Date: Wed, 12 Aug 2020 09:44:25 +0400 From: Gleb Popov <arrowd@freebsd.org> To: =?UTF-8?Q?T=C4=B3l_Coosemans?= <tijl@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, toolchain@freebsd.org Subject: Re: Undefined reference to __atomic_store_8 Message-ID: <CALH631mVop8dKbe775VdyjSeLA1sKEO-yDdufWa%2BiT2Vi8TEFg@mail.gmail.com> In-Reply-To: <CALH631kJhO3LTYqHsd%2Bu0c_KNegQVaphbyhrbtTxGLkLHQ9s3w@mail.gmail.com> References: <CALH631msD2jEwaH2ejzdE5RCYd=CA2GYV-o-%2BfC6sPSskXdkTg@mail.gmail.com> <20200807212855.GB2551@kib.kiev.ua> <CALH631=FxoEdp_akV-szxTuLVrZ4oFgHWKcWJNOGEuLwqJ3Hyw@mail.gmail.com> <20200808133000.GC2551@kib.kiev.ua> <CALH631nqdHq02xYM4cnOaPhmB5L3O5TJyZiKVNrUJu7kHg0uFQ@mail.gmail.com> <20200809143742.430764e7@FreeBSD.org> <20200809154312.GH2551@kib.kiev.ua> <CALH631=7M6J3L7AzgHqEwtyBo6f-9SPM4w3PDAULUACVwJMVyg@mail.gmail.com> <20200811235210.41049ad1@FreeBSD.org> <CALH631kJhO3LTYqHsd%2Bu0c_KNegQVaphbyhrbtTxGLkLHQ9s3w@mail.gmail.com>
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On Wed, Aug 12, 2020 at 9:21 AM Gleb Popov <arrowd@freebsd.org> wrote:
> Indeed, this looks like a culprit! When compiling using first command line
> (the long one) I get following warnings:
>
> /wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/ghc-prim/cbits/atomic.c:369:10:
> warning: misaligned atomic operation may incur significant performance
> penalty [-Watomic-alignment]
> return __atomic_load_n((StgWord64 *) x, __ATOMIC_SEQ_CST);
> ^
> /wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/ghc-prim/cbits/atomic.c:417:3:
> warning: misaligned atomic operation may incur significant performance
> penalty [-Watomic-alignment]
> __atomic_store_n((StgWord64 *) x, (StgWord64) val, __ATOMIC_SEQ_CST);
> ^
> 2 warnings generated.
>
> I guess this basically means "I'm emitting a call there". So, what's the
> correct fix in this case?
>
I just noticed that Clang emits these warnings (and the call instruction)
only for functions handling StgWord64 type. For the same code with
StgWord32, like
StgWord
hs_atomicread32(StgWord x)
{
#if HAVE_C11_ATOMICS
return __atomic_load_n((StgWord32 *) x, __ATOMIC_SEQ_CST);
#else
return __sync_add_and_fetch((StgWord32 *) x, 0);
#endif
}
no warning is emitted as well as no call.
How does clang infer alignment in these cases? What's so special about
StgWord64?
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