InstCombine, InstSimplify: (%X /s C1) /s C2 isn't always 0 when C1 * C2 overflow

consider:
C1 = INT_MIN
C2 = -1

C1 * C2 overflows without a doubt but consider the following:
%x = i32 INT_MIN

This means that (%X /s C1) is 1 and (%X /s C1) /s C2 is -1.

N. B.  Move the unsigned version of this transform to InstSimplify, it
doesn't create any new instructions.

This fixes PR21243.

llvm-svn: 219567

1 files changed, 4 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 249456fa6ea..8d004f2a0a2 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -727,11 +727,10 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
       // (X / C1) / C2  -> X / (C1*C2)
       if (Instruction::BinaryOps(LHS->getOpcode()) == I.getOpcode())
         if (ConstantInt *LHSRHS = dyn_cast<ConstantInt>(LHS->getOperand(1))) {
-          if (MultiplyOverflows(RHS, LHSRHS,
-                                I.getOpcode() == Instruction::SDiv))
-            return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
-          return BinaryOperator::Create(I.getOpcode(), LHS->getOperand(0),
-                                        ConstantExpr::getMul(RHS, LHSRHS));
+          if (!MultiplyOverflows(RHS, LHSRHS,
+                                 I.getOpcode() == Instruction::SDiv))
+            return BinaryOperator::Create(I.getOpcode(), LHS->getOperand(0),
+                                          ConstantExpr::getMul(RHS, LHSRHS));
         }
 
       Value *X;