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authorJonathan Wakely <jwakely@redhat.com>2021-10-06 20:05:48 +0100
committerJonathan Wakely <jwakely@redhat.com>2021-10-21 00:19:20 +0100
commit8a9a59311551e833ca064de44ac23b193e1b704d (patch)
tree1d91ec3b267277e45d849261406337c1c4fa5a3f /stdlib
parentaa783f9a7b774d67487daa9376095738aef5cf88 (diff)
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Add alloc_align attribute to memalign et al
GCC 4.9.0 added the alloc_align attribute to say that a function
argument specifies the alignment of the returned pointer. Clang supports
the attribute too. Using the attribute can allow a compiler to generate
better code if it knows the returned pointer has a minimum alignment.
See https://gcc.gnu.org/PR60092 for more details.

GCC implicitly knows the semantics of aligned_alloc and posix_memalign,
but not the obsolete memalign. As a result, GCC generates worse code
when memalign is used, compared to aligned_alloc.  Clang knows about
aligned_alloc and memalign, but not posix_memalign.

This change adds a new __attribute_alloc_align__ macro to <sys/cdefs.h>
and then uses it on memalign (where it helps GCC) and aligned_alloc
(where GCC and Clang already know the semantics, but it doesn't hurt)
and xposix_memalign. It can't be used on posix_memalign because that
doesn't return a pointer (the allocated pointer is returned via a void**
parameter instead).

Unlike the alloc_size attribute, alloc_align only allows a single
argument. That means the new __attribute_alloc_align__ macro doesn't
really need to be used with double parentheses to protect a comma
between its arguments. For consistency with __attribute_alloc_size__
this patch defines it the same way, so that double parentheses are
required.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
Diffstat (limited to 'stdlib')
-rw-r--r--stdlib/stdlib.h4
1 files changed, 3 insertions, 1 deletions
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index 74c00eee73..1feb381211 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -1,4 +1,5 @@
 /* Copyright (C) 1991-2021 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -589,7 +590,8 @@ extern int posix_memalign (void **__memptr, size_t __alignment, size_t __size)
 #ifdef __USE_ISOC11
 /* ISO C variant of aligned allocation.  */
 extern void *aligned_alloc (size_t __alignment, size_t __size)
-     __THROW __attribute_malloc__ __attribute_alloc_size__ ((2)) __wur;
+     __THROW __attribute_malloc__ __attribute_alloc_align__ ((1))
+     __attribute_alloc_size__ ((2)) __wur;
 #endif
 
 /* Abort execution and generate a core-dump.  */