Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -595,19 +595,25 @@
     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();
     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);
-    }
+    const unsigned MaxAmt = SExtSize - CISize;
+    if (CISize == ASize)
+      if (ShiftAmt <= MaxAmt)
+        return BinaryOperator::CreateAShr(A, ConstantInt::get(CI.getType(),
+                                                     std::min(ShiftAmt, ASize - 1)));
+    if (CISize > ASize)
+      if (SExt->hasOneUse() && ShiftAmt <= MaxAmt) {
+        Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
+        Shift->takeName(Src);
+        return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+      }
   }
 
   if (Instruction *I = shrinkBitwiseLogic(CI))
Index: test/Transforms/InstCombine/cast.ll
===================================================================
--- test/Transforms/InstCombine/cast.ll
+++ test/Transforms/InstCombine/cast.ll
@@ -1470,3 +1470,16 @@
   %D = trunc i96 %C to i32
   ret i32 %D
 }
+; Don't turn this in an `ashr`.
+; CHECK-LABEL: @PR33078(
+; CHECK-NEXT:    [[B:%.*]] = sext i3 %x to i16
+; CHECK-NEXT:    [[C:%.*]] = lshr i16 [[B]], 13
+; CHECK-NEXT:    [[D:%.*]] = trunc i16 [[C]] to i8
+; CHECK-NEXT:    ret i8 [[D]]
+
+define i8 @PR33078(i3 %x) {
+  %B = sext i3 %x to i16
+  %C = lshr i16 %B, 13
+  %D = trunc i16 %C to i8
+  ret i8 %D
+}