Uros Bizjak wrote:
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 <ubiz...@gmail.com>
Cc: Andrew Morton <a...@linux-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__
This is causing a build issue on ppc64le with a new patch replacing use
of unreachable() with __builtin_unreachable() in __WARN_FLAGS():
https://lore.kernel.org/linuxppc-dev/20220808114908.240813-2...@linux.ibm.com/
during RTL pass: combine
In file included from /linux/kernel/locking/rtmutex_api.c:9:
/linux/kernel/locking/rtmutex.c: In function '__rt_mutex_slowlock.constprop':
/linux/kernel/locking/rtmutex.c:1612:1: internal compiler error: in
purge_dead_edges, at cfgrtl.c:3369
1612 | }
| ^
0x142817c internal_error(char const*, ...)
???:0
0x5c8a1b fancy_abort(char const*, int, char const*)
???:0
0x72017f purge_all_dead_edges()
???:0
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <file:///usr/share/doc/gcc-11/README.Bugs> for instructions.
So, it looks like gcc still has issues with certain uses of asm goto.
- Naveen
