[AArch64] Drop poison-generating flags in genSubAdd2SubSub combiner

A miscompilation issue has been addressed with improved handling.

Fixes: #88950.

Author: antoniofrighetto

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -6924,19 +6924,26 @@ genSubAdd2SubSub(MachineFunction &MF, MachineRegisterInfo &MRI,
     assert((Opcode == AArch64::SUBWrr || Opcode == AArch64::SUBXrr) &&
            "Unexpected instruction opcode.");
 
+  uint32_t Flags = Root.mergeFlagsWith(*AddMI);
+  Flags &= ~MachineInstr::NoSWrap;
+  Flags &= ~MachineInstr::NoUWrap;
+
   MachineInstrBuilder MIB1 =
       BuildMI(MF, MIMetadata(Root), TII->get(Opcode), NewVR)
           .addReg(RegA, getKillRegState(RegAIsKill))
-          .addReg(RegB, getKillRegState(RegBIsKill));
+          .addReg(RegB, getKillRegState(RegBIsKill))
+          .setMIFlags(Flags);
   MachineInstrBuilder MIB2 =
       BuildMI(MF, MIMetadata(Root), TII->get(Opcode), ResultReg)
           .addReg(NewVR, getKillRegState(true))
-          .addReg(RegC, getKillRegState(RegCIsKill));
+          .addReg(RegC, getKillRegState(RegCIsKill))
+          .setMIFlags(Flags);
 
   InstrIdxForVirtReg.insert(std::make_pair(NewVR, 0));
   InsInstrs.push_back(MIB1);
   InsInstrs.push_back(MIB2);
   DelInstrs.push_back(AddMI);
+  DelInstrs.push_back(&Root);
 }
 
 /// When getMachineCombinerPatterns() finds potential patterns,
@@ -6966,13 +6973,13 @@ void AArch64InstrInfo::genAlternativeCodeSequence(
     // ==> (A - B) - C
     genSubAdd2SubSub(MF, MRI, TII, Root, InsInstrs, DelInstrs, 1,
                      InstrIdxForVirtReg);
-    break;
+    return;
   case AArch64MachineCombinerPattern::SUBADD_OP2:
     // A - (B + C)
     // ==> (A - C) - B
     genSubAdd2SubSub(MF, MRI, TII, Root, InsInstrs, DelInstrs, 2,
                      InstrIdxForVirtReg);
-    break;
+    return;
   case AArch64MachineCombinerPattern::MULADDW_OP1:
   case AArch64MachineCombinerPattern::MULADDX_OP1:
     // MUL I=A,B,0

llvm/test/CodeGen/AArch64/machine-combiner-subadd2.mir

@@ -237,3 +237,30 @@ body:              |
     RET_ReallyLR implicit $w0
 
 ...
+---
+# Drop nowrap flags in SUB
+
+# CHECK-LABEL: name: test8
+# CHECK:       %7:gpr64 = SUBXrr %1, %0
+# CHECK-NEXT:  %4:gpr64common = SUBXrr killed %7, killed %2
+
+name:            test8
+registers:
+  - { id: 0, class: gpr64 }
+  - { id: 1, class: gpr64 }
+  - { id: 2, class: gpr64common }
+  - { id: 3, class: gpr64 }
+  - { id: 4, class: gpr64common }
+  - { id: 5, class: gpr64 }
+body:              |
+  bb.0:
+    %1:gpr64 = COPY $x1
+    %0:gpr64 = COPY $x0
+    %2:gpr64common = ORRXri %0:gpr64, 4096
+    %3:gpr64 = ADDXrr killed %2:gpr64common, %0:gpr64
+    %4:gpr64common = nsw SUBSXrr %1:gpr64, killed %3:gpr64, implicit-def dead $nzcv
+    %5:gpr64 = SUBSXri %4:gpr64common, 0, 0, implicit-def $nzcv
+    $x0 = COPY %5:gpr64
+    RET_ReallyLR implicit $x0
+
+...