[llvm-dev] atomic ops are optimized with incorrect semantics .

Umesh Kalappa via llvm-dev
Tue, 11 Feb 2020 09:43:40 -0000

Thank you All and Nicolai,

This is what we get with ARM like


(CLANG 9.0.1.1)
1834 0000000080001ccc <bm>:
1835 80001ccc:¦ 52800028 ¦ mov¦w8, #0x1 ¦// #1
1836 80001cd0:¦ b9000008 ¦ str¦w8, [x0]
1837 80001cd4:¦ 17ffffff ¦ b¦ 80001cd0 <bm+0x4>

(CLANG 8.0.0.2)
1833 0000000080001c90 <bm>:
1834 80001c90:> 320003e8 > orr>w8, wzr, #0x1
1835 80001c94:> 885f7c1f > ldxr> wzr, [x0]
1836 80001c98:> 881f7c08 > stxr> wzr, w8, [x0]
1837 80001c9c:> 17fffffe > b> 80001c94 <bm+0x4>

may be in x86 ,you are right mov is atomic, but if we need to sync the
operation with the previous insn's then just having atomic doesn't help
right?


~Umesh

On Tue, Feb 11, 2020 at 2:08 AM Nicolai Hähnle <nhaehnle at gmail.com>
wrote:

> Hi Umesh,
>
> On Mon, Feb 10, 2020 at 2:06 PM Umesh Kalappa via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hi All,
> >
> > With the "https://gcc.godbolt.org/z/yBYTrd"  case .
> >
> > the atomic is converted to non atomic ops  for x86 like
> >
> > from
> > xchg dword ptr [100], eax
> > to
> > mov dword ptr [100], 1
> >
> > the pass is responsible for this tranformation was instCombine i.e
> InstCombiner::visitAtomicRMWInst
> >
> > which converts the IR like
> >  %0 = atomicrmw xchg i32* inttoptr (i64 100 to i32*), i32 1 monotonic
> > to
> >  store atomic i32 1, i32* inttoptr (i64 100 to i32*) monotonic, align
4
>
> Can you explain why you think this code generation is incorrect? A
> simple mov is an atomic store on x86 from what I understand - the
> godbolt example seems perfectly correct to me, what am I missing?
>
> Cheers,
> Nicolai
>
>
> >
> > which is valid for relax(monotonic) and release ordering as per the
code
> out there.
> >
> > we think that,its the inst lowering issue, where the atomic store was
> lowered to non-atomic store here.
> >
> > to work around we changed our case to strong ordering .
> >
> > and we are debugging the case with x86 and the same goes with an arm
too.
> >
> > Any suggestions or thoughts on the transformation? , will be helpful
to
> proceed further.
> >
> > FYI, the problem persists with LLVM-9, not with the LLVM-8.
> >
> > Thank you
> > ~Umesh
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> --
> Lerne, wie die Welt wirklich ist,
> aber vergiss niemals, wie sie sein sollte.
>
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