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PR middle-end/99295 - documentation on __attribute__((malloc)) is wrong

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gcc/ChangeLog:

	PR middle-end/99295
	* doc/extend.texi (attribute malloc): Reword and clarify nonaliasing
	property.
This commit is contained in:
Martin Sebor 2021-03-02 11:19:49 -07:00
parent 66ecb059c9
commit 397ed1dbff
1 changed files with 11 additions and 8 deletions
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19
gcc/doc/extend.texi
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@ -3240,7 +3240,9 @@ calls.
Attribute @code{malloc} indicates that a function is @code{malloc}-like,
i.e., that the pointer @var{P} returned by the function cannot alias any
other pointer valid when the function returns, and moreover no
pointers to valid objects occur in any storage addressed by @var{P}.
pointers to valid objects occur in any storage addressed by @var{P}. In
addition, the GCC predicts that a function with the attribute returns
non-null in most cases.
Independently, the form of the attribute with one or two arguments
associates @code{deallocator} as a suitable deallocation function for
@ -3248,13 +3250,14 @@ pointers returned from the @code{malloc}-like function. @var{ptr-index}
denotes the positional argument to which when the pointer is passed in
calls to @code{deallocator} has the effect of deallocating it.
Using the attribute with no arguments is designed to improve optimization.
The compiler predicts that a function with the attribute returns non-null
in most cases. Functions like @code{malloc} and @code{calloc} have this
property because they return a pointer to uninitialized or zeroed-out
storage. However, functions like @code{realloc} do not have this property,
as they may return pointers to storage containing pointers to existing
objects.
Using the attribute with no arguments is designed to improve optimization
by relying on the aliasing property it implies. Functions like @code{malloc}
and @code{calloc} have this property because they return a pointer to
uninitialized or zeroed-out, newly obtained storage. However, functions
like @code{realloc} do not have this property, as they may return pointers
to storage containing pointers to existing objects. Additionally, since
all such functions are assumed to return null only infrequently, callers
can be optimized based on that assumption.
Associating a function with a @var{deallocator} helps detect calls to
mismatched allocation and deallocation functions and diagnose them under