Where do you sugest to LLVM people that codegem could be improved? The bit scan forward and reverse both test for zero and do jumps (when you want zero defined), when they could be doing conditional moves because both instructions st the zero flag if the input is zero. Basically...

import ldc.intrinsics;
alias llvm_bsf = llvm_cttz;

void foo(int a)
{
    a = llvm_bsf(a,false);
    writeln(a);
}

compiles to this...

        test    ebx, ebx
        je      .LBB0_1
        bsf     ebx, ebx
        jmp     .LBB0_3
.LBB0_1:
        mov     ebx, 32
.LBB0_3:

where it could just be

        mov     edi,32
        bsf     ebx,ebx
        cmovz   ebx,edi

On Saturday, 27 April 2019 at 20:25:01 UTC, NaN wrote:
> Where do you sugest to LLVM people that codegem could be improved?

On their mailinglist or in their bug tracker.

> The bit scan forward and reverse both test for zero and do jumps (when you want zero defined), when they could be doing conditional moves because both instructions st the zero flag if the input is zero.

Two remarks:
1. Conditional move is not necessarily faster than branching
2. On recent CPUs `tzcnt` is the better instruction that has defined output for input 0

-Johan

On Sunday, 28 April 2019 at 11:22:57 UTC, Johan Engelen wrote:
> On Saturday, 27 April 2019 at 20:25:01 UTC, NaN wrote:
>>
> Two remarks:
> 1. Conditional move is not necessarily faster than branching
> 2. On recent CPUs `tzcnt` is the better instruction that has defined output for input 0

Unfortunately neither my CPU or myself are very recent.