[InstCombine] fold X urem C -> X < C ? X : X - C when C is big (PR28672)
We already have the udiv variant of this transform, so I think this is ok for
InstCombine too even though there is an increase in IR instructions. As the
tests and TODO comments show, the transform can lead to follow-on combines.
This should fix: https://llvm.org/bugs/show_bug.cgi?id=28672
Differential Revision: https://reviews.llvm.org/D24527
llvm-svn: 282209
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 7f12590..cd31c63 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1447,6 +1447,14 @@
return replaceInstUsesWith(I, Ext);
}
+ // X urem C -> X < C ? X : X - C, where C >= signbit.
+ const APInt *DivisorC;
+ if (match(Op1, m_APInt(DivisorC)) && DivisorC->isNegative()) {
+ Value *Cmp = Builder->CreateICmpULT(Op0, Op1);
+ Value *Sub = Builder->CreateSub(Op0, Op1);
+ return SelectInst::Create(Cmp, Op0, Sub);
+ }
+
return nullptr;
}