Messages in this thread | | | Date | Thu, 25 Aug 2022 16:00:52 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR 58670 |
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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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