InstSimplify: Simplify (X - (0 - Y)) if the second sub is NUW

If the NUW bit is set for 0 - Y, we know that all values for Y other
than 0 would produce a poison value.  This allows us to replace (0 - Y)
with 0 in the expression (X - (0 - Y)) which will ultimately leave us
with X.

This partially fixes PR20189.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214384 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InstructionSimplify.cpp

@@ -676,6 +676,18 @@ static Value *SimplifySubInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
   if (Op0 == Op1)
     return Constant::getNullValue(Op0->getType());
 
+  // X - (0 - Y) -> X if the second sub is NUW.
+  // If Y != 0, 0 - Y is a poison value.
+  // If Y == 0, 0 - Y simplifies to 0.
+  if (BinaryOperator::isNeg(Op1)) {
+    if (const auto *BO = dyn_cast<BinaryOperator>(Op1)) {
+      assert(BO->getOpcode() == Instruction::Sub &&
+             "Expected a subtraction operator!");
+      if (BO->hasNoUnsignedWrap())
+        return Op0;
+    }
+  }
+
   // (X + Y) - Z -> X + (Y - Z) or Y + (X - Z) if everything simplifies.
   // For example, (X + Y) - Y -> X; (Y + X) - Y -> X
   Value *X = nullptr, *Y = nullptr, *Z = Op1;

test/Transforms/InstSimplify/AndOrXor.ll

@@ -20,3 +20,11 @@ define i64 @pow2b(i32 %x) {
   ret i64 %e2
 ; CHECK: ret i64 %e
 }
+
+define i32 @sub_neg_nuw(i32 %x, i32 %y) {
+; CHECK-LABEL: @sub_neg_nuw(
+  %neg = sub nuw i32 0, %y
+  %sub = sub i32 %x, %neg
+  ret i32 %sub
+; CHECK: ret i32 %x
+}