The srem -> urem transform is not safe for any divisor that's not a power of two.

E.g. -5 % 5 is 0 with srem and 1 with urem.

Also addresses Frits van Bommel's comments.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120049 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 9 insertions, 11 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 7969dbf..e52f201 100644
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -54,19 +54,17 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
     if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
       return Res;
 
-  // X shift (A srem B) -> X shift (A urem B) iff B is positive.
+  // X shift (A srem B) -> X shift (A and B-1) iff B is a power of 2.
   // Because shifts by negative values are undefined.
   if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op1))
-    if (BO->getOpcode() == Instruction::SRem && BO->getType()->isIntegerTy()) {
-      // Make sure the divisor's sign bit is zero.
-      APInt Mask = APInt::getSignBit(BO->getType()->getPrimitiveSizeInBits());
-      if (MaskedValueIsZero(BO->getOperand(1), Mask)) {
-        Value *URem = Builder->CreateURem(BO->getOperand(0), BO->getOperand(1),
-                                          BO->getName());
-        I.setOperand(1, URem);
-        return &I;
-      }
-    }
+    if (BO->hasOneUse() && BO->getOpcode() == Instruction::SRem)
+      if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
+        if (CI->getValue().isPowerOf2()) {
+          Constant *C = ConstantInt::get(BO->getType(), CI->getValue()-1);
+          Value *Rem = Builder->CreateAnd(BO->getOperand(0), C, BO->getName());
+          I.setOperand(1, Rem);
+          return &I;
+        }
 
   return 0;
 }