From: Richard Biener <rguenther@suse.de>
Date: Thu, 23 Nov 2023 07:54:56 +0000 (+0100)
Subject: middle-end/32667 - document cpymem and memcpy exact overlap requirement
X-Git-Tag: basepoints/gcc-15~4367
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7758cb4b53e8a33642709402ce582f769eb9fd18;p=thirdparty%2Fgcc.git

middle-end/32667 - document cpymem and memcpy exact overlap requirement

The following amends the cpymem documentation to mention that exact
overlap needs to be handled gracefully, also noting that the target
runtime is expected to behave the same way where -ffreestanding
docs mention the set of routines required.

	PR middle-end/32667
	* doc/md.texi (cpymem): Document that exact overlap of source
	and destination needs to work.
	* doc/standards.texi (ffreestanding): Mention memcpy is required
	to handle the exact overlap case.
---

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index df6725ffc9c2..536ce997f019 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -6915,8 +6915,9 @@ individually copied data units in the block.
 
 The @code{cpymem@var{m}} patterns need not give special consideration
 to the possibility that the source and destination strings might
-overlap. These patterns are used to do inline expansion of
-@code{__builtin_memcpy}.
+overlap.  An exception is the case where source and destination are
+equal, this case needs to be handled correctly.
+These patterns are used to do inline expansion of @code{__builtin_memcpy}.
 
 @cindex @code{movmem@var{m}} instruction pattern
 @item @samp{movmem@var{m}}
diff --git a/gcc/doc/standards.texi b/gcc/doc/standards.texi
index 4eb43f14f904..6eebb9426f3d 100644
--- a/gcc/doc/standards.texi
+++ b/gcc/doc/standards.texi
@@ -184,7 +184,9 @@ GNU C library).  @xref{Standard Libraries,,Standard Libraries}.
 Most of the compiler support routines used by GCC are present in
 @file{libgcc}, but there are a few exceptions.  GCC requires the
 freestanding environment provide @code{memcpy}, @code{memmove},
-@code{memset} and @code{memcmp}.
+@code{memset} and @code{memcmp}.  Contrary to the standards
+covering @code{memcpy} GCC expects the case of an exact overlap
+of source and destination to work and not invoke undefined behavior.
 Finally, if @code{__builtin_trap} is used, and the target does
 not implement the @code{trap} pattern, then GCC emits a call
 to @code{abort}.