(1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2
rdar://problem/6501862
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132316 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index a9b60d3..94b619b 100644
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -135,6 +135,23 @@
return BinaryOperator::CreateAdd(Add, Builder->CreateMul(C1, CI));
}
}
+
+ // (1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2
+ // The "* 2" thus becomes a potential shifting opportunity.
+ {
+ const APInt & Val = CI->getValue();
+ const APInt &PosVal = Val.abs();
+ if (Val.isNegative() && PosVal.isPowerOf2()) {
+ Value *X = 0;
+ if (match(Op0, m_Sub(m_One(), m_Value(X)))) {
+ // ConstantInt::get(Op0->getType(), 2);
+ Value *Sub = Builder->CreateSub(X, ConstantInt::get(X->getType(), 1),
+ "dec1");
+ return BinaryOperator::CreateMul(Sub, ConstantInt::get(X->getType(),
+ PosVal));
+ }
+ }
+ }
}
// Simplify mul instructions with a constant RHS.