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

[craig.topper]

Summary

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.

[RKSimon]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

static SDValue combineADDCARRYDiamond(DAGCombiner &Combiner, SelectionDAG &DAG,
 
   return SDValue();
 }
 
 SDValue DAGCombiner::visitADDCARRYLike(SDValue N0, SDValue N1, SDValue CarryIn,
                                        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);
 
   /**
    * When one of the addcarry argument is itself a carry, we may be facing
    * a diamond carry propagation. In which case we try to transform the DAG
    * to ensure linear carry propagation if that is possible.

llvm/test/CodeGen/X86/add-of-carry.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=i686-unknown-unknown | FileCheck %s
 
 ; These tests use adc/sbb in place of set+add/sub. Should this transform
 ; be enabled by micro-architecture rather than as part of generic lowering/isel?
 
 ; <rdar://problem/8449754>
 
 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
   %inc = zext i1 %cmp to i32
   %z.0 = add i32 %add4, %inc
   ret i32 %z.0
 }