David Zarzycki via llvm-dev
2019-Sep-13 08:02 UTC
[llvm-dev] Side-channel resistant values
> 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 <mailto: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 <https://godbolt.org/z/S46I_q> Dave -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190913/88b2e14f/attachment.html>
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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190913/f79ead99/attachment.html>
I don't think the X86 cmov converter pass knows about unpredictable? Do we even preserve that metadata into Machine IR? There's also this frontend bug with the builtin getting translated to IR https://bugs.llvm.org/show_bug.cgi?id=40031 ~Craig On Fri, Sep 13, 2019 at 2:18 PM Sanjay Patel via llvm-dev < llvm-dev at lists.llvm.org> 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 >> > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190913/78042f04/attachment.html>
David Zarzycki via llvm-dev
2019-Sep-14 05:19 UTC
[llvm-dev] Side-channel resistant values
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