Messages in this thread | | | From | Uros Bizjak <> | Date | Thu, 25 Aug 2022 13:34:22 +0200 | Subject | Re: [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR 58670 |
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On Thu, Aug 25, 2022 at 12:30 PM Naveen N. Rao <naveen.n.rao@linux.ibm.com> wrote: > > 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.
This looks like a powerpc specific bug, and has nothing to do with PR58560 (asm goto miscompilation). If this is indeed a target specific bug, it should be worked around in arch/powerpc/include/asm/compiler.h, but please also report it to the GCC bugzilla.
Uros.
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