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>

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;