[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

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 7f125905029..cd31c63ff28 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1447,6 +1447,14 @@ Instruction *InstCombiner::visitURem(BinaryOperator &I) {
     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;
 }