[llvm] [SPIRV] LLVM IR DIScope nodes are now emitted after their dependencies. (PR #227361)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -833,13 +835,160 @@ MCRegister SPIRVNonSemanticDebugHandler::findOrEmitOpTypeInt32(
   return Reg;
 }
 
-std::optional<MCRegister> SPIRVNonSemanticDebugHandler::emitDebugTypePointer(
+// Each node emits its dependencies first. E.g. for `void f() { struct L; }`,
+// node L emits f, then L. For `struct S { S *p; }`, S* is a back edge, so S
+// is emitted without member p.
+SPIRVNonSemanticDebugHandler::EmitResult
+SPIRVNonSemanticDebugHandler::getOrCreateDebugScope(const DIScope *S) {
+  if (!S || FailedScopes.contains(S))
+    return EmitResult::unsupported();
+  if (auto Reg = lookupOptReg(DebugScopeRegs, S))
+    return EmitResult::emitted(*Reg);
+
+  // Mark the node as in progress while its dependencies are resolved/emitted.
+  // This also prevents cycles from happening.
+  if (!ScopesInProgress.insert(S).second)
+    return EmitResult::inProgress();
+  EmitResult R = emitDebugScope(S);
+  ScopesInProgress.erase(S);
+  assert((!R || !GlobalNSDIEnabled) &&
+         "debug type or scope created after module-scope NSDI emission");
+  if (R)
+    DebugScopeRegs[S] = R.Reg;
+  else if (R.Status == EmitStatus::Unsupported)
+    FailedScopes.insert(S);
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jmmartinez wrote:

The algorithm is clear; it seems a bit unavoidable the big diff.

Though, I have an idea:
* I think `FailedScopes` is redundant with `ScopesInProgress`. When we "fail" we could just leave the scope in `ScopesInProgress`. Since we treat them in a very similar way (a scope that fails is a scope that doesn't end).
* Then, we only need 2 states in `EmitResult`: succeed and in progress.
* That means that we can just remove the state and simply use the `MCRegister()` as a "failed to remove" value.
* If we allow `DebugScopeRegs` to also hold "in progress / failed" scopes, we can simply use `DebugScopeRegs` to keep the full state without adding `ScopesInProgress`.

Is my train of thought right? Or did I derail?

https://github.com/llvm/llvm-project/pull/227361