1 files changed, 13 insertions, 6 deletions
diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 001a4bcf16f3..f4bf5221f6a2 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -585,6 +585,7 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     return CastInst::CreateIntegerCast(Shift, DestTy, false);
   }
 
+  // FIXME: We should canonicalize to zext/trunc and remove this transform.
   // Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type
   // conversion.
   // It works because bits coming from sign extension have the same value as
@@ -595,18 +596,24 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     Value *SExt = cast<Instruction>(Src)->getOperand(0);
     const unsigned SExtSize = SExt->getType()->getPrimitiveSizeInBits();
     const unsigned ASize = A->getType()->getPrimitiveSizeInBits();
+    const unsigned CISize = CI.getType()->getPrimitiveSizeInBits();
+    const unsigned MaxAmt = SExtSize - std::max(CISize, ASize);
     unsigned ShiftAmt = Cst->getZExtValue();
+
     // This optimization can be only performed when zero bits generated by
     // the original lshr aren't pulled into the value after truncation, so we
     // can only shift by values no larger than the number of extension bits.
     // FIXME: Instead of bailing when the shift is too large, use and to clear
     // the extra bits.
-    if (SExt->hasOneUse() && ShiftAmt <= SExtSize - ASize) {
-      // If shifting by the size of the original value in bits or more, it is
-      // being filled with the sign bit, so shift by ASize-1 to avoid ub.
-      Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
-      Shift->takeName(Src);
-      return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+    if (ShiftAmt <= MaxAmt) {
+      if (CISize == ASize)
+        return BinaryOperator::CreateAShr(A, ConstantInt::get(CI.getType(),
+                                          std::min(ShiftAmt, ASize - 1)));
+      if (SExt->hasOneUse()) {
+        Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
+        Shift->takeName(Src);
+        return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+      }
     }
   }