optimize uadd(x, cst) into a comparison when the normal

result is dead.  This is required for my next patch to not
regress the testsuite.

llvm-svn: 122181

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index d53f3291e71..f3a5e724c61 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -523,6 +523,21 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
         return InsertValueInst::Create(Struct, Add, 0);
       }
     }
+    
+    // If the normal result of the add is dead, and the RHS is a constant, we
+    // can transform this into a range comparison.
+    // overflow = uadd a, -4  -->  overflow = icmp ugt a, 3
+    if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS))
+      if (ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(II->use_back()))
+        if (II->hasOneUse() && EVI->getNumIndices() == 1 && !EVI->use_empty() &&
+            *EVI->idx_begin() == 1) {  // Extract of overflow result.
+          Builder->SetInsertPoint(EVI);
+          Value *R = Builder->CreateICmpUGT(LHS, ConstantExpr::getNot(CI));
+          R->takeName(EVI);
+          ReplaceInstUsesWith(*EVI, R);
+          return II;
+        }
+    
   }
   // FALL THROUGH uadd into sadd
   case Intrinsic::sadd_with_overflow:
@@ -550,6 +565,7 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
         return InsertValueInst::Create(Struct, II->getArgOperand(0), 0);
       }
     }
+      
     break;
   case Intrinsic::usub_with_overflow:
   case Intrinsic::ssub_with_overflow: