[PATCH] D37686: [DAG] Consolidating Instruction->SDNode Flags propagation in one class for better code management.

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In https://reviews.llvm.org/D37686#877987, @hfinkel wrote:

> While `fast` does imply `nnan`, and `nnan` does propagate backward, and it also implies `arcp`, and `arcp` does not propagate backward. `arcp` applied only to the instruction to which it's attached. In this case,  we're allowed to use a reciprocal approximation to the division, and not the sqrt. However, we could argue that using the rsqrt is like doing the sqrt exactly and then just approximating the division. If there are no other users of the sqrt itself, there seems to be little semantic difference.


I thought we leaned to the more conservative (less propagating) model in IR. Not sure if that would be different here in the DAG or if rsqrt is a special case. Either way, it doesn't affect the logic at node creation time?

Here's a sqrt multi-use case to think about:

  define float @multiuse_strict_sqrt(float %x, i1 %cmp) {
    %y = call float @llvm.sqrt.f32(float %x)
    %z = fdiv fast float 1.0,  %y
    br i1 %cmp, label %t, label %f
  
  t:
    ret float %z
  f:
    %add = fadd float %y, 1.0
    ret float %add
  }

Should this be:

  ## BB#0:
  	sqrtss	%xmm0, %xmm0 <--- shared sqrt op because sqrt is treated as strict
  	testb	$1, %dil
  	je	LBB0_2
  ## BB#1:                                ## %t
  	movss	LCPI0_0(%rip), %xmm1  
  	divss	%xmm0, %xmm1
  	movaps	%xmm1, %xmm0
  	retq
  LBB0_2:                                 ## %f
  	addss	LCPI0_0(%rip), %xmm0
  	retq

Or:

  ## BB#0:
  	testb	$1, %dil
  	je	LBB0_2
  ## BB#1:                                ## %t
  	vrsqrtss	%xmm0, %xmm0, %xmm1   <--- fast sqrt is assumed part of fast div
  	vmulss	%xmm1, %xmm0, %xmm0
  	vmulss	%xmm1, %xmm0, %xmm0
  	vaddss	LCPI0_0(%rip), %xmm0, %xmm0
  	vmulss	LCPI0_1(%rip), %xmm1, %xmm1
  	vmulss	%xmm0, %xmm1, %xmm0
  	retq
  LBB0_2:                                 ## %f
  	vsqrtss	%xmm0, %xmm0, %xmm0  <--- strict sqrt only applies on this path
  	vaddss	LCPI0_2(%rip), %xmm0, %xmm0
  	retq


https://reviews.llvm.org/D37686