The patch titled
Subject: compiler-gcc.h: remove ancient workaround for gcc PR 58670
has been added to the -mm mm-nonmm-unstable branch. Its filename is
compiler-gcch-remove-ancient-workaround-for-gcc-pr-58670.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/compiler-gcch-remove-ancient-workaround-for-gcc-pr-58670.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: compiler-gcc.h: remove ancient workaround for gcc PR 58670
Date: Fri, 24 Jun 2022 16:14:12 +0200
The workaround for 'asm goto' miscompilation introduces a compiler barrier
quirk that inhibits many useful compiler optimizations. For example,
__try_cmpxchg_user compiles to:
11375: 41 8b 4d 00 mov 0x0(%r13),%ecx
11379: 41 8b 02 mov (%r10),%eax
1137c: f0 0f b1 0a lock cmpxchg %ecx,(%rdx)
11380: 0f 94 c2 sete %dl
11383: 84 d2 test %dl,%dl
11385: 75 c4 jne 1134b <...>
11387: 41 89 02 mov %eax,(%r10)
where the barrier inhibits flags propagation from asm when compiled with
gcc-12.
When the mentioned quirk is removed, the following code is generated:
11553: 41 8b 4d 00 mov 0x0(%r13),%ecx
11557: 41 8b 02 mov (%r10),%eax
1155a: f0 0f b1 0a lock cmpxchg %ecx,(%rdx)
1155e: 74 c9 je 11529 <...>
11560: 41 89 02 mov %eax,(%r10)
The refered compiler bug:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
was fixed for gcc-4.8.2.
Current minimum required version of GCC is version 5.1 which has the above
'asm goto' miscompilation fixed, so remove the workaround.
Link: https://lkml.kernel.org/r/20220624141412.72274-1-ubizjak@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/compiler-gcc.h | 11 -----------
1 file changed, 11 deletions(-)
--- a/include/linux/compiler-gcc.h~compiler-gcch-remove-ancient-workaround-for-gcc-pr-58670
+++ a/include/linux/compiler-gcc.h
@@ -66,17 +66,6 @@
__builtin_unreachable(); \
} while (0)
-/*
- * GCC 'asm goto' miscompiles certain code sequences:
- *
- * http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
- *
- * Work it around via a compiler barrier quirk suggested by Jakub Jelinek.
- *
- * (asm goto is automatically volatile - the naming reflects this.)
- */
-#define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)
-
#if defined(CONFIG_ARCH_USE_BUILTIN_BSWAP)
#define __HAVE_BUILTIN_BSWAP32__
#define __HAVE_BUILTIN_BSWAP64__
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
compiler-gcch-remove-ancient-workaround-for-gcc-pr-58670.patch
