[LLVMdev] passing vector of booleans to functions

[Roland Leißa]

Hi all,

I'm currently trying to figure out the best way to pass vector of
booleans to other functions. Take this small example:

define <4 x float> @vcmp_add(<4 x float> %a, <4 x float> %b) {
entry:
  %cmp = fcmp olt <4 x float> %a, %b
  %add = fadd <4 x float> %a, %b
  %sel = select <4 x i1> %cmp, <4 x float> %add, <4 x float> %a
  ret <4 x float> %sel
}

I will get (on SSE):
	movaps	%xmm0, %xmm2
	cmpltps	%xmm1, %xmm0
	addps	%xmm2, %xmm1
	blendvps	%xmm1, %xmm2
	movaps	%xmm2, %xmm0
	ret

great :)
But now, let us try to pass a mask to a function.

define <4 x float> @masked_add_1(<4 x i1> %mask, <4 x float> %a, <4 x float> %b) {
entry:
  %add = fadd <4 x float> %a, %b
  %sel = select <4 x i1> %mask, <4 x float> %add, <4 x float> %a
  ret <4 x float> %sel
}

I will get:

addps   %xmm1, %xmm2
pslld   $31, %xmm0
blendvps    %xmm2, %xmm1
movaps  %xmm1, %xmm0
ret

While this is correct and works, I'm unhappy with the pssld. Apparently,
LLVM uses a <4 x i32> to hold the <4 x i1> while the LSB holds the mask
bit. But blendvps expects the MSB as mask bit and therefore the shift.

OK, let's try better. This time, I will directly use <4 x i32>:

define <4 x float> @masked_add_32(<4 x i32> %mask, <4 x float> %a, <4 x float> %b) 
{
entry:
  %add = fadd <4 x float> %a, %b
  %trunc = trunc <4 x i32> %mask to <4 x i1>
  %sel = select <4 x i1> %trunc, <4 x float> %add, <4 x float> %a
  ret <4 x float> %sel
}

But damn, I have to truncate the mask in order to use the select. So in
the end, LLVM will produce the same code as above. So what code do I
have to use, in order to get rid of the shift? 

If there would be a way to somehow tell LLVM that each element of %mask
is guaranteed to be 0xFFFFFFFF or 0x0...

Thanks,
Roland