From 42158f3eeaf0468efd46b4df28ff2658eff4e5b6 Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 20 Aug 2014 07:17:31 +0000
Subject: [PATCH] InstCombine: Annotate sub with nuw when we prove it's safe

We can prove that a 'sub' can be a 'sub nuw' if the left-hand side is
negative and the right-hand side is non-negative.

llvm-svn: 216045
---
 llvm/lib/Transforms/InstCombine/InstCombine.h  |  1 +
 .../InstCombine/InstCombineAddSub.cpp          | 18 ++++++++++++++++++
 llvm/test/Transforms/InstCombine/sub.ll        | 14 +++++++++++++-
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombine.h b/llvm/lib/Transforms/InstCombine/InstCombine.h
index 8cb634c8a3ec..9e1cc3cb4d2f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombine.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombine.h
@@ -251,6 +251,7 @@ private:
   bool WillNotOverflowSignedAdd(Value *LHS, Value *RHS);
   bool WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS);
   bool WillNotOverflowSignedSub(Value *LHS, Value *RHS);
+  bool WillNotOverflowUnsignedSub(Value *LHS, Value *RHS);
   Value *EmitGEPOffset(User *GEP);
   Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
   Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 22566ceb050c..e7e6cdd63d15 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -988,6 +988,20 @@ bool InstCombiner::WillNotOverflowSignedSub(Value *LHS, Value *RHS) {
   return false;
 }
 
+/// \brief Return true if we can prove that:
+///    (sub LHS, RHS)  === (sub nuw LHS, RHS)
+bool InstCombiner::WillNotOverflowUnsignedSub(Value *LHS, Value *RHS) {
+  // If the LHS is negative and the RHS is non-negative, no unsigned wrap.
+  bool LHSKnownNonNegative, LHSKnownNegative;
+  bool RHSKnownNonNegative, RHSKnownNegative;
+  ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, 0);
+  ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, 0);
+  if (LHSKnownNegative && RHSKnownNonNegative)
+    return true;
+
+  return false;
+}
+
 // Checks if any operand is negative and we can convert add to sub.
 // This function checks for following negative patterns
 //   ADD(XOR(OR(Z, NOT(C)), C)), 1) == NEG(AND(Z, C))
@@ -1660,6 +1674,10 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
     Changed = true;
     I.setHasNoSignedWrap(true);
   }
+  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedSub(Op0, Op1)) {
+    Changed = true;
+    I.setHasNoUnsignedWrap(true);
+  }
 
   return Changed ? &I : nullptr;
 }
diff --git a/llvm/test/Transforms/InstCombine/sub.ll b/llvm/test/Transforms/InstCombine/sub.ll
index 0d310a8deeb1..c786ba44ea73 100644
--- a/llvm/test/Transforms/InstCombine/sub.ll
+++ b/llvm/test/Transforms/InstCombine/sub.ll
@@ -506,6 +506,18 @@ define i4 @test42(i4 %x, i4 %y) {
 ; CHECK-LABEL: @test42(
 ; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
 ; CHECK-NEXT: [[AND1:%.*]] = and i4 %x, 7
-; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 %a, %b
+; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 [[AND]], [[AND1]]
+; CHECK: ret i4 [[RET]]
+}
+
+define i4 @test43(i4 %x, i4 %y) {
+  %a = or i4 %x, -8
+  %b = and i4 %y, 7
+  %c = sub i4 %a, %b
+  ret i4 %c
+; CHECK-LABEL: @test43(
+; CHECK-NEXT: [[OR:%.*]] = or i4 %x, -8
+; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7
+; CHECK-NEXT: [[RET:%.*]] = sub nuw i4 [[OR]], [[AND]]
 ; CHECK: ret i4 [[RET]]
 }