1 files changed, 31 insertions, 0 deletions
diff --git a/lib/Transforms/InstCombine/InstCombineSelect.cpp b/lib/Transforms/InstCombine/InstCombineSelect.cpp
index c958cde..c44fe9d 100644
--- a/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -329,6 +329,37 @@ Instruction *InstCombiner::visitSelectInstWithICmp(SelectInst &SI,
       }
     }
 
+  // Transform (X >s -1) ? C1 : C2 --> ((X >>s 31) & (C2 - C1)) + C1
+  // and       (X <s  0) ? C2 : C1 --> ((X >>s 31) & (C2 - C1)) + C1
+  // FIXME: Type and constness constraints could be lifted, but we have to
+  //        watch code size carefully. We should consider xor instead of
+  //        sub/add when we decide to do that.
+  if (const IntegerType *Ty = dyn_cast<IntegerType>(CmpLHS->getType())) {
+    if (TrueVal->getType() == Ty) {
+      if (ConstantInt *Cmp = dyn_cast<ConstantInt>(CmpRHS)) {
+        ConstantInt *C1 = NULL, *C2 = NULL;
+        if (Pred == ICmpInst::ICMP_SGT && Cmp->isAllOnesValue()) {
+          C1 = dyn_cast<ConstantInt>(TrueVal);
+          C2 = dyn_cast<ConstantInt>(FalseVal);
+        } else if (Pred == ICmpInst::ICMP_SLT && Cmp->isNullValue()) {
+          C1 = dyn_cast<ConstantInt>(FalseVal);
+          C2 = dyn_cast<ConstantInt>(TrueVal);
+        }
+        if (C1 && C2) {
+          // This shift results in either -1 or 0.
+          Value *AShr = Builder->CreateAShr(CmpLHS, Ty->getBitWidth()-1);
+
+          // Check if we can express the operation with a single or.
+          if (C2->isAllOnesValue())
+            return ReplaceInstUsesWith(SI, Builder->CreateOr(AShr, C1));
+
+          Value *And = Builder->CreateAnd(AShr, C2->getValue()-C1->getValue());
+          return ReplaceInstUsesWith(SI, Builder->CreateAdd(And, C1));
+        }
+      }
+    }
+  }
+
   if (CmpLHS == TrueVal && CmpRHS == FalseVal) {
     // Transform (X == Y) ? X : Y  -> Y
     if (Pred == ICmpInst::ICMP_EQ)