[InstCombine] Fold x & (-1 >> y) u< x to x u> (-1 >> y)

https://bugs.llvm.org/show_bug.cgi?id=38123
https://rise4fun.com/Alive/ocb

This pattern is not commutative. But InstSimplify will
already have taken care of the 'commutative' variant.

llvm-svn: 337098

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index f89dca42a55..eec8fadedb2 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2897,6 +2897,11 @@ static Value *foldICmpWithLowBitMaskedVal(ICmpInst &I,
     assert(X == I.getOperand(1) && "instsimplify took care of commut. variant");
     DstPred = ICmpInst::Predicate::ICMP_ULE;
     break;
+  case ICmpInst::Predicate::ICMP_ULT:
+    //  x & (-1 >> y) u< x    ->    x u> (-1 >> y)
+    assert(X == I.getOperand(1) && "instsimplify took care of commut. variant");
+    DstPred = ICmpInst::Predicate::ICMP_UGT;
+    break;
   // TODO: more folds are possible, https://bugs.llvm.org/show_bug.cgi?id=38123
   default:
     return nullptr;