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SubjectRe: [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR 58670
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 <ubizjak@gmail.com>
> Cc: Andrew Morton <akpm@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-sv@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

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