[llvm-dev] Ignored branch predictor hints

[David Zarzycki via llvm-dev]

I’d wager that the if-else chain is being converted to a "switch statement” during an optimization pass and the __builtin_expect() hint is lost. Can you file a bug? https://bugs.llvm.org


> On May 9, 2018, at 1:57 PM, Dávid Bolvanský via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
> #define likely(x)       __builtin_expect((x),1)
> 
> // switch like
> char * b(int e) {
>     if (likely(e == 0))
>         return "0";
>     else if (e == 1)
>         return "1";
>     else return "f";
> }
> GCC correctly prefers the first case:
> b(int):
>   mov eax, OFFSET FLAT:.LC0
>   test edi, edi
>   jne .L7
>   ret
> 
> But Clang seems to ignore _builtin_expect hints in this case.
> b(int): # @b(int)
>   cmp edi, 1
>   mov eax, offset .L.str.1
>   mov ecx, offset .L.str.2
>   cmove rcx, rax
>   test edi, edi
>   mov eax, offset .L.str
>   cmovne rax, rcx
>   ret
> https://godbolt.org/g/tuAVT7 <https://godbolt.org/g/tuAVT7>_______________________________________________
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