From 8a9a59311551e833ca064de44ac23b193e1b704d Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Wed, 6 Oct 2021 20:05:48 +0100 Subject: [PATCH] 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 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 Reviewed-by: Carlos O'Donell Tested-by: Carlos O'Donell --- malloc/malloc.h | 5 +++-- misc/sys/cdefs.h | 10 ++++++++++ stdlib/stdlib.h | 4 +++- support/support.h | 5 +++-- 4 files changed, 19 insertions(+), 5 deletions(-) diff --git a/malloc/malloc.h b/malloc/malloc.h index 2df0b38..0e8a0cf 100644 --- a/malloc/malloc.h +++ b/malloc/malloc.h @@ -1,5 +1,6 @@ /* Prototypes and definition for malloc implementation. Copyright (C) 1996-2021 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 @@ -64,8 +65,8 @@ extern void free (void *__ptr) __THROW; /* Allocate SIZE bytes allocated to ALIGNMENT bytes. */ extern void *memalign (size_t __alignment, size_t __size) - __THROW __attribute_malloc__ __attribute_alloc_size__ ((2)) __wur - __attr_dealloc_free; + __THROW __attribute_malloc__ __attribute_alloc_align__ ((1)) + __attribute_alloc_size__ ((2)) __wur __attr_dealloc_free; /* Allocate SIZE bytes on a page boundary. */ extern void *valloc (size_t __size) __THROW __attribute_malloc__ diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h index c353d2e..ab57d4a 100644 --- a/misc/sys/cdefs.h +++ b/misc/sys/cdefs.h @@ -1,4 +1,5 @@ /* Copyright (C) 1992-2021 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 @@ -290,6 +291,15 @@ # define __attribute_alloc_size__(params) /* Ignore. */ #endif +/* Tell the compiler which argument to an allocation function + indicates the alignment of the allocation. */ +#if __GNUC_PREREQ (4, 9) || __glibc_has_attribute (__alloc_align__) +# define __attribute_alloc_align__(param) \ + __attribute__ ((__alloc_align__ param)) +#else +# define __attribute_alloc_align__(param) /* Ignore. */ +#endif + /* At some point during the gcc 2.96 development the `pure' attribute for functions was introduced. We don't want to use it unconditionally (although this would be possible) since it generates warnings. */ diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h index 74c00ee..1feb381 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. */ diff --git a/support/support.h b/support/support.h index 0ee454d..10a4a36 100644 --- a/support/support.h +++ b/support/support.h @@ -1,5 +1,6 @@ /* Common extra functions. Copyright (C) 2016-2021 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 @@ -104,8 +105,8 @@ extern void *xrealloc (void *o, size_t n) extern char *xstrdup (const char *) __attribute_malloc__ __attr_dealloc_free __returns_nonnull; void *xposix_memalign (size_t alignment, size_t n) - __attribute_malloc__ __attribute_alloc_size__ ((2)) __attr_dealloc_free - __returns_nonnull; + __attribute_malloc__ __attribute_alloc_align__ ((1)) + __attribute_alloc_size__ ((2)) __attr_dealloc_free __returns_nonnull; char *xasprintf (const char *format, ...) __attribute__ ((format (printf, 1, 2), malloc)) __attr_dealloc_free __returns_nonnull; -- 2.7.4