[glibc/maskray/relr] Add alloc_align attribute to memalign et al

[glibc/maskray/relr] Add alloc_align attribute to memalign et al

[Fangrui Song]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=8a9a59311551e833ca064de44ac23b193e1b704d

commit 8a9a59311551e833ca064de44ac23b193e1b704d
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 6 20:05:48 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>
    
    Reviewed-by: Carlos O'Donell <carlos@redhat.com>
    Tested-by: Carlos O'Donell <carlos@redhat.com>

Diff:
---
 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 2df0b38050..0e8a0cf051 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 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.  */
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index 74c00eee73..1feb381211 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 0ee454da6d..10a4a36848 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;