path: root/manual/manual/cmds/native.etex

\chapter{Native-code compilation (ocamlopt)} \label{c:nativecomp}
\pdfchapter{Native-code compilation (ocamlopt)}
%HEVEA\cutname{native.html}

This chapter describes the OCaml high-performance
native-code compiler "ocamlopt", which compiles OCaml source files to
native code object files and link these object files to produce
standalone executables.

The native-code compiler is only available on certain platforms.
It produces code that runs faster than the bytecode produced by
"ocamlc", at the cost of increased compilation time and executable code
size. Compatibility with the bytecode compiler is extremely high: the
same source code should run identically when compiled with "ocamlc" and
"ocamlopt".

It is not possible to mix native-code object files produced by "ocamlopt"
with bytecode object files produced by "ocamlc": a program must be
compiled entirely with "ocamlopt" or entirely with "ocamlc". Native-code
object files produced by "ocamlopt" cannot be loaded in the toplevel
system "ocaml".

\section{Overview of the compiler}

The "ocamlopt" command has a command-line interface very close to that
of "ocamlc". It accepts the same types of arguments, and processes them
sequentially:

\begin{itemize}
\item
Arguments ending in ".mli" are taken to be source files for
compilation unit interfaces. Interfaces specify the names exported by
compilation units: they declare value names with their types, define
public data types, declare abstract data types, and so on. From the
file \var{x}".mli", the "ocamlopt" compiler produces a compiled interface
in the file \var{x}".cmi". The interface produced is identical to that
produced by the bytecode compiler "ocamlc".

\item
Arguments ending in ".ml" are taken to be source files for compilation
unit implementations. Implementations provide definitions for the
names exported by the unit, and also contain expressions to be
evaluated for their side-effects.  From the file \var{x}".ml", the "ocamlopt"
compiler produces two files: \var{x}".o", containing native object code,
and \var{x}".cmx", containing extra information for linking and
optimization of the clients of the unit. The compiled implementation
should always be referred to under the name \var{x}".cmx" (when given
a ".o" or ".obj" file, "ocamlopt" assumes that it contains code compiled from C,
not from OCaml).

The implementation is checked against the interface file \var{x}".mli"
(if it exists) as described in the manual for "ocamlc"
(chapter~\ref{c:camlc}).

\item
Arguments ending in ".cmx" are taken to be compiled object code.  These
files are linked together, along with the object files obtained
by compiling ".ml" arguments (if any), and the OCaml standard
library, to produce a native-code executable program. The order in
which ".cmx" and ".ml" arguments are presented on the command line is
relevant: compilation units are initialized in that order at
run-time, and it is a link-time error to use a component of a unit
before having initialized it. Hence, a given \var{x}".cmx" file must come
before all ".cmx" files that refer to the unit \var{x}.

\item
Arguments ending in ".cmxa" are taken to be libraries of object code.
Such a library packs in two files (\var{lib}".cmxa" and \var{lib}".a"/".lib")
a set of object files (".cmx" and ".o"/".obj" files). Libraries are build with
"ocamlopt -a" (see the description of the "-a" option below). The object
files contained in the library are linked as regular ".cmx" files (see
above), in the order specified when the library was built. The only
difference is that if an object file contained in a library is not
referenced anywhere in the program, then it is not linked in.

\item
Arguments ending in ".c" are passed to the C compiler, which generates
a ".o"/".obj" object file. This object file is linked with the program.

\item
Arguments ending in ".o", ".a" or ".so" (".obj", ".lib" and ".dll"
under Windows) are assumed to be C object files and
libraries. They are linked with the program.

\end{itemize}

The output of the linking phase is a regular Unix or Windows
executable file. It does not need "ocamlrun" to run.

\section{Options}

The following command-line options are recognized by "ocamlopt".
The options "-pack", "-a", "-shared", "-c" and "-output-obj" are mutually
exclusive.

\begin{options}

\item["-a"]
Build a library (".cmxa" and ".a"/".lib" files) with the object files
(".cmx" and ".o"/".obj" files) given on the command line, instead of
linking them into an executable file. The name of the library must be
set with the "-o" option.

If "-cclib" or "-ccopt" options are passed on the command
line, these options are stored in the resulting ".cmxa" library.  Then,
linking with this library automatically adds back the
"-cclib" and "-ccopt" options as if they had been provided on the
command line, unless the "-noautolink" option is given.

\item["-absname"]
Force error messages to show absolute paths for file names.

\item["-annot"]
Dump detailed information about the compilation (types, bindings,
tail-calls, etc).  The information for file \var{src}".ml"
is put into file \var{src}".annot".  In case of a type error, dump
all the information inferred by the type-checker before the error.
The \var{src}".annot" file can be used with the emacs commands given in
"emacs/caml-types.el" to display types and other annotations
interactively.

\item["-bin-annot"]
Dump detailed information about the compilation (types, bindings,
tail-calls, etc) in binary format. The information for file \var{src}".ml"
is put into file \var{src}".cmt".  In case of a type error, dump
all the information inferred by the type-checker before the error.
The "*.cmt" files produced by "-bin-annot" contain more information
and are much more compact than the files produced by "-annot".

\item["-c"]
Compile only. Suppress the linking phase of the
compilation. Source code files are turned into compiled files, but no
executable file is produced. This option is useful to
compile modules separately.

\item["-cc" \var{ccomp}]
Use \var{ccomp} as the C linker called to build the final executable
and as the C compiler for compiling ".c" source files.

\item["-cclib" "-l"\var{libname}]
Pass the "-l"\var{libname} option to the linker. This causes the given
C library to be linked with the program.

\item["-ccopt" \var{option}]
Pass the given option to the C compiler and linker. For instance,
"-ccopt -L"\var{dir} causes the C linker to search for C libraries in
directory \var{dir}.

\item["-compact"]
Optimize the produced code for space rather than for time. This
results in slightly smaller but slightly slower programs. The default is to
optimize for speed.

\item["-config"]
Print the version number of "ocamlopt" and a detailed summary of its
configuration, then exit.

\item["-for-pack" \var{module-path}]
Generate an object file (".cmx" and ".o"/".obj" files) that can later be
included
as a sub-module (with the given access path) of a compilation unit
constructed with "-pack".  For instance, "ocamlopt -for-pack P -c A.ml"
will generate "a.cmx" and "a.o" files that can later be used with
"ocamlopt -pack -o P.cmx a.cmx".

\item["-g"]
Add debugging information while compiling and linking. This option is
required in order to produce stack backtraces when
the program terminates on an uncaught exception (see
section~\ref{ocamlrun-options}).

\item["-i"]
Cause the compiler to print all defined names (with their inferred
types or their definitions) when compiling an implementation (".ml"
file).  No compiled files (".cmo" and ".cmi" files) are produced.
This can be useful to check the types inferred by the
compiler. Also, since the output follows the syntax of interfaces, it
can help in writing an explicit interface (".mli" file) for a file:
just redirect the standard output of the compiler to a ".mli" file,
and edit that file to remove all declarations of unexported names.

\item["-I" \var{directory}]
Add the given directory to the list of directories searched for
compiled interface files (".cmi"), compiled object code files
(".cmx"), and libraries (".cmxa"). By default, the current directory
is searched first, then the standard library directory. Directories
added with "-I" are searched after the current directory, in the order
in which they were given on the command line, but before the standard
library directory. See also option "-nostdlib".

If the given directory starts with "+", it is taken relative to the
standard library directory.  For instance, "-I +labltk" adds the
subdirectory "labltk" of the standard library to the search path.

\item["-impl" \var{filename}]
Compile the file \var{filename} as an implementation file, even if its
extension is not ".ml".

\item["-inline" \var{n}]
Set aggressiveness of inlining to \var{n}, where \var{n} is a positive
integer. Specifying "-inline 0" prevents all functions from being
inlined, except those whose body is smaller than the call site. Thus,
inlining causes no expansion in code size. The default aggressiveness,
"-inline 1", allows slightly larger functions to be inlined, resulting
in a slight expansion in code size. Higher values for the "-inline"
option cause larger and larger functions to become candidate for
inlining, but can result in a serious increase in code size.

\item["-intf" \var{filename}]
Compile the file \var{filename} as an interface file, even if its
extension is not ".mli".

\item["-intf-suffix" \var{string}]
Recognize file names ending with \var{string} as interface files
(instead of the default ".mli").

\item["-labels"]
Labels are not ignored in types, labels may be used in applications,
and labelled parameters can be given in any order.  This is the default.

\item["-linkall"]
Force all modules contained in libraries to be linked in. If this
flag is not given, unreferenced modules are not linked in. When
building a library ("-a" flag), setting the "-linkall" flag forces all
subsequent links of programs involving that library to link all the
modules contained in the library.

\item["-no-app-funct"]
Deactivates the applicative behaviour of functors. With this option,
each functor application generates new types in its result and
applying the same functor twice to the same argument yields two
incompatible structures.

\item["-noassert"]
Do not compile assertion checks.  Note that the special form
"assert false" is always compiled because it is typed specially.
This flag has no effect when linking already-compiled files.

\item["-noautolink"]
When linking ".cmxa" libraries, ignore "-cclib" and "-ccopt"
options potentially contained in the libraries (if these options were
given when building the libraries).  This can be useful if a library
contains incorrect specifications of C libraries or C options; in this
case, during linking, set "-noautolink" and pass the correct C
libraries and options on the command line.

\item["-nodynlink"]
Allow the compiler to use some optimizations that are valid only for code
that is never dynlinked.

\item["-nolabels"]
Ignore non-optional labels in types. Labels cannot be used in
applications, and parameter order becomes strict.

\item["-nostdlib"]
Do not automatically add the standard library directory the list of
directories searched for compiled interface files (".cmi"), compiled
object code files (".cmx"), and libraries (".cmxa"). See also option
"-I".

\item["-o" \var{exec-file}]
Specify the name of the output file produced by the linker. The
default output name is "a.out" under Unix and "camlprog.exe" under
Windows. If the "-a" option is given, specify the name of the library
produced.  If the "-pack" option is given, specify the name of the
packed object file produced.  If the "-output-obj" option is given,
specify the name of the output file produced. If the "-shared" option
is given, specify the name of plugin file produced.

\item["-output-obj"]
Cause the linker to produce a C object file instead of an executable
file. This is useful to wrap OCaml code as a C library,
callable from any C program. See chapter~\ref{c:intf-c},
section~\ref{s:embedded-code}. The name of the output object file
must be set with the "-o" option.
This option can also be used to produce a compiled shared/dynamic
library (".so" extension, ".dll" under Windows).

\item["-p"]
Generate extra code to write profile information when the program is
executed.  The profile information can then be examined with the
analysis program "gprof".  (See chapter~\ref{c:profiler} for more
information on profiling.)  The "-p" option must be given both at
compile-time and at link-time.  Linking object files not compiled with
"-p" is possible, but results in less precise profiling.

\begin{unix} See the Unix manual page for "gprof(1)" for more
information about the profiles.

Full support for "gprof" is only available for certain platforms
(currently: Intel x86 32 and 64 bits under Linux, BSD and MacOS X).
On other platforms, the "-p" option will result in a less precise
profile (no call graph information, only a time profile).
\end{unix}

\begin{windows}
The "-p" option does not work under Windows.
\end{windows}

\item["-pack"]
Build an object file (".cmx" and ".o"/".obj" files) and its associated compiled
interface (".cmi") that combines the ".cmx" object
files given on the command line, making them appear as sub-modules of
the output ".cmx" file.  The name of the output ".cmx" file must be
given with the "-o" option.  For instance,
\begin{verbatim}
        ocamlopt -pack -o P.cmx A.cmx B.cmx C.cmx
\end{verbatim}
generates compiled files "P.cmx", "P.o" and "P.cmi" describing a
compilation unit having three sub-modules "A", "B" and "C",
corresponding to the contents of the object files "A.cmx", "B.cmx" and
"C.cmx".  These contents can be referenced as "P.A", "P.B" and "P.C"
in the remainder of the program.

The ".cmx" object files being combined must have been compiled with
the appropriate "-for-pack" option.  In the example above,
"A.cmx", "B.cmx" and "C.cmx" must have been compiled with
"ocamlopt -for-pack P".

Multiple levels of packing can be achieved by combining "-pack" with
"-for-pack".  Consider the following example:
\begin{verbatim}
        ocamlopt -for-pack P.Q -c A.ml
        ocamlopt -pack -o Q.cmx -for-pack P A.cmx
        ocamlopt -for-pack P -c B.ml
        ocamlopt -pack -o P.cmx Q.cmx B.cmx
\end{verbatim}
The resulting "P.cmx" object file has sub-modules "P.Q", "P.Q.A"
and "P.B".

\item["-pp" \var{command}]
Cause the compiler to call the given \var{command} as a preprocessor
for each source file. The output of \var{command} is redirected to
an intermediate file, which is compiled. If there are no compilation
errors, the intermediate file is deleted afterwards.

\item["-ppx" \var{command}]
After parsing, pipe the abstract syntax tree through the preprocessor
\var{command}. The module "Ast_mapper", described in
chapter~\ref{Ast-underscoremapper}, implements the external interface
of a preprocessor.

\item["-principal"]
Check information path during type-checking, to make sure that all
types are derived in a principal way. All programs accepted in
"-principal" mode are also accepted in default mode with equivalent
types, but different binary signatures.

\item["-rectypes"]
Allow arbitrary recursive types during type-checking.  By default,
only recursive types where the recursion goes through an object type
are supported. Note that once you have created an interface using this
flag, you must use it again for all dependencies.

\item["-runtime-variant" \var{suffix}]
Add the \var{suffix} string to the name of the runtime library used by
the program.  Currently, only one such suffix is supported: "d", and
only if the OCaml compiler was configured with option
"-with-debug-runtime".  This suffix gives the debug version of the
runtime, which is useful for debugging pointer problems in low-level
code such as C stubs.

\item["-S"]
Keep the assembly code produced during the compilation. The assembly
code for the source file \var{x}".ml" is saved in the file \var{x}".s".

\item["-shared"]
Build a plugin (usually ".cmxs") that can be dynamically loaded with
the "Dynlink" module. The name of the plugin must be
set with the "-o" option. A plugin can include a number of OCaml
modules and libraries, and extra native objects (".o", ".obj", ".a",
".lib" files). Building native plugins is only supported for some
operating system. Under some systems (currently,
only Linux AMD 64), all the OCaml code linked in a plugin must have
been compiled without the "-nodynlink" flag. Some constraints might also
apply to the way the extra native objects have been compiled (under
Linux AMD 64, they must contain only position-independent code).

\item["-safe-string"]
Enforce the separation between types "string" and "bytes",
thereby making strings read-only. This will become the default in
a future version of OCaml.

\item["-short-paths"]
When a type is visible under several module-paths, use the shortest
one when printing the type's name in inferred interfaces and error and
warning messages. Identifier names starting with an underscore "_" or
containing double underscores "__" incur a penalty of $+10$ when computing
their length.

\item["-strict-sequence"]
Force the left-hand part of each sequence to have type unit.

\item["-strict-formats"]
Reject invalid formats that were accepted in legacy format
implementations. You should use this flag to detect and fix such
invalid formats, as they will be rejected by future OCaml versions.

\item["-thread"]
Compile or link multithreaded programs, in combination with the
system "threads" library described in chapter~\ref{c:threads}.

\item["-unsafe"]
Turn bound checking off for array and string accesses (the "v.(i)" and
"s.[i]" constructs). Programs compiled with "-unsafe" are therefore
faster, but unsafe: anything can happen if the program accesses an
array or string outside of its bounds.  Additionally, turn off the
check for zero divisor in integer division and modulus operations.
With "-unsafe", an integer division (or modulus) by zero can halt the
program or continue with an unspecified result instead of raising a
"Division_by_zero" exception.

\item["-unsafe-string"]
Identify the types "string" and "bytes",
thereby making strings writable. For reasons of backward compatibility,
this is the default setting for the moment, but this will change in a future
version of OCaml.

\item["-v"]
Print the version number of the compiler and the location of the
standard library directory, then exit.

\item["-verbose"]
Print all external commands before they are executed, in particular
invocations of the assembler, C compiler, and linker.

\item["-version" or "-vnum"]
Print the version number of the compiler in short form (e.g. "3.11.0"),
then exit.

\item["-w" \var{warning-list}]
Enable, disable, or mark as fatal the warnings specified by the argument
\var{warning-list}.
Each warning can be {\em enabled} or {\em disabled}, and each warning
can be {\em fatal} or {\em non-fatal}.
If a warning is disabled, it isn't displayed and doesn't affect
compilation in any way (even if it is fatal).  If a warning is
enabled, it is displayed normally by the compiler whenever the source
code triggers it.  If it is enabled and fatal, the compiler will also
stop with an error after displaying it.

The \var{warning-list} argument is a sequence of warning specifiers,
with no separators between them.  A warning specifier is one of the
following:

\begin{options}
\item["+"\var{num}] Enable warning number \var{num}.
\item["-"\var{num}] Disable warning number \var{num}.
\item["@"\var{num}] Enable and mark as fatal warning number \var{num}.
\item["+"\var{num1}..\var{num2}] Enable warnings in the given range.
\item["-"\var{num1}..\var{num2}] Disable warnings in the given range.
\item["@"\var{num1}..\var{num2}] Enable and mark as fatal warnings in
the given range.
\item["+"\var{letter}] Enable the set of warnings corresponding to
\var{letter}. The letter may be uppercase or lowercase.
\item["-"\var{letter}] Disable the set of warnings corresponding to
\var{letter}. The letter may be uppercase or lowercase.
\item["@"\var{letter}] Enable and mark as fatal the set of warnings
corresponding to \var{letter}. The letter may be uppercase or
lowercase.
\item[\var{uppercase-letter}] Enable the set of warnings corresponding
to \var{uppercase-letter}.
\item[\var{lowercase-letter}] Disable the set of warnings corresponding
to \var{lowercase-letter}.
\end{options}

Warning numbers and letters which are out of the range of warnings
that are currently defined are ignored. The warning are as follows.
\begin{options}
\input{warnings-help.tex}
\end{options}

The default setting is "-w +a-4-6-7-9-27-29-32..39-41..42-44-45".
It is displayed by "ocamlopt -help".
Note that warnings 5 and 10 are not always triggered, depending on
the internals of the type checker.

\item["-warn-error" \var{warning-list}]
Mark as fatal the warnings specified in the argument \var{warning-list}.
The compiler will stop with an error when one of these warnings is
emitted. The \var{warning-list} has the same meaning as for
the "-w" option: a "+" sign (or an uppercase letter) marks the
corresponding warnings as fatal, a "-"
sign (or a lowercase letter) turns them back into non-fatal warnings,
and a "@" sign both enables and marks as fatal the corresponding
warnings.

Note: it is not recommended to use warning sets (i.e. letters) as
arguments to "-warn-error"
in production code, because this can break your build when future versions
of OCaml add some new warnings.

The default setting is "-warn-error -a" (all warnings are non-fatal).

\item["-warn-help"]
Show the description of all available warning numbers.

\item["-where"]
Print the location of the standard library, then exit.

\item["-" \var{file}]
Process \var{file} as a file name, even if it starts with a dash (-)
character.

\item["-help" or "--help"]
Display a short usage summary and exit.
%
\end{options}

\noindent
On native Windows, the following environment variable is also consulted:

\begin{options}
\item["OCAML_FLEXLINK"]  Alternative executable to use instead of the
configured value. Primarily used for bootstrapping.
\end{options}

\paragraph{Options for the IA32 architecture}
The IA32 code generator (Intel Pentium, AMD Athlon) supports the
following additional option:

\begin{options}
\item["-ffast-math"] Use the IA32 instructions to compute
trigonometric and exponential functions, instead of calling the
corresponding library routines.  The functions affected are:
"atan", "atan2", "cos", "log", "log10", "sin", "sqrt" and "tan".
The resulting code runs faster, but the range of supported arguments
and the precision of the result can be reduced.  In particular,
trigonometric operations "cos", "sin", "tan" have their range reduced to
$[-2^{64}, 2^{64}]$.
\end{options}

\paragraph{Options for the AMD64 architecture}
The AMD64 code generator (64-bit versions of Intel Pentium and AMD
Athlon) supports the following additional options:

\begin{options}
\item["-fPIC"] Generate position-independent machine code.  This is
the default.
\item["-fno-PIC"] Generate position-dependent machine code.
\end{options}

\paragraph{Options for the Sparc architecture}
The Sparc code generator supports the following additional options:
\begin{options}
\item["-march=v8"] Generate SPARC version 8 code.
\item["-march=v9"] Generate SPARC version 9 code.
\end{options}
The default is to generate code for SPARC version 7, which runs on all
SPARC processors.

\section{Common errors}

The error messages are almost identical to those of "ocamlc".
See section~\ref{s:comp-errors}.

\section{Running executables produced by ocamlopt}

Executables generated by "ocamlopt" are native, stand-alone executable
files that can be invoked directly.  They do
not depend on the "ocamlrun" bytecode runtime system nor on
dynamically-loaded C/OCaml stub libraries.

During execution of an "ocamlopt"-generated executable,
the following environment variables are also consulted:
\begin{options}
\item["OCAMLRUNPARAM"]  Same usage as in "ocamlrun"
  (see section~\ref{ocamlrun-options}), except that option "l"
  is ignored (the operating system's stack size limit
  is used instead).
\item["CAMLRUNPARAM"]  If "OCAMLRUNPARAM" is not found in the
  environment, then "CAMLRUNPARAM" will be used instead.  If
  "CAMLRUNPARAM" is not found, then the default values will be used.
\end{options}

\section{Compatibility with the bytecode compiler}
\label{s:compat-native-bytecode}

This section lists the known incompatibilities between the bytecode
compiler and the native-code compiler. Except on those points, the two
compilers should generate code that behave identically.

\begin{itemize}

\item Signals are detected only when the program performs an
allocation in the heap. That is, if a signal is delivered while in a
piece of code that does not allocate, its handler will not be called
until the next heap allocation.

\item Stack overflow, typically caused by excessively deep recursion,
is handled in one of the following ways, depending on the
platform used:
\begin{itemize}
\item By raising a "Stack_overflow" exception, like the bytecode
  compiler does.  (IA32/Linux, AMD64/Linux, PowerPC/MacOSX, MS Windows
  32-bit ports).
\item By aborting the program on a ``segmentation fault'' signal.
(All other Unix systems.)
\item By terminating the program silently.
(MS Windows 64 bits).
\end{itemize}

\item On IA32 processors only (Intel and AMD x86 processors in 32-bit
mode), some intermediate results in floating-point computations are
kept in extended precision rather than being rounded to double
precision like the bytecode compiler always does.  Floating-point
results can therefore differ slightly between bytecode and native code.

\end{itemize}