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...opinion, the original C code should have produced a “select” and give the backend the opportunity to optimise it if required. I think that the frontend should perform only target independent optimisations. You didn't specify how you compile that code. We could also get: https://godbolt.org/z/B-5lj1 Which can actually be folded further to just long test(long a, long b) { return a * b; } Is "test" actually an implementation of a 64-bit-wide multiplication compiler-rt builtin? Then i'd think the main problem is that it is being optimized in the first place, you could end up...

...should have produced a “select” and give the > backend the opportunity to optimise it if required. I think that the > frontend should perform only target independent optimisations. > > > You didn't specify how you compile that code. > We could also get: https://godbolt.org/z/B-5lj1 > Which can actually be folded further to just > long test(long a, long b) { > return a * b; > } > Is "test" actually an implementation of a 64-bit-wide multiplication > compiler-rt builtin? > Then i'd think the main problem is that it is being optimized in the &...

...al C code should have produced a “select” and give the backend the opportunity to optimise it if required. I think that the frontend should perform only target independent optimisations. > > > You didn't specify how you compile that code. > We could also get: https://godbolt.org/z/B-5lj1 > Which can actually be folded further to just > long test(long a, long b) { > return a * b; > } > Is "test" actually an implementation of a 64-bit-wide multiplication > compiler-rt builtin? > Then i'd think the main problem is that it is being optimized in t...

...select” and give the backend the opportunity to optimise it if required. I think that the frontend should perform only target independent optimisations. >>> >>> >>> You didn't specify how you compile that code. >>> We could also get: https://godbolt.org/z/B-5lj1 <https://godbolt.org/z/B-5lj1> >>> Which can actually be folded further to just >>> long test(long a, long b) { >>> return a * b; >>> } >>> Is "test" actually an implementation of a 64-bit-wide multiplication >>> compiler-rt bui...

...the backend the opportunity to optimise it if required. I think that the frontend should perform only target independent optimisations. >>>> >>>> >>>> You didn't specify how you compile that code. >>>> We could also get: https://godbolt.org/z/B-5lj1 <https://godbolt.org/z/B-5lj1> >>>> Which can actually be folded further to just >>>> long test(long a, long b) { >>>> return a * b; >>>> } >>>> Is "test" actually an implementation of a 64-bit-wide multiplication >>...

Changing the order of the checks in CodeGenPrepare::optimizeSelectInst() sounds good to me. But you may need to go further for optimum performance. For example, we may be canonicalizing math/logic IR patterns into 'select' such as in the recent: https://reviews.llvm.org/D67799 So if you want those to become ALU ops again rather than branches, then you need to do the transform later in