[llvm] [NFC][CodeGen] Refactor subregister index verification for MIR (PR #181921)

[Jay Foad via llvm-commits]

================
@@ -2818,28 +2818,29 @@ MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
           return;
         }
       }
-      if (MONum < MCID.getNumOperands()) {
-        if (const TargetRegisterClass *DRC = TII->getRegClass(MCID, MONum)) {
-          if (SubIdx) {
-            const TargetRegisterClass *SuperRC =
-                TRI->getLargestLegalSuperClass(RC, *MF);
-            if (!SuperRC) {
-              report("No largest legal super class exists.", MO, MONum);
-              return;
-            }
-            DRC = TRI->getMatchingSuperRegClass(SuperRC, DRC, SubIdx);
-            if (!DRC) {
-              report("No matching super-reg register class.", MO, MONum);
-              return;
-            }
-          }
-          if (!RC->hasSuperClassEq(DRC)) {
-            report("Illegal virtual register for instruction", MO, MONum);
-            OS << "Expected a " << TRI->getRegClassName(DRC)
-               << " register, but got a " << TRI->getRegClassName(RC)
-               << " register\n";
-          }
-        }
+      if (MONum >= MCID.getNumOperands())
+        break;
+      const TargetRegisterClass *DRC = TII->getRegClass(MCID, MONum);
+      if (!DRC)
+        break;
+
+      // If SubIdx is used, validate that RC with SubIdx can be used for an
+      // operand of class DRC. This is valid if for every register in RC, the
+      // register obtained by applying SubIdx to it is in DRC, i.e.,
+      // getMatchingSuperRegClass(RC, DRC, SubIdx) returns RC.
+      if (SubIdx && TRI->getMatchingSuperRegClass(RC, DRC, SubIdx) != RC) {
+        report("Illegal virtual register for instruction", MO, MONum);
+        OS << TRI->getRegClassName(RC) << "." << TRI->getSubRegIndexName(SubIdx)
+           << " cannot be used for " << TRI->getRegClassName(DRC)
+           << " operands.";
+      }
+
+      // If no SubIdx is used, just that that RC is a sub-class of DRC.
----------------
jayfoad wrote:

Did you mean:
```suggestion
      // If no SubIdx is used, just test that RC is a sub-class of DRC.
```

https://github.com/llvm/llvm-project/pull/181921