[InstCombine] Combine neg of shl of sub (PR44529)
Fixes https://bugs.llvm.org/show_bug.cgi?id=44529. We already have
a combine to sink a negation through a left-shift, but it currently
only works if the shift operand is negatable without creating any
instructions. This patch introduces freelyNegateValue() as a more
powerful extension of dyn_castNegVal(), which allows negating a
value as long as this doesn't end up increasing instruction count.
Specifically, this patch adds support for negating A-B to B-A.
This mechanism could in the future be extended to handle general
negation chains that a) start at a proper 0-X negation and b) only
require one operand to be freely negatable. This would end up as a
weaker form of D68408 aimed at the most obviously profitable subset
that eliminates a negation entirely.
Differential Revision: https://reviews.llvm.org/D72978
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index d707dbf..3f842f9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1985,7 +1985,7 @@
// 0 - (X << Y) -> (-X << Y) when X is freely negatable.
if (match(Op1, m_Shl(m_Value(X), m_Value(Y))) && match(Op0, m_Zero()))
- if (Value *XNeg = dyn_castNegVal(X))
+ if (Value *XNeg = freelyNegateValue(X))
return BinaryOperator::CreateShl(XNeg, Y);
// Subtracting -1/0 is the same as adding 1/0: