[InstCombine] remove constant by inverting compare + logic (PR27105)

https://llvm.org/bugs/show_bug.cgi?id=27105

We can check if all bits outside of a constant mask are set with a 
single constant.

As noted in the bug report, although this form should be considered the
canonical IR, backends may want to transform this into an 'andn' / 'andc' 
comparison against zero because that could be a single machine instruction.

Differential Revision: http://reviews.llvm.org/D18842

llvm-svn: 266362

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 9b963fd6f12..c97b67eff24 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2248,6 +2248,15 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
           Constant *NotCI = ConstantExpr::getNot(RHS);
           if (!ConstantExpr::getAnd(BOC, NotCI)->isNullValue())
             return replaceInstUsesWith(ICI, Builder->getInt1(isICMP_NE));
+
+          // Comparing if all bits outside of a constant mask are set?
+          // Replace (X | C) == -1 with (X & ~C) == ~C.
+          // This removes the -1 constant.
+          if (BO->hasOneUse() && RHS->isAllOnesValue()) {
+            Constant *NotBOC = ConstantExpr::getNot(BOC);
+            Value *And = Builder->CreateAnd(BO->getOperand(0), NotBOC);
+            return new ICmpInst(ICI.getPredicate(), And, NotBOC);
+          }
         }
         break;