[PATCH] D46505: [DAGcombine] Teach the combiner about -a = ~a + 1

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

We should have minimal dedicated tests for these patterns. Something like this:

  ; simple fold - no overflow intrinsic/node
  define i32 @inc_not(i32 %a) {
    %nota = xor i32 %a, -1
    %r = add i32 %nota, 1
    ret i32 %r
  }
  
  %ov32 = type { i32, i1 }
  declare %ov32 @llvm.uadd.with.overflow.i32(i32, i32)
  
  define void @uaddo1_not(i32 %a, i32* %p0, i1* %p1) {
    %nota = xor i32 %a, -1
    %uaddo = call %ov32 @llvm.uadd.with.overflow.i32(i32 %nota, i32 1)
    %r0 = extractvalue %ov32 %uaddo, 0
    %r1 = extractvalue %ov32 %uaddo, 1
    store i32 %r0, i32* %p0
    store i1 %r1, i1* %p1
    ret void
  }
  
  ; another test for addcarry



================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:2057-2059
+  if (N0.getOpcode() == ISD::XOR &&
+      isOneConstantOrOneSplatConstant(N1) &&
+      isAllOnesConstantOrAllOnesSplatConstant(N0.getOperand(1)))
----------------
Use llvm::isBitwiseNot()? I don't think we care about vectors in the motivating cases, but you could extend that to handle a vector constant if it matters.

On 2nd thought, do we need this? It's not necessary for the test shown here, and instcombine already does that transform. If the pattern is created here in the DAG, it should have its own test (ie, this could be an independent patch).


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:2264-2266
+  if (N0.getOpcode() == ISD::XOR &&
+      isOneConstantOrOneSplatConstant(N1) &&
+      isAllOnesConstantOrAllOnesSplatConstant(N0.getOperand(1))) {
----------------
Same as above - use isBitwiseNot()?


Repository:
  rL LLVM

https://reviews.llvm.org/D46505