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asm-generic: unaligned: remove byteshift helpers

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In theory, compilers should be able to work this out themselves so we
can use a simpler version based on the swab() helpers.

I have verified that this works on all supported compiler versions
(gcc-4.9 and up, clang-10 and up). Looking at the object code produced by
gcc-11, I found that the impact is mostly a change in inlining decisions
that lead to slightly larger code.

In other cases, this version produces explicit byte swaps in place of
separate byte access, or comparing against pre-swapped constants.

While the source code is clearly simpler, I have not seen an indication
of the new version actually producing better code on Arm, so maybe
we want to skip this after all. From what I can tell, gcc recognizes
the byteswap pattern in the byteshift.h header and can turn it into
explicit instructions, but it does not turn a __builtin_bswap32() back
into individual bytes when that would result in better output, e.g.
when storing a byte-reversed constant.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
This commit is contained in:
Arnd Bergmann 2021-05-08 00:07:51 +02:00
parent f12d3ff3f4
commit 0652035a57
6 changed files with 60 additions and 146 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/arm/include/asm/unaligned.h
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@ -10,13 +10,11 @@
#if defined(__LITTLE_ENDIAN)
# include <linux/unaligned/le_struct.h>
# include <linux/unaligned/be_byteshift.h>
# include <linux/unaligned/generic.h>
# define get_unaligned __get_unaligned_le
# define put_unaligned __put_unaligned_le
#elif defined(__BIG_ENDIAN)
# include <linux/unaligned/be_struct.h>
# include <linux/unaligned/le_byteshift.h>
# include <linux/unaligned/generic.h>
# define get_unaligned __get_unaligned_be
# define put_unaligned __put_unaligned_be

2
include/asm-generic/unaligned.h
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@ -16,7 +16,6 @@
#if defined(__LITTLE_ENDIAN)
# ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
# include <linux/unaligned/le_struct.h>
# include <linux/unaligned/be_byteshift.h>
# endif
# include <linux/unaligned/generic.h>
# define get_unaligned __get_unaligned_le
@ -24,7 +23,6 @@
#elif defined(__BIG_ENDIAN)
# ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
# include <linux/unaligned/be_struct.h>
# include <linux/unaligned/le_byteshift.h>
# endif
# include <linux/unaligned/generic.h>
# define get_unaligned __get_unaligned_be

71
include/linux/unaligned/be_byteshift.h
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@ -1,71 +0,0 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_UNALIGNED_BE_BYTESHIFT_H
#define _LINUX_UNALIGNED_BE_BYTESHIFT_H
#include <linux/types.h>
static inline u16 __get_unaligned_be16(const u8 *p)
{
return p[0] << 8 | p[1];
}
static inline u32 __get_unaligned_be32(const u8 *p)
{
return p[0] << 24 | p[1] << 16 | p[2] << 8 | p[3];
}
static inline u64 __get_unaligned_be64(const u8 *p)
{
return (u64)__get_unaligned_be32(p) << 32 |
__get_unaligned_be32(p + 4);
}
static inline void __put_unaligned_be16(u16 val, u8 *p)
{
*p++ = val >> 8;
*p++ = val;
}
static inline void __put_unaligned_be32(u32 val, u8 *p)
{
__put_unaligned_be16(val >> 16, p);
__put_unaligned_be16(val, p + 2);
}
static inline void __put_unaligned_be64(u64 val, u8 *p)
{
__put_unaligned_be32(val >> 32, p);
__put_unaligned_be32(val, p + 4);
}
static inline u16 get_unaligned_be16(const void *p)
{
return __get_unaligned_be16(p);
}
static inline u32 get_unaligned_be32(const void *p)
{
return __get_unaligned_be32(p);
}
static inline u64 get_unaligned_be64(const void *p)
{
return __get_unaligned_be64(p);
}
static inline void put_unaligned_be16(u16 val, void *p)
{
__put_unaligned_be16(val, p);
}
static inline void put_unaligned_be32(u32 val, void *p)
{
__put_unaligned_be32(val, p);
}
static inline void put_unaligned_be64(u64 val, void *p)
{
__put_unaligned_be64(val, p);
}
#endif /* _LINUX_UNALIGNED_BE_BYTESHIFT_H */

30
include/linux/unaligned/be_struct.h
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@ -34,4 +34,34 @@ static inline void put_unaligned_be64(u64 val, void *p)
__put_unaligned_cpu64(val, p);
}
static inline u16 get_unaligned_le16(const void *p)
{
return swab16(__get_unaligned_cpu16((const u8 *)p));
}
static inline u32 get_unaligned_le32(const void *p)
{
return swab32(__get_unaligned_cpu32((const u8 *)p));
}
static inline u64 get_unaligned_le64(const void *p)
{
return swab64(__get_unaligned_cpu64((const u8 *)p));
}
static inline void put_unaligned_le16(u16 val, void *p)
{
__put_unaligned_cpu16(swab16(val), p);
}
static inline void put_unaligned_le32(u32 val, void *p)
{
__put_unaligned_cpu32(swab32(val), p);
}
static inline void put_unaligned_le64(u64 val, void *p)
{
__put_unaligned_cpu64(swab64(val), p);
}
#endif /* _LINUX_UNALIGNED_BE_STRUCT_H */

71
include/linux/unaligned/le_byteshift.h
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@ -1,71 +0,0 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_UNALIGNED_LE_BYTESHIFT_H
#define _LINUX_UNALIGNED_LE_BYTESHIFT_H
#include <linux/types.h>
static inline u16 __get_unaligned_le16(const u8 *p)
{
return p[0] | p[1] << 8;
}
static inline u32 __get_unaligned_le32(const u8 *p)
{
return p[0] | p[1] << 8 | p[2] << 16 | p[3] << 24;
}
static inline u64 __get_unaligned_le64(const u8 *p)
{
return (u64)__get_unaligned_le32(p + 4) << 32 |
__get_unaligned_le32(p);
}
static inline void __put_unaligned_le16(u16 val, u8 *p)
{
*p++ = val;
*p++ = val >> 8;
}
static inline void __put_unaligned_le32(u32 val, u8 *p)
{
__put_unaligned_le16(val >> 16, p + 2);
__put_unaligned_le16(val, p);
}
static inline void __put_unaligned_le64(u64 val, u8 *p)
{
__put_unaligned_le32(val >> 32, p + 4);
__put_unaligned_le32(val, p);
}
static inline u16 get_unaligned_le16(const void *p)
{
return __get_unaligned_le16(p);
}
static inline u32 get_unaligned_le32(const void *p)
{
return __get_unaligned_le32(p);
}
static inline u64 get_unaligned_le64(const void *p)
{
return __get_unaligned_le64(p);
}
static inline void put_unaligned_le16(u16 val, void *p)
{
__put_unaligned_le16(val, p);
}
static inline void put_unaligned_le32(u32 val, void *p)
{
__put_unaligned_le32(val, p);
}
static inline void put_unaligned_le64(u64 val, void *p)
{
__put_unaligned_le64(val, p);
}
#endif /* _LINUX_UNALIGNED_LE_BYTESHIFT_H */

30
include/linux/unaligned/le_struct.h
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@ -34,4 +34,34 @@ static inline void put_unaligned_le64(u64 val, void *p)
__put_unaligned_cpu64(val, p);
}
static inline u16 get_unaligned_be16(const void *p)
{
return swab16(__get_unaligned_cpu16((const u8 *)p));
}
static inline u32 get_unaligned_be32(const void *p)
{
return swab32(__get_unaligned_cpu32((const u8 *)p));
}
static inline u64 get_unaligned_be64(const void *p)
{
return swab64(__get_unaligned_cpu64((const u8 *)p));
}
static inline void put_unaligned_be16(u16 val, void *p)
{
__put_unaligned_cpu16(swab16(val), p);
}
static inline void put_unaligned_be32(u32 val, void *p)
{
__put_unaligned_cpu32(swab32(val), p);
}
static inline void put_unaligned_be64(u64 val, void *p)
{
__put_unaligned_cpu64(swab64(val), p);
}
#endif /* _LINUX_UNALIGNED_LE_STRUCT_H */