DAGCombine: Make sure we only eliminate trunc/extend when the scales of truncation and extension match.

This fixes PR33368.

Reviewer: rksimon

Differential Revision:  https://reviews.llvm.org/D34069

llvm-svn: 306345

1 files changed, 9 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d02dcb6f443..d1a5a98607c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -15013,6 +15013,11 @@ static SDValue combineTruncationShuffle(ShuffleVectorSDNode *SVN,
   unsigned NumElts = VT.getVectorNumElements();
   unsigned EltSizeInBits = VT.getScalarSizeInBits();
   unsigned ExtSrcSizeInBits = N00.getScalarValueSizeInBits();
+  unsigned ExtDstSizeInBits = N0.getScalarValueSizeInBits();
+
+  if (ExtDstSizeInBits % ExtSrcSizeInBits != 0)
+    return SDValue();
+  unsigned ExtScale = ExtDstSizeInBits / ExtSrcSizeInBits;
 
   // (v4i32 truncate_vector_inreg(v2i64)) == shuffle<0,2-1,-1>
   // (v8i16 truncate_vector_inreg(v4i32)) == shuffle<0,2,4,6,-1,-1,-1,-1>
@@ -15034,11 +15039,10 @@ static SDValue combineTruncationShuffle(ShuffleVectorSDNode *SVN,
   if (EltSizeInBits != ExtSrcSizeInBits)
     return SDValue();
 
-  // Attempt to match a 'truncate_vector_inreg' shuffle, we just search for
-  // power-of-2 truncations as they are the most likely.
-  for (unsigned Scale = 2; Scale < NumElts; Scale *= 2)
-    if (isTruncate(Scale))
-      return DAG.getBitcast(VT, N00);
+  // We can remove *extend_vector_inreg only if the truncation happens at
+  // the same scale as the extension.
+  if (isTruncate(ExtScale))
+    return DAG.getBitcast(VT, N00);
 
   return SDValue();
 }