[llvm-dev] Side-channel resistant values

[Finkel, Hal J. via llvm-dev]

On 9/13/19 4:41 PM, Craig Topper via llvm-dev wrote:
I don't think the X86 cmov converter pass knows about unpredictable? Do we even preserve that metadata into Machine IR?


AFAIK, no. It doesn't make it past the SDAGBuilder (mostly, it is used in CGP in order to prevent the select -> branch conversion). It also, in both SDAGBuilder and CGP, prevents splitting of logical operations feeding conditional branches - splitting by forming more branches).

 -Hal


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<mailto: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<mailto:llvm-dev at lists.llvm.org>> wrote:


On Sep 13, 2019, at 10:45 AM, Chandler Carruth <chandlerc at gmail.com<mailto: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

Dave
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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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