linux-kernel - [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR 58670

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Message-Id: <20220624141412.72274-1-ubizjak@gmail.com>
Date:   Fri, 24 Jun 2022 16:14:12 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR 58670

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.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
---
 include/linux/compiler-gcc.h | 11 -----------
 1 file changed, 11 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index a0c55eeaeaf1..9b157b71036f 100644
--- a/include/linux/compiler-gcc.h
+++ b/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__
-- 
2.35.3