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Add alloc_align attribute to memalign et al

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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>
This commit is contained in:
Jonathan Wakely
2021-10-06 20:05:48 +01:00
parent aa783f9a7b
commit 8a9a593115
4 changed files with 19 additions and 5 deletions
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4
stdlib/stdlib.h
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@@ -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. */