[llvm] [InstSimplify] Add basic simplifications for vp.reverse (PR #144112)

[Philip Reames via llvm-commits]

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@@ -25,26 +23,23 @@ define <vscale x 4 x i32> @rev_of_rev_diffevl(<vscale x 4 x i32> %a, i32 %evl) {
 
 define <vscale x 4 x i32> @rev_of_poison(i32 %evl) {
 ; CHECK-LABEL: @rev_of_poison(
-; CHECK-NEXT:    [[REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> poison, <vscale x 4 x i1> splat (i1 true), i32 [[EVL:%.*]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[REV]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> poison
 ;
   %rev = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse(<vscale x 4 x i32> poison, <vscale x 4 x i1> splat (i1 true), i32 %evl)
   ret <vscale x 4 x i32> %rev
 }
 
 define <vscale x 4 x i32> @rev_of_undef(i32 %evl) {
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preames wrote:

I want to push back a bit here.  There's a big difference between removing undef from tests where the undef is the result of reduction, or doesn't reflect test intention.  This case is specifically testing correct behavior of an undef fold.  I would hope we are *not* removing such cases, that would seem actively undesirable.  

https://github.com/llvm/llvm-project/pull/144112