[PATCH] Add alloc_align attribute to memalign et al
Jonathan Wakely
jwakely@redhat.com
Thu Sep 30 10:19:37 GMT 2021
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commit ec065256995fc8756972291a79011320fa180649
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Thu Sep 30 10:48:40 2021 +0100
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>
diff --git a/malloc/malloc.h b/malloc/malloc.h
index 2df0b38050..78d3ad82b2 100644
--- a/malloc/malloc.h
+++ b/malloc/malloc.h
@@ -64,8 +64,8 @@ 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 4dac9d264d..7b2a9bd3b0 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -243,6 +243,15 @@
# 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 0481c12355..45df9a12c6 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -589,7 +589,8 @@ 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 837a806531..91c7178f06 100644
--- a/support/support.h
+++ b/support/support.h
@@ -98,8 +98,8 @@ 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;
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