[X86] Prevent several calls to ISD::isConstantSplatVector from returning a narrower APInt than the original scalar type

ISD::isConstantSplatVector can shrink to the smallest splat width. But we don't check the size of the resulting APInt at all. This can cause us to misinterpret the results.

This patch just adds a flag to prevent the APInt from changing width.

Fixes PR34271.

Differential Revision: https://reviews.llvm.org/D36996

llvm-svn: 311429

1 files changed, 7 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 508316a1fad..6489ca57b33 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -116,7 +116,8 @@ bool ConstantFPSDNode::isValueValidForType(EVT VT,
 //                              ISD Namespace
 //===----------------------------------------------------------------------===//
 
-bool ISD::isConstantSplatVector(const SDNode *N, APInt &SplatVal) {
+bool ISD::isConstantSplatVector(const SDNode *N, APInt &SplatVal,
+                                bool AllowShrink) {
   auto *BV = dyn_cast<BuildVectorSDNode>(N);
   if (!BV)
     return false;
@@ -124,9 +125,11 @@ bool ISD::isConstantSplatVector(const SDNode *N, APInt &SplatVal) {
   APInt SplatUndef;
   unsigned SplatBitSize;
   bool HasUndefs;
-  EVT EltVT = N->getValueType(0).getVectorElementType();
-  return BV->isConstantSplat(SplatVal, SplatUndef, SplatBitSize, HasUndefs) &&
-         EltVT.getSizeInBits() >= SplatBitSize;
+  unsigned EltSize = N->getValueType(0).getVectorElementType().getSizeInBits();
+  unsigned MinSplatBits = AllowShrink ? 0 : EltSize;
+  return BV->isConstantSplat(SplatVal, SplatUndef, SplatBitSize, HasUndefs,
+                             MinSplatBits) &&
+         EltSize >= SplatBitSize;
 }
 
 // FIXME: AllOnes and AllZeros duplicate a lot of code. Could these be