[PATCH] D64190: [DAGCombiner] Don't combine (addcarry (uaddo X, Y), 0, Carry) -> (addcarry X, Y, Carry) if the Carry comes from the uaddo.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: spatel, RKSimon, deadalnix.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

The uaddo won't be removed and the addcarry will still be
dependent on the uaddo. So we'll just increase the use count
of X and Y and potentially require a COPY.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D64190

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/test/CodeGen/X86/add-of-carry.ll


Index: llvm/test/CodeGen/X86/add-of-carry.ll
===================================================================
--- llvm/test/CodeGen/X86/add-of-carry.ll
+++ llvm/test/CodeGen/X86/add-of-carry.ll
@@ -9,11 +9,9 @@
 define i32 @test1(i32 %sum, i32 %x) nounwind readnone ssp {
 ; CHECK-LABEL: test1:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %ecx
 ; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; CHECK-NEXT:    movl %eax, %edx
-; CHECK-NEXT:    addl %ecx, %edx
-; CHECK-NEXT:    adcl %ecx, %eax
+; CHECK-NEXT:    addl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:    adcl $0, %eax
 ; CHECK-NEXT:    retl
   %add4 = add i32 %x, %sum
   %cmp = icmp ult i32 %add4, %x
Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2966,8 +2966,11 @@
                                        SDNode *N) {
   // Iff the flag result is dead:
   // (addcarry (add|uaddo X, Y), 0, Carry) -> (addcarry X, Y, Carry)
+  // Don't do this if the Carry comes from the uaddo. It won't remove the uaddo
+  // or the dependency between the instructions.
   if ((N0.getOpcode() == ISD::ADD ||
-       (N0.getOpcode() == ISD::UADDO && N0.getResNo() == 0)) &&
+       (N0.getOpcode() == ISD::UADDO && N0.getResNo() == 0 &&
+        N0.getValue(1) != CarryIn)) &&
       isNullConstant(N1) && !N->hasAnyUseOfValue(1))
     return DAG.getNode(ISD::ADDCARRY, SDLoc(N), N->getVTList(),
                        N0.getOperand(0), N0.getOperand(1), CarryIn);


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