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>

1 files changed, 10 insertions, 0 deletions

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index c353d2ec4d..ab57d4a065 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.  */