On Tue, Nov 17, 2015 at 11:32 AM, Craig Topper <craig.top...@gmail.com> wrote:
> craig.topper added a subscriber: craig.topper. > craig.topper added a comment. > > The summary of why this is ok is slightly misleading. I read this as reason why programs might try to use this, not why this change here is correct. (This change is ok as it has an explicit 0 check as you say, so it'll only be efficient once you target machines with BMI) > The backend won't try to encode the TZCNT instruction when the BMI feature > is not enabled. Notice this just maps the to the generic non-x86 specific > __builtin_ctz. The backend will emit a regular BSF instruction if BMI is > not supported. There will be a zero check around it unless the backend can > prove that the input is not 0. > > > Repository: > rL LLVM > > http://reviews.llvm.org/D14748 > > > >
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