[SystemZ] Fix addcarry of addcarry of const carry (PR42606)

[nikic]

Summary

This fixes https://bugs.llvm.org/show_bug.cgi?id=42606 by extending D64213. Instead of only checking if the carry comes from a matching operation, we now check the full chain of carries. Otherwise we might custom lower the outermost addcarry, but then generically legalize an inner addcarry.

This feels a bit hacky...

[uweigand]

I agree it's a bit hacky, but I don't see any better solution either. In any case, the change is not wrong -- everything rejected should be rejected. Given that this fixes a real problem, I'm fine with the patch. LGTM.

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

SDValue SystemZTargetLowering::lowerXALUO(SDValue Op,
 
   SDValue SetCC = emitSETCC(DAG, DL, Result.getValue(1), CCValid, CCMask);
   if (N->getValueType(1) == MVT::i1)
     SetCC = DAG.getNode(ISD::TRUNCATE, DL, MVT::i1, SetCC);
 
   return DAG.getNode(ISD::MERGE_VALUES, DL, N->getVTList(), Result, SetCC);
 }
 
+static bool isAddCarryChain(SDValue Carry) {
+  while (Carry.getOpcode() == ISD::ADDCARRY)
+    Carry = Carry.getOperand(2);
+  return Carry.getOpcode() == ISD::UADDO;
+}
+
+static bool isSubBorrowChain(SDValue Carry) {
+  while (Carry.getOpcode() == ISD::SUBCARRY)
+    Carry = Carry.getOperand(2);
+  return Carry.getOpcode() == ISD::USUBO;
+}
+
 // Lower ADDCARRY/SUBCARRY nodes.
 SDValue SystemZTargetLowering::lowerADDSUBCARRY(SDValue Op,
                                                 SelectionDAG &DAG) const {
 
   SDNode *N = Op.getNode();
   MVT VT = N->getSimpleValueType(0);
 
   // Let legalize expand this if it isn't a legal type yet.
   if (!DAG.getTargetLoweringInfo().isTypeLegal(VT))
     return SDValue();
 
   SDValue LHS = N->getOperand(0);
   SDValue RHS = N->getOperand(1);
   SDValue Carry = Op.getOperand(2);
   SDLoc DL(N);
   unsigned BaseOp = 0;
   unsigned CCValid = 0;
   unsigned CCMask = 0;
 
   switch (Op.getOpcode()) {
   default: llvm_unreachable("Unknown instruction!");
   case ISD::ADDCARRY:
-    if (Carry.getOpcode() != ISD::UADDO && Carry.getOpcode() != ISD::ADDCARRY)
+    if (!isAddCarryChain(Carry))
       return SDValue();
 
     BaseOp = SystemZISD::ADDCARRY;
     CCValid = SystemZ::CCMASK_LOGICAL;
     CCMask = SystemZ::CCMASK_LOGICAL_CARRY;
     break;
   case ISD::SUBCARRY:
-    if (Carry.getOpcode() != ISD::USUBO && Carry.getOpcode() != ISD::SUBCARRY)
+    if (!isSubBorrowChain(Carry))
       return SDValue();
 
     BaseOp = SystemZISD::SUBCARRY;
     CCValid = SystemZ::CCMASK_LOGICAL;
     CCMask = SystemZ::CCMASK_LOGICAL_BORROW;
     break;
   }
 

llvm/test/CodeGen/SystemZ/pr42606.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+define i64 @test(i64 %lo, i64 %hi) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    la %r0, 0(%r2,%r2)
+; CHECK-NEXT:    clgr %r0, %r2
+; CHECK-NEXT:    ipm %r0
+; CHECK-NEXT:    la %r1, 1(%r2,%r2)
+; CHECK-NEXT:    cghi %r1, 0
+; CHECK-NEXT:    ipm %r1
+; CHECK-NEXT:    afi %r1, -268435456
+; CHECK-NEXT:    srl %r1, 31
+; CHECK-NEXT:    rosbg %r1, %r0, 63, 63, 36
+; CHECK-NEXT:    algfr %r3, %r1
+; CHECK-NEXT:    lgr %r2, %r3
+; CHECK-NEXT:    br %r14
+  %tmp = tail call { i64, i1 } @llvm.uadd.with.overflow.i64(i64 %lo, i64 1)
+  %tmp1 = extractvalue { i64, i1 } %tmp, 0
+  %tmp2 = extractvalue { i64, i1 } %tmp, 1
+  %tmp3 = zext i1 %tmp2 to i64
+  %tmp4 = tail call { i64, i1 } @llvm.uadd.with.overflow.i64(i64 %lo, i64 %tmp1)
+  %tmp5 = extractvalue { i64, i1 } %tmp4, 1
+  %tmp6 = zext i1 %tmp5 to i64
+  %spec.select.i = add i64 0, %hi
+  %tmp7 = add i64 %spec.select.i, %tmp3
+  %tmp8 = add i64 %tmp7, %tmp6
+  ret i64 %tmp8
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare { i64, i1 } @llvm.uadd.with.overflow.i64(i64, i64) #0
+
+attributes #0 = { nounwind readnone speculatable }