[llvm] [WebAssembly] Avoid scanning unrelated debug values (NFCI) (PR #218378)

[Heejin Ahn via llvm-commits]

================
@@ -79,44 +88,65 @@ WebAssemblyDebugValueManager::getSinkableDebugValues(
     MachineInstr *Insert) const {
   if (DbgValues.empty())
     return {};
-  // DBG_VALUEs between Def and Insert
+
+  // If Def and Insert are in different BBs, we only handle a simple case in
+  // which Insert's BB is a successor of Def's BB.
+  if (Def->getParent() != Insert->getParent() &&
+      !Def->getParent()->isSuccessor(Insert->getParent()))
+    return {};
+
+  SmallDenseSet<std::pair<const DILocalVariable *, const DILocation *>, 4>
+      OurVars;
+  for (MachineInstr *DV : DbgValues)
+    OurVars.insert({DV->getDebugVariable(), DV->getDebugLoc()->getInlinedAt()});
+  auto IsRelevantDbgValue = [&](const MachineInstr &MI) {
+    return MI.isDebugValue() &&
+           OurVars.count(
+               {MI.getDebugVariable(), MI.getDebugLoc()->getInlinedAt()});
+  };
----------------
aheejin wrote:

Is this pre-filtering mainly to reduce the size of `DbgValuesInBetween`? If so, it would be nice to write this in the PR description too.

---

And is there a reason we only comapre `getDebugVariable` and `getInlinedAt` here, given that we later compare all three (`getDebugVariable`, `getInlinedAt`, and `getDebugExpression`) later in the loop like this?
```cpp
    DebugVariable Var(DV->getDebugVariable(), DV->getDebugExpression(),
                      DV->getDebugLoc()->getInlinedAt());
    auto It = SeenDbgVarToDbgValues.find(Var);
    if (It == SeenDbgVarToDbgValues.end()) {
      SinkableDbgValues.push_back(DV);
      continue;
    }
```

How about we just create `OurVars` like
```cpp
     SmallDenseSet<DebugVariable, 4> OurVars;
     for (MachineInstr *DV : DbgValues)
       OurVars.insert(DebugVariable(DV->getDebugVariable(), DV->getDebugExpression(),
   DV->getDebugLoc()->getInlinedAt()));
```
and later we can do like
```cpp
     if (OurVars.count(Var) && !llvm::is_contained(DbgValues, &MI))
       SeenDbgVarToDbgValues[Var].push_back(&MI);
```


https://github.com/llvm/llvm-project/pull/218378