[PATCH] D33391: [DAGCombine] fold (add/uaddo (xor a, -1), 1) -> (sub 0, a)

[Amaury SECHET via Phabricator via llvm-commits]

deadalnix created this revision.

As per title. This is the very straightforward 2 complement subtraction transaform.


https://reviews.llvm.org/D33391

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/subcarry.ll


Index: test/CodeGen/X86/subcarry.ll
===================================================================
--- test/CodeGen/X86/subcarry.ll
+++ test/CodeGen/X86/subcarry.ll
@@ -6,13 +6,12 @@
 define %S @negate(%S* nocapture readonly %this) {
 ; CHECK-LABEL: negate:
 ; CHECK:       # BB#0: # %entry
-; CHECK-NEXT:    movq (%rsi), %rax
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    subq (%rsi), %rax
 ; CHECK-NEXT:    movq 8(%rsi), %rcx
-; CHECK-NEXT:    notq %rax
-; CHECK-NEXT:    addq $1, %rax
+; CHECK-NEXT:    movq 16(%rsi), %rdx
 ; CHECK-NEXT:    notq %rcx
 ; CHECK-NEXT:    adcq $0, %rcx
-; CHECK-NEXT:    movq 16(%rsi), %rdx
 ; CHECK-NEXT:    notq %rdx
 ; CHECK-NEXT:    adcq $0, %rdx
 ; CHECK-NEXT:    movq 24(%rsi), %rsi
Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1961,6 +1961,12 @@
       VT.isInteger() && DAG.haveNoCommonBitsSet(N0, N1))
     return DAG.getNode(ISD::OR, DL, VT, N0, N1);
 
+  // fold (add (xor a, -1), 1) -> (sub 0, a)
+  if (N0.getOpcode() == ISD::XOR &&
+      isOneConstantOrOneSplatConstant(N1) &&
+      isAllOnesConstantOrAllOnesSplatConstant(N0.getOperand(1)))
+    return DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), N0.getOperand(0));
+
   if (SDValue Combined = visitADDLike(N0, N1, N))
     return Combined;
 
@@ -2080,6 +2086,12 @@
     return CombineTo(N, DAG.getNode(ISD::ADD, DL, VT, N0, N1),
                      DAG.getConstant(0, DL, CarryVT));
 
+  // fold (uaddo (xor a, -1), 1) -> (sub 0, a)
+  if (N0.getOpcode() == ISD::XOR &&
+      isOneConstantOrOneSplatConstant(N1) &&
+      isAllOnesConstantOrAllOnesSplatConstant(N0.getOperand(1)))
+    return DAG.getNode(ISD::USUBO, DL, N->getVTList(), DAG.getConstant(0, DL, VT), N0.getOperand(0));
+
   if (SDValue Combined = visitUADDOLike(N0, N1, N))
     return Combined;
 


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