Fix a random missed optimization by making InstCombine more aggressive when determining which bits are demanded by
a comparison against a constant.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123203 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 6c6d26c..985d912 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1693,6 +1693,45 @@
return ExtractValueInst::Create(Call, 1, "uadd.overflow");
}
+// DemandedBitsLHSMask - When performing a comparison against a constant,
+// it is possible that not all the bits in the LHS are demanded. This helper
+// method computes the mask that IS demanded.
+static APInt DemandedBitsLHSMask(ICmpInst &I,
+ unsigned BitWidth, bool isSignCheck) {
+ if (isSignCheck)
+ return APInt::getSignBit(BitWidth);
+
+ ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand(1));
+ if (!CI) return APInt::getAllOnesValue(BitWidth);
+
+ APInt RHS = CI->getValue();
+ APInt Mask(BitWidth, 0);
+
+ switch (I.getPredicate()) {
+ // For a UGT comparison, we don't care about any bits that
+ // correspond to the trailing ones of the comparand. The value of these
+ // bits doesn't impact the outcome of the comparison, because any value
+ // greater than the RHS must differ in a bit higher than these due to carry.
+ case ICmpInst::ICMP_UGT: {
+ unsigned trailingOnes = RHS.countTrailingOnes();
+ APInt lowBitsSet = APInt::getLowBitsSet(BitWidth, trailingOnes);
+ return ~lowBitsSet;
+ }
+
+ // Similarly, for a ULT comparison, we don't care about the trailing zeros.
+ // Any value less than the RHS must differ in a higher bit because of carries.
+ case ICmpInst::ICMP_ULT: {
+ unsigned trailingZeros = RHS.countTrailingZeros();
+ APInt lowBitsSet = APInt::getLowBitsSet(BitWidth, trailingZeros);
+ return ~lowBitsSet;
+ }
+
+ default:
+ return APInt::getAllOnesValue(BitWidth);
+ }
+
+ return Mask;
+}
Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
bool Changed = false;
@@ -1830,8 +1869,7 @@
APInt Op1KnownZero(BitWidth, 0), Op1KnownOne(BitWidth, 0);
if (SimplifyDemandedBits(I.getOperandUse(0),
- isSignBit ? APInt::getSignBit(BitWidth)
- : APInt::getAllOnesValue(BitWidth),
+ DemandedBitsLHSMask(I, BitWidth, isSignBit),
Op0KnownZero, Op0KnownOne, 0))
return &I;
if (SimplifyDemandedBits(I.getOperandUse(1),