[InstCombine] shrink type of sdiv if dividend is sexted and constant divisor is small enough (PR28153)
This should fix PR28153:
https://llvm.org/bugs/show_bug.cgi?id=28153
Differential Revision: http://reviews.llvm.org/D21769
llvm-svn: 273951
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 4e5bf78..788097f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1150,6 +1150,23 @@
Value *ShAmt = ConstantInt::get(Op1->getType(), Op1C->exactLogBase2());
return BinaryOperator::CreateExactAShr(Op0, ShAmt, I.getName());
}
+
+ // If the dividend is sign-extended and the constant divisor is small enough
+ // to fit in the source type, shrink the division to the narrower type:
+ // (sext X) sdiv C --> sext (X sdiv C)
+ Value *Op0Src;
+ if (match(Op0, m_OneUse(m_SExt(m_Value(Op0Src)))) &&
+ Op0Src->getType()->getScalarSizeInBits() >= Op1C->getMinSignedBits()) {
+
+ // In the general case, we need to make sure that the dividend is not the
+ // minimum signed value because dividing that by -1 is UB. But here, we
+ // know that the -1 divisor case is already handled above.
+
+ Constant *NarrowDivisor =
+ ConstantExpr::getTrunc(cast<Constant>(Op1), Op0Src->getType());
+ Value *NarrowOp = Builder->CreateSDiv(Op0Src, NarrowDivisor);
+ return new SExtInst(NarrowOp, Op0->getType());
+ }
}
if (Constant *RHS = dyn_cast<Constant>(Op1)) {