch_subseg -Subsection (subsegment) number of this frag chain. -@item fix_root, fix_tail -(Defined only if @code{BFD_ASSEMBLER} is defined). Point to first and last -@code{fixS} structures associated with this subsection. -@item frch_obstack -Not currently used. Intended to be used for frag allocation for this -subsection. This should reduce frag generation caused by switching sections. -@item frch_frag_now -The current frag for this subsegment. -@end table - -A @code{frchainS} corresponds to a subsection; each section has a list of -@code{frchainS} records associated with it. In most cases, only one subsection -of each section is used, so the list will only be one element long, but any -processing of frag chains should be prepared to deal with multiple chains per -section. - -After the input files have been completely processed, and no more frags are to -be generated, the frag chains are joined into one per section for further -processing. After this point, it is safe to operate on one chain per section. - -The assembler always has a current frag, named @code{frag_now}. More space is -allocated for the current frag using the @code{frag_more} function; this -returns a pointer to the amount of requested space. Relaxing is done using -variant frags allocated by @code{frag_var} or @code{frag_variant} -(@pxref{Relaxation}). - -@node GAS processing -@section What GAS does when it runs -@cindex internals, overview - -This is a quick look at what an assembler run looks like. - -@itemize @bullet -@item -The assembler initializes itself by calling various init routines. - -@item -For each source file, the @code{read_a_source_file} function reads in the file -and parses it. The global variable @code{input_line_pointer} points to the -current text; it is guaranteed to be correct up to the end of the line, but not -farther. - -@item -For each line, the assembler passes labels to the @code{colon} function, and -isolates the first word. If it looks like a pseudo-op, the word is looked up -in the pseudo-op hash table @code{po_hash} and dispatched to a pseudo-op -routine. Otherwise, the target dependent @code{md_assemble} routine is called -to parse the instruction. - -@item -When pseudo-ops or instructions output data, they add it to a frag, calling -@code{frag_more} to get space to store it in. - -@item -Pseudo-ops and instructions can also output fixups created by @code{fix_new} or -@code{fix_new_exp}. - -@item -For certain targets, instructions can create variant frags which are used to -store relaxation information (@pxref{Relaxation}). - -@item -When the input file is finished, the @code{write_object_file} routine is -called. It assigns addresses to all the frags (@code{relax_segment}), resolves -all the fixups (@code{fixup_segment}), resolves all the symbol values (using -@code{resolve_symbol_value}), and finally writes out the file (in the -@code{BFD_ASSEMBLER} case, this is done by simply calling @code{bfd_close}). -@end itemize - -@node Porting GAS -@section Porting GAS -@cindex porting - -Each GAS target specifies two main things: the CPU file and the object format -file. Two main switches in the @file{configure.in} file handle this. The -first switches on CPU type to set the shell variable @code{cpu_type}. The -second switches on the entire target to set the shell variable @code{fmt}. - -The configure script uses the value of @code{cpu_type} to select two files in -the @file{config} directory: @file{tc-@var{CPU}.c} and @file{tc-@var{CPU}.h}. -The configuration process will create a file named @file{targ-cpu.h} in the -build directory which includes @file{tc-@var{CPU}.h}. - -The configure script also uses the value of @code{fmt} to select two files: -@file{obj-@var{fmt}.c} and @file{obj-@var{fmt}.h}. The configuration process -will create a file named @file{obj-format.h} in the build directory which -includes @file{obj-@var{fmt}.h}. - -You can also set the emulation in the configure script by setting the @code{em} -variable. Normally the default value of @samp{generic} is fine. The -configuration process will create a file named @file{targ-env.h} in the build -directory which includes @file{te-@var{em}.h}. - -Porting GAS to a new CPU requires writing the @file{tc-@var{CPU}} files. -Porting GAS to a new object file format requires writing the -@file{obj-@var{fmt}} files. There is sometimes some interaction between these -two files, but it is normally minimal. - -The best approach is, of course, to copy existing files. The documentation -below assumes that you are looking at existing files to see usage details. - -These interfaces have grown over time, and have never been carefully thought -out or designed. Nothing about the interfaces described here is cast in stone. -It is possible that they will change from one version of the assembler to the -next. Also, new macros are added all the time as they are needed. - -@menu -* CPU backend:: Writing a CPU backend -* Object format backend:: Writing an object format backend -* Emulations:: Writing emulation files -@end menu - -@node CPU backend -@subsection Writing a CPU backend -@cindex CPU backend -@cindex @file{tc-@var{CPU}} - -The CPU backend files are the heart of the assembler. They are the only parts -of the assembler which actually know anything about the instruction set of the -processor. - -You must define a reasonably small list of macros and functions in the CPU -backend files. You may define a large number of additional macros in the CPU -backend files, not all of which are documented here. You must, of course, -define macros in the @file{.h} file, which is included by every assembler -source file. You may define the functions as macros in the @file{.h} file, or -as functions in the @file{.c} file. - -@table @code -@item TC_@var{CPU} -@cindex TC_@var{CPU} -By convention, you should define this macro in the @file{.h} file. For -example, @file{tc-m68k.h} defines @code{TC_M68K}. You might have to use this -if it is necessary to add CPU specific code to the object format file. - -@item TARGET_FORMAT -This macro is the BFD target name to use when creating the output file. This -will normally depend upon the @code{OBJ_@var{FMT}} macro. - -@item TARGET_ARCH -This macro is the BFD architecture to pass to @code{bfd_set_arch_mach}. - -@item TARGET_MACH -This macro is the BFD machine number to pass to @code{bfd_set_arch_mach}. If -it is not defined, GAS will use 0. - -@item TARGET_BYTES_BIG_ENDIAN -You should define this macro to be non-zero if the target is big endian, and -zero if the target is little endian. - -@item md_shortopts -@itemx md_longopts -@itemx md_longopts_size -@itemx md_parse_option -@itemx md_show_usage -@cindex md_shortopts -@cindex md_longopts -@cindex md_longopts_size -@cindex md_parse_option -@cindex md_show_usage -GAS uses these variables and functions during option processing. -@code{md_shortopts} is a @code{const char *} which GAS adds to the machine -independent string passed to @code{getopt}. @code{md_longopts} is a -@code{struct option []} which GAS adds to the machine independent long options -passed to @code{getopt}; you may use @code{OPTION_MD_BASE}, defined in -@file{as.h}, as the start of a set of long option indices, if necessary. -@code{md_longopts_size} is a @code{size_t} holding the size @code{md_longopts}. -GAS will call @code{md_parse_option} whenever @code{getopt} returns an -unrecognized code, presumably indicating a special code value which appears in -@code{md_longopts}. GAS will call @code{md_show_usage} when a usage message is -printed; it should print a description of the machine specific options. - -@item md_begin -@cindex md_begin -GAS will call this function at the start of the assembly, after the command -line arguments have been parsed and all the machine independent initializations -have been completed. - -@item md_cleanup -@cindex md_cleanup -If you define this macro, GAS will call it at the end of each input file. - -@item md_assemble -@cindex md_assemble -GAS will call this function for each input line which does not contain a -pseudo-op. The argument is a null terminated string. The function should -assemble the string as an instruction with operands. Normally -@code{md_assemble} will do this by calling @code{frag_more} and writing out -some bytes (@pxref{Frags}). @code{md_assemble} will call @code{fix_new} to -create fixups as needed (@pxref{Fixups}). Targets which need to do special -purpose relaxation will call @code{frag_var}. - -@item md_pseudo_table -@cindex md_pseudo_table -This is a const array of type @code{pseudo_typeS}. It is a mapping from -pseudo-op names to functions. You should use this table to implement -pseudo-ops which are specific to the CPU. - -@item tc_conditional_pseudoop -@cindex tc_conditional_pseudoop -If this macro is defined, GAS will call it with a @code{pseudo_typeS} argument. -It should return non-zero if the pseudo-op is a conditional which controls -whether code is assembled, such as @samp{.if}. GAS knows about the normal -conditional pseudo-ops,and you should normally not have to define this macro. - -@item comment_chars -@cindex comment_chars -This is a null terminated @code{const char} array of characters which start a -comment. - -@item tc_comment_chars -@cindex tc_comment_chars -If this macro is defined, GAS will use it instead of @code{comment_chars}. - -@item tc_symbol_chars -@cindex tc_symbol_chars -If this macro is defined, it is a pointer to a null terminated list of -characters which may appear in an operand. GAS already assumes that all -alphanumberic characters, and @samp{$}, @samp{.}, and @samp{_} may appear in an -operand (see @samp{symbol_chars} in @file{app.c}). This macro may be defined -to treat additional characters as appearing in an operand. This affects the -way in which GAS removes whitespace before passing the string to -@samp{md_assemble}. - -@item line_comment_chars -@cindex line_comment_chars -This is a null terminated @code{const char} array of characters which start a -comment when they appear at the start of a line. - -@item line_separator_chars -@cindex line_separator_chars -This is a null terminated @code{const char} array of characters which separate -lines (semicolon and newline are such characters by default, and need not be -listed in this array). - -@item EXP_CHARS -@cindex EXP_CHARS -This is a null terminated @code{const char} array of characters which may be -used as the exponent character in a floating point number. This is normally -@code{"eE"}. - -@item FLT_CHARS -@cindex FLT_CHARS -This is a null terminated @code{const char} array of characters which may be -used to indicate a floating point constant. A zero followed by one of these -characters is assumed to be followed by a floating point number; thus they -operate the way that @code{0x} is used to indicate a hexadecimal constant. -Usually this includes @samp{r} and @samp{f}. - -@item LEX_AT -@cindex LEX_AT -You may define this macro to the lexical type of the @kbd{@}} character. The -default is zero. - -Lexical types are a combination of @code{LEX_NAME} and @code{LEX_BEGIN_NAME}, -both defined in @file{read.h}. @code{LEX_NAME} indicates that the character -may appear in a name. @code{LEX_BEGIN_NAME} indicates that the character may -appear at the beginning of a nem. - -@item LEX_BR -@cindex LEX_BR -You may define this macro to the lexical type of the brace characters @kbd{@{}, -@kbd{@}}, @kbd{[}, and @kbd{]}. The default value is zero. - -@item LEX_PCT -@cindex LEX_PCT -You may define this macro to the lexical type of the @kbd{%} character. The -default value is zero. - -@item LEX_QM -@cindex LEX_QM -You may define this macro to the lexical type of the @kbd{?} character. The -default value it zero. - -@item LEX_DOLLAR -@cindex LEX_DOLLAR -You may define this macro to the lexical type of the @kbd{$} character. The -default value is @code{LEX_NAME | LEX_BEGIN_NAME}. - -@item SINGLE_QUOTE_STRINGS -@cindex SINGLE_QUOTE_STRINGS -If you define this macro, GAS will treat single quotes as string delimiters. -Normally only double quotes are accepted as string delimiters. - -@item NO_STRING_ESCAPES -@cindex NO_STRING_ESCAPES -If you define this macro, GAS will not permit escape sequences in a string. - -@item ONLY_STANDARD_ESCAPES -@cindex ONLY_STANDARD_ESCAPES -If you define this macro, GAS will warn about the use of nonstandard escape -sequences in a string. - -@item md_start_line_hook -@cindex md_start_line_hook -If you define this macro, GAS will call it at the start of each line. - -@item LABELS_WITHOUT_COLONS -@cindex LABELS_WITHOUT_COLONS -If you define this macro, GAS will assume that any text at the start of a line -is a label, even if it does not have a colon. - -@item TC_START_LABEL -@cindex TC_START_LABEL -You may define this macro to control what GAS considers to be a label. The -default definition is to accept any name followed by a colon character. - -@item NO_PSEUDO_DOT -@cindex NO_PSEUDO_DOT -If you define this macro, GAS will not require pseudo-ops to start with a -@kbd{.} character. - -@item TC_EQUAL_IN_INSN -@cindex TC_EQUAL_IN_INSN -If you define this macro, it should return nonzero if the instruction is -permitted to contain an @kbd{=} character. GAS will use this to decide if a -@kbd{=} is an assignment or an instruction. - -@item TC_EOL_IN_INSN -@cindex TC_EOL_IN_INSN -If you define this macro, it should return nonzero if the current input line -pointer should be treated as the end of a line. - -@item md_parse_name -@cindex md_parse_name -If this macro is defined, GAS will call it for any symbol found in an -expression. You can define this to handle special symbols in a special way. -If a symbol always has a certain value, you should normally enter it in the -symbol table, perhaps using @code{reg_section}. - -@item md_undefined_symbol -@cindex md_undefined_symbol -GAS will call this function when a symbol table lookup fails, before it -creates a new symbol. Typically this would be used to supply symbols whose -name or value changes dynamically, possibly in a context sensitive way. -Predefined symbols with fixed values, such as register names or condition -codes, are typically entered directly into the symbol table when @code{md_begin} -is called. - -@item md_operand -@cindex md_operand -GAS will call this function for any expression that can not be recognized. -When the function is called, @code{input_line_pointer} will point to the start -of the expression. - -@item tc_unrecognized_line -@cindex tc_unrecognized_line -If you define this macro, GAS will call it when it finds a line that it can not -parse. - -@item md_do_align -@cindex md_do_align -You may define this macro to handle an alignment directive. GAS will call it -when the directive is seen in the input file. For example, the i386 backend -uses this to generate efficient nop instructions of varying lengths, depending -upon the number of bytes that the alignment will skip. - -@item HANDLE_ALIGN -@cindex HANDLE_ALIGN -You may define this macro to do special handling for an alignment directive. -GAS will call it at the end of the assembly. - -@item md_flush_pending_output -@cindex md_flush_pending_output -If you define this macro, GAS will call it each time it skips any space because of a -space filling or alignment or data allocation pseudo-op. - -@item TC_PARSE_CONS_EXPRESSION -@cindex TC_PARSE_CONS_EXPRESSION -You may define this macro to parse an expression used in a data allocation -pseudo-op such as @code{.word}. You can use this to recognize relocation -directives that may appear in such directives. - -@item BITFIELD_CONS_EXPRESSION -@cindex BITFIELD_CONS_EXPRESSION -If you define this macro, GAS will recognize bitfield instructions in data -allocation pseudo-ops, as used on the i960. - -@item REPEAT_CONS_EXPRESSION -@cindex REPEAT_CONS_EXPRESSION -If you define this macro, GAS will recognize repeat counts in data allocation -pseudo-ops, as used on the MIPS. - -@item md_cons_align -@cindex md_cons_align -You may define this macro to do any special alignment before a data allocation -pseudo-op. - -@item TC_CONS_FIX_NEW -@cindex TC_CONS_FIX_NEW -You may define this macro to generate a fixup for a data allocation pseudo-op. - -@item TC_INIT_FIX_DATA (@var{fixp}) -@cindex TC_INIT_FIX_DATA -A C statement to initialize the target specific fields of fixup @var{fixp}. -These fields are defined with the @code{TC_FIX_TYPE} macro. - -@item TC_FIX_DATA_PRINT (@var{stream}, @var{fixp}) -@cindex TC_FIX_DATA_PRINT -A C statement to output target specific debugging information for -fixup @var{fixp} to @var{stream}. This macro is called by @code{print_fixup}. - -@item TC_FRAG_INIT (@var{fragp}) -@cindex TC_FRAG_INIT -A C statement to initialize the target specific fields of frag @var{fragp}. -These fields are defined with the @code{TC_FRAG_TYPE} macro. - -@item md_number_to_chars -@cindex md_number_to_chars -This should just call either @code{number_to_chars_bigendian} or -@code{number_to_chars_littleendian}, whichever is appropriate. On targets like -the MIPS which support options to change the endianness, which function to call -is a runtime decision. On other targets, @code{md_number_to_chars} can be a -simple macro. - -@item md_reloc_size -@cindex md_reloc_size -This variable is only used in the original version of gas (not -@code{BFD_ASSEMBLER} and not @code{MANY_SEGMENTS}). It holds the size of a -relocation entry. - -@item WORKING_DOT_WORD -@itemx md_short_jump_size -@itemx md_long_jump_size -@itemx md_create_short_jump -@itemx md_create_long_jump -@cindex WORKING_DOT_WORD -@cindex md_short_jump_size -@cindex md_long_jump_size -@cindex md_create_short_jump -@cindex md_create_long_jump -If @code{WORKING_DOT_WORD} is defined, GAS will not do broken word processing -(@pxref{Broken words}). Otherwise, you should set @code{md_short_jump_size} to -the size of a short jump (a jump that is just long enough to jump around a long -jmp) and @code{md_long_jump_size} to the size of a long jump (a jump that can -go anywhere in the function), You should define @code{md_create_short_jump} to -create a short jump around a long jump, and define @code{md_create_long_jump} -to create a long jump. - -@item md_estimate_size_before_relax -@cindex md_estimate_size_before_relax -This function returns an estimate of the size of a @code{rs_machine_dependent} -frag before any relaxing is done. It may also create any necessary -relocations. - -@item md_relax_frag -@cindex md_relax_frag -This macro may be defined to relax a frag. GAS will call this with the frag -and the change in size of all previous frags; @code{md_relax_frag} should -return the change in size of the frag. @xref{Relaxation}. - -@item TC_GENERIC_RELAX_TABLE -@cindex TC_GENERIC_RELAX_TABLE -If you do not define @code{md_relax_frag}, you may define -@code{TC_GENERIC_RELAX_TABLE} as a table of @code{relax_typeS} structures. The -machine independent code knows how to use such a table to relax PC relative -references. See @file{tc-m68k.c} for an example. @xref{Relaxation}. - -@item md_prepare_relax_scan -@cindex md_prepare_relax_scan -If defined, it is a C statement that is invoked prior to scanning -the relax table. - -@item LINKER_RELAXING_SHRINKS_ONLY -@cindex LINKER_RELAXING_SHRINKS_ONLY -If you define this macro, and the global variable @samp{linkrelax} is set -(because of a command line option, or unconditionally in @code{md_begin}), a -@samp{.align} directive will cause extra space to be allocated. The linker can -then discard this space when relaxing the section. - -@item md_convert_frag -@cindex md_convert_frag -GAS will call this for each rs_machine_dependent fragment. -The instruction is completed using the data from the relaxation pass. -It may also create any necessary relocations. -@xref{Relaxation}. - -@item md_apply_fix -@cindex md_apply_fix -GAS will call this for each fixup. It should store the correct value in the -object file. @code{fixup_segment} performs a generic overflow check on the -@code{valueT *val} argument after @code{md_apply_fix} returns. If the overflow -check is relevant for the target machine, then @code{md_apply_fix} should -modify @code{valueT *val}, typically to the value stored in the object file. - -@item TC_HANDLES_FX_DONE -@cindex TC_HANDLES_FX_DONE -If this macro is defined, it means that @code{md_apply_fix} correctly sets the -@code{fx_done} field in the fixup. - -@item tc_gen_reloc -@cindex tc_gen_reloc -A @code{BFD_ASSEMBLER} GAS will call this to generate a reloc. GAS will pass -the resulting reloc to @code{bfd_install_relocation}. This currently works -poorly, as @code{bfd_install_relocation} often does the wrong thing, and -instances of @code{tc_gen_reloc} have been written to work around the problems, -which in turns makes it difficult to fix @code{bfd_install_relocation}. - -@item RELOC_EXPANSION_POSSIBLE -@cindex RELOC_EXPANSION_POSSIBLE -If you define this macro, it means that @code{tc_gen_reloc} may return multiple -relocation entries for a single fixup. In this case, the return value of -@code{tc_gen_reloc} is a pointer to a null terminated array. - -@item MAX_RELOC_EXPANSION -@cindex MAX_RELOC_EXPANSION -You must define this if @code{RELOC_EXPANSION_POSSIBLE} is defined; it -indicates the largest number of relocs which @code{tc_gen_reloc} may return for -a single fixup. - -@item tc_fix_adjustable -@cindex tc_fix_adjustable -You may define this macro to indicate whether a fixup against a locally defined -symbol should be adjusted to be against the section symbol. It should return a -non-zero value if the adjustment is acceptable. - -@item MD_PCREL_FROM_SECTION -@cindex MD_PCREL_FROM_SECTION -If you define this macro, it should return the offset between the address of a -PC relative fixup and the position from which the PC relative adjustment should -be made. On many processors, the base of a PC relative instruction is the next -instruction, so this macro would return the length of an instruction. - -@item md_pcrel_from -@cindex md_pcrel_from -This is the default value of @code{MD_PCREL_FROM_SECTION}. The difference is -that @code{md_pcrel_from} does not take a section argument. - -@item tc_frob_label -@cindex tc_frob_label -If you define this macro, GAS will call it each time a label is defined. - -@item md_section_align -@cindex md_section_align -GAS will call this function for each section at the end of the assembly, to -permit the CPU backend to adjust the alignment of a section. - -@item tc_frob_section -@cindex tc_frob_section -If you define this macro, a @code{BFD_ASSEMBLER} GAS will call it for each -section at the end of the assembly. - -@item tc_frob_file_before_adjust -@cindex tc_frob_file_before_adjust -If you define this macro, GAS will call it after the symbol values are -resolved, but before the fixups have been changed from local symbols to section -symbols. - -@item tc_frob_symbol -@cindex tc_frob_symbol -If you define this macro, GAS will call it for each symbol. You can indicate -that the symbol should not be included in the object file by definining this -macro to set its second argument to a non-zero value. - -@item tc_frob_file -@cindex tc_frob_file -If you define this macro, GAS will call it after the symbol table has been -completed, but before the relocations have been generated. - -@item tc_frob_file_after_relocs -If you define this macro, GAS will call it after the relocs have been -generated. - -@item LISTING_HEADER -A string to use on the header line of a listing. The default value is simply -@code{"GAS LISTING"}. - -@item LISTING_WORD_SIZE -The number of bytes to put into a word in a listing. This affects the way the -bytes are clumped together in the listing. For example, a value of 2 might -print @samp{1234 5678} where a value of 1 would print @samp{12 34 56 78}. The -default value is 4. - -@item LISTING_LHS_WIDTH -The number of words of data to print on the first line of a listing for a -particular source line, where each word is @code{LISTING_WORD_SIZE} bytes. The -default value is 1. - -@item LISTING_LHS_WIDTH_SECOND -Like @code{LISTING_LHS_WIDTH}, but applying to the second and subsequent line -of the data printed for a particular source line. The default value is 1. - -@item LISTING_LHS_CONT_LINES -The maximum number of continuation lines to print in a listing for a particular -source line. The default value is 4. - -@item LISTING_RHS_WIDTH -The maximum number of characters to print from one line of the input file. The -default value is 100. -@end table - -@node Object format backend -@subsection Writing an object format backend -@cindex object format backend -@cindex @file{obj-@var{fmt}} - -As with the CPU backend, the object format backend must define a few things, -and may define some other things. The interface to the object format backend -is generally simpler; most of the support for an object file format consists of -defining a number of pseudo-ops. - -The object format @file{.h} file must include @file{targ-cpu.h}. - -This section will only define the @code{BFD_ASSEMBLER} version of GAS. It is -impossible to support a new object file format using any other version anyhow, -as the original GAS version only supports a.out, and the @code{MANY_SEGMENTS} -GAS version only supports COFF. - -@table @code -@item OBJ_@var{format} -@cindex OBJ_@var{format} -By convention, you should define this macro in the @file{.h} file. For -example, @file{obj-elf.h} defines @code{OBJ_ELF}. You might have to use this -if it is necessary to add object file format specific code to the CPU file. - -@item obj_begin -If you define this macro, GAS will call it at the start of the assembly, after -the command line arguments have been parsed and all the machine independent -initializations have been completed. - -@item obj_app_file -@cindex obj_app_file -If you define this macro, GAS will invoke it when it sees a @code{.file} -pseudo-op or a @samp{#} line as used by the C preprocessor. - -@item OBJ_COPY_SYMBOL_ATTRIBUTES -@cindex OBJ_COPY_SYMBOL_ATTRIBUTES -You should define this macro to copy object format specific information from -one symbol to another. GAS will call it when one symbol is equated to -another. - -@item obj_fix_adjustable -@cindex obj_fix_adjustable -You may define this macro to indicate whether a fixup against a locally defined -symbol should be adjusted to be against the section symbol. It should return a -non-zero value if the adjustment is acceptable. - -@item obj_sec_sym_ok_for_reloc -@cindex obj_sec_sym_ok_for_reloc -You may define this macro to indicate that it is OK to use a section symbol in -a relocateion entry. If it is not, GAS will define a new symbol at the start -of a section. - -@item EMIT_SECTION_SYMBOLS -@cindex EMIT_SECTION_SYMBOLS -You should define this macro with a zero value if you do not want to include -section symbols in the output symbol table. The default value for this macro -is one. - -@item obj_adjust_symtab -@cindex obj_adjust_symtab -If you define this macro, GAS will invoke it just before setting the symbol -table of the output BFD. For example, the COFF support uses this macro to -generate a @code{.file} symbol if none was generated previously. - -@item SEPARATE_STAB_SECTIONS -@cindex SEPARATE_STAB_SECTIONS -You may define this macro to indicate that stabs should be placed in separate -sections, as in ELF. - -@item INIT_STAB_SECTION -@cindex INIT_STAB_SECTION -You may define this macro to initialize the stabs section in the output file. - -@item OBJ_PROCESS_STAB -@cindex OBJ_PROCESS_STAB -You may define this macro to do specific processing on a stabs entry. - -@item obj_frob_section -@cindex obj_frob_section -If you define this macro, GAS will call it for each section at the end of the -assembly. - -@item obj_frob_file_before_adjust -@cindex obj_frob_file_before_adjust -If you define this macro, GAS will call it after the symbol values are -resolved, but before the fixups have been changed from local symbols to section -symbols. - -@item obj_frob_symbol -@cindex obj_frob_symbol -If you define this macro, GAS will call it for each symbol. You can indicate -that the symbol should not be included in the object file by definining this -macro to set its second argument to a non-zero value. - -@item obj_frob_file -@cindex obj_frob_file -If you define this macro, GAS will call it after the symbol table has been -completed, but before the relocations have been generated. - -@item obj_frob_file_after_relocs -If you define this macro, GAS will call it after the relocs have been -generated. - -@item SET_SECTION_RELOCS (@var{sec}, @var{relocs}, @var{n}) -@cindex SET_SECTION_RELOCS -If you define this, it will be called after the relocations have been set for -the section @var{sec}. The list of relocations is in @var{relocs}, and the -number of relocations is in @var{n}. This is only used with -@code{BFD_ASSEMBLER}. -@end table - -@node Emulations -@subsection Writing emulation files - -Normally you do not have to write an emulation file. You can just use -@file{te-generic.h}. - -If you do write your own emulation file, it must include @file{obj-format.h}. - -An emulation file will often define @code{TE_@var{EM}}; this may then be used -in other files to change the output. - -@node Relaxation -@section Relaxation -@cindex relaxation - -@dfn{Relaxation} is a generic term used when the size of some instruction or -data depends upon the value of some symbol or other data. - -GAS knows to relax a particular type of PC relative relocation using a table. -You can also define arbitrarily complex forms of relaxation yourself. - -@menu -* Relaxing with a table:: Relaxing with a table -* General relaxing:: General relaxing -@end menu - -@node Relaxing with a table -@subsection Relaxing with a table - -If you do not define @code{md_relax_frag}, and you do define -@code{TC_GENERIC_RELAX_TABLE}, GAS will relax @code{rs_machine_dependent} frags -based on the frag subtype and the displacement to some specified target -address. The basic idea is that several machines have different addressing -modes for instructions that can specify different ranges of values, with -successive modes able to access wider ranges, including the entirety of the -previous range. Smaller ranges are assumed to be more desirable (perhaps the -instruction requires one word instead of two or three); if this is not the -case, don't describe the smaller-range, inferior mode. - -The @code{fr_subtype} field of a frag is an index into a CPU-specific -relaxation table. That table entry indicates the range of values that can be -stored, the number of bytes that will have to be added to the frag to -accomodate the addressing mode, and the index of the next entry to examine if -the value to be stored is outside the range accessible by the current -addressing mode. The @code{fr_symbol} field of the frag indicates what symbol -is to be accessed; the @code{fr_offset} field is added in. - -If the @code{TC_PCREL_ADJUST} macro is defined, which currently should only happen -for the NS32k family, the @code{TC_PCREL_ADJUST} macro is called on the frag to -compute an adjustment to be made to the displacement. - -The value fitted by the relaxation code is always assumed to be a displacement -from the current frag. (More specifically, from @code{fr_fix} bytes into the -frag.) -@ignore -This seems kinda silly. What about fitting small absolute values? I suppose -@code{md_assemble} is supposed to take care of that, but if the operand is a -difference between symbols, it might not be able to, if the difference was not -computable yet. -@end ignore - -The end of the relaxation sequence is indicated by a ``next'' value of 0. This -means that the first entry in the table can't be used. - -For some configurations, the linker can do relaxing within a section of an -object file. If call instructions of various sizes exist, the linker can -determine which should be used in each instance, when a symbol's value is -resolved. In order for the linker to avoid wasting space and having to insert -no-op instructions, it must be able to expand or shrink the section contents -while still preserving intra-section references and meeting alignment -requirements. - -For the i960 using b.out format, no expansion is done; instead, each -@samp{.align} directive causes extra space to be allocated, enough that when -the linker is relaxing a section and removing unneeded space, it can discard -some or all of this extra padding and cause the following data to be correctly -aligned. - -For the H8/300, I think the linker expands calls that can't reach, and doesn't -worry about alignment issues; the cpu probably never needs any significant -alignment beyond the instruction size. - -The relaxation table type contains these fields: - -@table @code -@item long rlx_forward -Forward reach, must be non-negative. -@item long rlx_backward -Backward reach, must be zero or negative. -@item rlx_length -Length in bytes of this addressing mode. -@item rlx_more -Index of the next-longer relax state, or zero if there is no next relax state. -@end table - -The relaxation is done in @code{relax_segment} in @file{write.c}. The -difference in the length fields between the original mode and the one finally -chosen by the relaxing code is taken as the size by which the current frag will -be increased in size. For example, if the initial relaxing mode has a length -of 2 bytes, and because of the size of the displacement, it gets upgraded to a -mode with a size of 6 bytes, it is assumed that the frag will grow by 4 bytes. -(The initial two bytes should have been part of the fixed portion of the frag, -since it is already known that they will be output.) This growth must be -effected by @code{md_convert_frag}; it should increase the @code{fr_fix} field -by the appropriate size, and fill in the appropriate bytes of the frag. -(Enough space for the maximum growth should have been allocated in the call to -frag_var as the second argument.) - -If relocation records are needed, they should be emitted by -@code{md_estimate_size_before_relax}. This function should examine the target -symbol of the supplied frag and correct the @code{fr_subtype} of the frag if -needed. When this function is called, if the symbol has not yet been defined, -it will not become defined later; however, its value may still change if the -section it is in gets relaxed. - -Usually, if the symbol is in the same section as the frag (given by the -@var{sec} argument), the narrowest likely relaxation mode is stored in -@code{fr_subtype}, and that's that. - -If the symbol is undefined, or in a different section (and therefore moveable -to an arbitrarily large distance), the largest available relaxation mode is -specified, @code{fix_new} is called to produce the relocation record, -@code{fr_fix} is increased to include the relocated field (remember, this -storage was allocated when @code{frag_var} was called), and @code{frag_wane} is -called to convert the frag to an @code{rs_fill} frag with no variant part. -Sometimes changing addressing modes may also require rewriting the instruction. -It can be accessed via @code{fr_opcode} or @code{fr_fix}. - -Sometimes @code{fr_var} is increased instead, and @code{frag_wane} is not -called. I'm not sure, but I think this is to keep @code{fr_fix} referring to -an earlier byte, and @code{fr_subtype} set to @code{rs_machine_dependent} so -that @code{md_convert_frag} will get called. - -@node General relaxing -@subsection General relaxing - -If using a simple table is not suitable, you may implement arbitrarily complex -relaxation semantics yourself. For example, the MIPS backend uses this to emit -different instruction sequences depending upon the size of the symbol being -accessed. - -When you assemble an instruction that may need relaxation, you should allocate -a frag using @code{frag_var} or @code{frag_variant} with a type of -@code{rs_machine_dependent}. You should store some sort of information in the -@code{fr_subtype} field so that you can figure out what to do with the frag -later. - -When GAS reaches the end of the input file, it will look through the frags and -work out their final sizes. - -GAS will first call @code{md_estimate_size_before_relax} on each -@code{rs_machine_dependent} frag. This function must return an estimated size -for the frag. - -GAS will then loop over the frags, calling @code{md_relax_frag} on each -@code{rs_machine_dependent} frag. This function should return the change in -size of the frag. GAS will keep looping over the frags until none of the frags -changes size. - -@node Broken words -@section Broken words -@cindex internals, broken words -@cindex broken words - -Some compilers, including GCC, will sometimes emit switch tables specifying -16-bit @code{.word} displacements to branch targets, and branch instructions -that load entries from that table to compute the target address. If this is -done on a 32-bit machine, there is a chance (at least with really large -functions) that the displacement will not fit in 16 bits. The assembler -handles this using a concept called @dfn{broken words}. This idea is well -named, since there is an implied promise that the 16-bit field will in fact -hold the specified displacement. - -If broken word processing is enabled, and a situation like this is encountered, -the assembler will insert a jump instruction into the instruction stream, close -enough to be reached with the 16-bit displacement. This jump instruction will -transfer to the real desired target address. Thus, as long as the @code{.word} -value really is used as a displacement to compute an address to jump to, the -net effect will be correct (minus a very small efficiency cost). If -@code{.word} directives with label differences for values are used for other -purposes, however, things may not work properly. For targets which use broken -words, the @samp{-K} option will warn when a broken word is discovered. - -The broken word code is turned off by the @code{WORKING_DOT_WORD} macro. It -isn't needed if @code{.word} emits a value large enough to contain an address -(or, more correctly, any possible difference between two addresses). - -@node Internal functions -@section Internal functions - -This section describes basic internal functions used by GAS. - -@menu -* Warning and error messages:: Warning and error messages -* Hash tables:: Hash tables -@end menu - -@node Warning and error messages -@subsection Warning and error messages - -@deftypefun @{@} int had_warnings (void) -@deftypefunx @{@} int had_errors (void) -Returns non-zero if any warnings or errors, respectively, have been printed -during this invocation. -@end deftypefun - -@deftypefun @{@} void as_perror (const char *@var{gripe}, const char *@var{filename}) -Displays a BFD or system error, then clears the error status. -@end deftypefun - -@deftypefun @{@} void as_tsktsk (const char *@var{format}, ...) -@deftypefunx @{@} void as_warn (const char *@var{format}, ...) -@deftypefunx @{@} void as_bad (const char *@var{format}, ...) -@deftypefunx @{@} void as_fatal (const char *@var{format}, ...) -These functions display messages about something amiss with the input file, or -internal problems in the assembler itself. The current file name and line -number are printed, followed by the supplied message, formatted using -@code{vfprintf}, and a final newline. - -An error indicated by @code{as_bad} will result in a non-zero exit status when -the assembler has finished. Calling @code{as_fatal} will result in immediate -termination of the assembler process. -@end deftypefun - -@deftypefun @{@} void as_warn_where (char *@var{file}, unsigned int @var{line}, const char *@var{format}, ...) -@deftypefunx @{@} void as_bad_where (char *@var{file}, unsigned int @var{line}, const char *@var{format}, ...) -These variants permit specification of the file name and line number, and are -used when problems are detected when reprocessing information saved away when -processing some earlier part of the file. For example, fixups are processed -after all input has been read, but messages about fixups should refer to the -original filename and line number that they are applicable to. -@end deftypefun - -@deftypefun @{@} void fprint_value (FILE *@var{file}, valueT @var{val}) -@deftypefunx @{@} void sprint_value (char *@var{buf}, valueT @var{val}) -These functions are helpful for converting a @code{valueT} value into printable -format, in case it's wider than modes that @code{*printf} can handle. If the -type is narrow enough, a decimal number will be produced; otherwise, it will be -in hexadecimal. The value itself is not examined to make this determination. -@end deftypefun - -@node Hash tables -@subsection Hash tables -@cindex hash tables - -@deftypefun @{@} @{struct hash_control *@} hash_new (void) -Creates the hash table control structure. -@end deftypefun - -@deftypefun @{@} void hash_die (struct hash_control *) -Destroy a hash table. -@end deftypefun - -@deftypefun @{@} PTR hash_delete (struct hash_control *, const char *) -Deletes entry from the hash table, returns the value it had. -@end deftypefun - -@deftypefun @{@} PTR hash_replace (struct hash_control *, const char *, PTR) -Updates the value for an entry already in the table, returning the old value. -If no entry was found, just returns NULL. -@end deftypefun - -@deftypefun @{@} @{const char *@} hash_insert (struct hash_control *, const char *, PTR) -Inserting a value already in the table is an error. -Returns an error message or NULL. -@end deftypefun - -@deftypefun @{@} @{const char *@} hash_jam (struct hash_control *, const char *, PTR) -Inserts if the value isn't already present, updates it if it is. -@end deftypefun - -@node Test suite -@section Test suite -@cindex test suite - -The test suite is kind of lame for most processors. Often it only checks to -see if a couple of files can be assembled without the assembler reporting any -errors. For more complete testing, write a test which either examines the -assembler listing, or runs @code{objdump} and examines its output. For the -latter, the TCL procedure @code{run_dump_test} may come in handy. It takes the -base name of a file, and looks for @file{@var{file}.d}. This file should -contain as its initial lines a set of variable settings in @samp{#} comments, -in the form: - -@example - #@var{varname}: @var{value} -@end example - -The @var{varname} may be @code{objdump}, @code{nm}, or @code{as}, in which case -it specifies the options to be passed to the specified programs. Exactly one -of @code{objdump} or @code{nm} must be specified, as that also specifies which -program to run after the assembler has finished. If @var{varname} is -@code{source}, it specifies the name of the source file; otherwise, -@file{@var{file}.s} is used. If @var{varname} is @code{name}, it specifies the -name of the test to be used in the @code{pass} or @code{fail} messages. - -The non-commented parts of the file are interpreted as regular expressions, one -per line. Blank lines in the @code{objdump} or @code{nm} output are skipped, -as are blank lines in the @code{.d} file; the other lines are tested to see if -the regular expression matches the program output. If it does not, the test -fails. - -Note that this means the tests must be modified if the @code{objdump} output -style is changed. - -@bye -@c Local Variables: -@c fill-column: 79 -@c End: |