[InstCombine] usub.sat(a, b) + b => umax(a, b) (PR42178)
Fixes https://bugs.llvm.org/show_bug.cgi?id=42178 by folding
usub.sat(a, b) + b to umax(a, b). The backend will expand umax
back to usubsat if that is profitable.
We may also want to handle uadd.sat(a, b) - b in the future.
Differential Revision: https://reviews.llvm.org/D63060
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index a45fdc7..42322e3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1425,6 +1425,14 @@
if (Instruction *SatAdd = foldToUnsignedSaturatedAdd(I))
return SatAdd;
+ // usub.sat(A, B) + A => umax(A, B)
+ if (match(&I, m_c_BinOp(
+ m_OneUse(m_Intrinsic<Intrinsic::usub_sat>(m_Value(A), m_Value(B))),
+ m_Deferred(B)))) {
+ return replaceInstUsesWith(I,
+ Builder.CreateIntrinsic(Intrinsic::umax, {I.getType()}, {A, B}));
+ }
+
return Changed ? &I : nullptr;
}