[llvm-dev] Side-channel resistant values

[Sanjay Patel via llvm-dev]

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>