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Diffstat (limited to 'gcc/doc/tree-ssa.texi')
| -rw-r--r-- | gcc/doc/tree-ssa.texi | 35 |
1 files changed, 32 insertions, 3 deletions
diff --git a/gcc/doc/tree-ssa.texi b/gcc/doc/tree-ssa.texi index 659431b0274..ebb85a05769 100644 --- a/gcc/doc/tree-ssa.texi +++ b/gcc/doc/tree-ssa.texi @@ -41,6 +41,7 @@ passes for GIMPLE@. * SSA Operands:: SSA names referenced by GIMPLE statements. * SSA:: Static Single Assignment representation. * Alias analysis:: Representing aliased loads and stores. +* Memory model:: Memory model used by the middle-end. @end menu @node Annotations @@ -108,9 +109,9 @@ full object that they represent. For instance, given Since @code{a} and @code{b} are non-aliased locals, the statement @code{a = b} will have one real definition and one real use because -variable @code{b} is completely modified with the contents of -variable @code{a}. Real definition are also known as @dfn{killing -definitions}. Similarly, the use of @code{a} reads all its bits. +variable @code{a} is completely modified with the contents of +variable @code{b}. Real definition are also known as @dfn{killing +definitions}. Similarly, the use of @code{b} reads all its bits. In contrast, virtual operands are used with variables that can have a partial or ambiguous reference. This includes structures, arrays, @@ -892,3 +893,31 @@ providing its aliasing VDEF. The walk stops if asked to. @end enumerate + +@node Memory model +@section Memory model +@cindex memory model + +The memory model used by the middle-end models that of the C/C++ +languages. The middle-end has the notion of an effective type +of a memory region which is used for type-based alias analysis. + +The following is a refinement of ISO C99 6.5/6, clarifying the block copy case +to follow common sense and extending the concept of a dynamic effective +type to objects with a declared type as required for C++. + +@smallexample +The effective type of an object for an access to its stored value is +the declared type of the object or the effective type determined by +a previous store to it. If a value is stored into an object through +an lvalue having a type that is not a character type, then the +type of the lvalue becomes the effective type of the object for that +access and for subsequent accesses that do not modify the stored value. +If a value is copied into an object using @code{memcpy} or @code{memmove}, +or is copied as an array of character type, then the effective type +of the modified object for that access and for subsequent accesses that +do not modify the value is undetermined. For all other accesses to an +object, the effective type of the object is simply the type of the +lvalue used for the access. +@end smallexample + |