InstCombine: Optimize -x s< cst

Summary:
This gets rid of a sub instruction by moving the negation to the
constant when valid.

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3773

llvm-svn: 208827

2 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 9424ad2b853..02e8bf10135 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2971,6 +2971,16 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         BO0->hasOneUse() && BO1->hasOneUse())
       return new ICmpInst(Pred, D, B);
 
+    // icmp (0-X) < cst --> x > -cst
+    if (NoOp0WrapProblem && ICmpInst::isSigned(Pred)) {
+      Value *X;
+      if (match(BO0, m_Neg(m_Value(X))))
+        if (ConstantInt *RHSC = dyn_cast<ConstantInt>(Op1))
+          if (!RHSC->isMinValue(/*isSigned=*/true))
+            return new ICmpInst(I.getSwappedPredicate(), X,
+                                ConstantExpr::getNeg(RHSC));
+    }
+
     BinaryOperator *SRem = nullptr;
     // icmp (srem X, Y), Y
     if (BO0 && BO0->getOpcode() == Instruction::SRem &&
diff --git a/llvm/test/Transforms/InstCombine/icmp.ll b/llvm/test/Transforms/InstCombine/icmp.ll
index 12a4744cc0f..f45897cd3ee 100644
--- a/llvm/test/Transforms/InstCombine/icmp.ll
+++ b/llvm/test/Transforms/InstCombine/icmp.ll
@@ -1356,3 +1356,12 @@ define i1 @icmp_ashr_ashr_ne(i32 %a, i32 %b) nounwind {
  %z = icmp ne i32 %x, %y
  ret i1 %z
 }
+
+; CHECK-LABEL: @icmp_neg_cst_slt
+; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp sgt i32 %a, 10
+; CHECK-NEXT: ret i1 [[CMP]]
+define i1 @icmp_neg_cst_slt(i32 %a) {
+  %1 = sub nsw i32 0, %a
+  %2 = icmp slt i32 %1, -10
+  ret i1 %2
+}