[gcc r10-10009] Backport from GCC 11 [PR99295].

[gcc r10-10009] Backport from GCC 11 [PR99295].

[Martin Sebor]

https://gcc.gnu.org/g:9d9cebd9205c7e28bd4d76c39dd9a643018791af

commit r10-10009-g9d9cebd9205c7e28bd4d76c39dd9a643018791af
Author: Martin Sebor <msebor@redhat.com>
Date:   Wed Jul 28 11:43:33 2021 -0600

    Backport from GCC 11 [PR99295].
    
    gcc/ChangeLog:
    
            PR c/99295
            * doc/extend.texi (attribute malloc): Reword and clarify nonaliasing
            property.

Diff:
---
 gcc/doc/extend.texi | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index afa87d89d5f..dd04dd27264 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -3228,15 +3228,18 @@ calls.
 This tells the compiler 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}.
-
-Using this attribute can improve optimization.  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 can return a
-pointer to storage containing pointers.
+pointers to valid objects occur in any storage addressed by @var{P}. In
+addition, GCC predicts that a function with the attribute returns
+non-null in most cases.
+
+Using the attribute 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.
 
 @item no_icf
 @cindex @code{no_icf} function attribute