[PATCH] D31960: [InstSimplify] fold identity shuffles (recursing if needed)

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: lib/Analysis/InstructionSimplify.cpp:4098
+  if (MaskVal == -1)
+    return nullptr;
+
----------------
filcab wrote:
> But we might end up with shuffles we'd like to remove if we always skip shuffles with undef, no?
> Do you want to add a shuffle similar to your `identity_mask` tests which shows that we don't remove if there's an undef (assuming it's as easy as making one of the last shuffle mask's elements an undef. No need to go out of your way for a negative test)?
> 
Yes, this is conservative as a first step, so it will miss some patterns. I wanted to make sure that I have my undef story straight before enhancing this (eg, D31980). 

Also, what is the expected behavior in this case?

  define <4 x float> @elts_test_vpermilvar_ps(<4 x float> %a0, i32 %a1) {
  ; CHECK-LABEL: @elts_test_vpermilvar_ps(
  ; CHECK-NEXT:    ret <4 x float> %a0
  ;
  %1 = insertelement <4 x i32> <i32 0, i32 1, i32 2, i32 3>, i32 %a1, i32 3
  %2 = tail call <4 x float> @llvm.x86.avx.vpermilvar.ps(<4 x float> %a0, <4 x i32> %1)
  %3 = shufflevector <4 x float> %2, <4 x float> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 undef>
  ret <4 x float> %3
  }

If we simplify the shuffle to %2, we can no longer eliminate the x86 vperm?


https://reviews.llvm.org/D31960