[llvm-dev] Side-channel resistant values

Chandler Carruth via llvm-dev
2019-09-14T05:35:38Z

The x86 backend is extremely aggressive in turning cmov with memory
operands into branches because that is often faster even for poorly
predicted branches due to the forced stall in the cmov.

On Fri, Sep 13, 2019 at 11:19 PM David Zarzycki <dave at znu.io> wrote:

> I’m struggling to find cases where __builtin_unpredictable() works at all.
> Even if we ignore cmp/br into switch conversion, it still doesn’t work:
>
> int test_cmov(int left, int right, int *alt) {
>         return __builtin_unpredictable(left < right) ? *alt : 999;
> }
>
> Should generate:
>
> test_cmov:
>         movl    $999, %eax
>         cmpl    %esi, %edi
>         cmovll  (%rdx), %eax
>         retq
>
> But currently generates:
>
> test_cmov:
>         movl    $999, %eax
>         cmpl    %esi, %edi
>         jge     .LBB0_2
>         movl    (%rdx), %eax
> .LBB0_2:
>         retq
>
>
>
> > On Sep 14, 2019, at 12:18 AM, Sanjay Patel <spatel at
rotateright.com>
> wrote:
> >
> > I'm not sure if this is the entire problem, but SimplifyCFG loses
the
> 'unpredictable' metadata when it converts a set of cmp/br into a
switch:
> > https://godbolt.org/z/neLzN3
> >
> > Filed here:
> > https://bugs.llvm.org/show_bug.cgi?id=43313
> >
> > On Fri, Sep 13, 2019 at 4:02 AM David Zarzycki via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> >
> >> On Sep 13, 2019, at 10:45 AM, Chandler Carruth <chandlerc at
gmail.com>
> wrote:
> >>
> >> On Fri, Sep 13, 2019 at 1:33 AM David Zarzycki via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >> Hi Chandler,
> >>
> >> The data-invariant feature sounds great but what about the general
> case? When performance tuning code, people sometimes need the ability to
> reliably generate CMOV, and right now the best advice is either “use inline
> assembly” or “keep refactoring until CMOV is emited” (and hope that future
> compilers continue to generate CMOV).
> >>
> >> Given that a patch already exists to reliably generate CMOV, are
there
> any good arguments against adding the feature?
> >>
> >> For *performance* tuning, the builtin that Hal mentioned is IMO
the
> correct design.
> >>
> >> Is there some reason why it doesn't work?
> >
> > I wasn’t aware of __builtin_unpredictable() until now and I haven’t
> debugged why it doesn’t work, but here are a couple examples, one using the
> ternary operator and one using a switch statement:
> >
> > https://godbolt.org/z/S46I_q
> >
> > Dave
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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