InstructionSimplify: Relanding r301766

Summary:
Re-applying r301766 with a fix to a typo and a regression test.

The log message for r301766 was:
==================================================================================
    InstructionSimplify: Canonicalize shuffle operands. NFC-ish.

    Summary:
     Apply canonicalization rules:
        1. Input vectors with no elements selected from can be replaced with undef.
        2. If only one input vector is constant it shall be the second one.

    This allows constant-folding to cover more ad-hoc simplifications that
    were in place and avoid duplication for RHS and LHS checks.

    There are more rules we may want to add in the future when we see a
    justification. e.g. mask elements that select undef elements can be
    replaced with undef.
==================================================================================

Reviewers: spatel, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32863

llvm-svn: 302373

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstSimplify/shufflevector.ll b/llvm/test/Transforms/InstSimplify/shufflevector.ll
index 6af0db8e5a4..cc49ae3554c 100644
--- a/llvm/test/Transforms/InstSimplify/shufflevector.ll
+++ b/llvm/test/Transforms/InstSimplify/shufflevector.ll
@@ -233,3 +233,17 @@ define <8 x i64> @PR30630(<8 x i64> %x) {
   ret <8 x i64> %s7
 }
 
+; This case covers internal canonicalization of shuffles with one constant input vector.
+
+;FIXME: Another issue exposed here, this whole function could be simplified to:
+;         ret <2 x float> zeroinitializer
+define <2 x float> @PR32872(<2 x float> %x) {
+; CHECK-LABEL: @PR32872(
+; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <2 x float> [[X:%.*]], <2 x float> zeroinitializer, <4 x i32> <i32 2, i32 2, i32 0, i32 1>
+; CHECK-NEXT:    [[TMP4:%.*]] = shufflevector <4 x float> zeroinitializer, <4 x float> [[TMP1]], <2 x i32> <i32 4, i32 5>
+; CHECK-NEXT:    ret <2 x float> [[TMP4]]
+;
+  %tmp1 = shufflevector <2 x float> %x, <2 x float> zeroinitializer, <4 x i32> <i32 2, i32 2, i32 0, i32 1>
+  %tmp4 = shufflevector <4 x float> zeroinitializer, <4 x float> %tmp1, <2 x i32> <i32 4, i32 5>
+  ret <2 x float> %tmp4
+}