[clang] [Clang] Consider reachability for file-scope warnings on initializers (PR #163885)

[Yanzuo Liu via cfe-commits]

================
@@ -2724,6 +2724,80 @@ static void flushDiagnostics(Sema &S, const sema::FunctionScopeInfo *fscope) {
     S.Diag(D.Loc, D.PD);
 }
 
+void sema::AnalysisBasedWarnings::FlushDiagnostics(
+    const SmallVector<clang::sema::PossiblyUnreachableDiag, 4> PUDs) {
+  for (const auto &D : PUDs)
+    S.Diag(D.Loc, D.PD);
+}
+
+void sema::AnalysisBasedWarnings::EmitPossiblyUnreachableDiags(
+    AnalysisDeclContext &AC,
+    SmallVector<clang::sema::PossiblyUnreachableDiag, 4> PUDs) {
+
+  if (PUDs.empty()) {
+    return;
+  }
+
+  bool Analyzed = false;
+
+  for (const auto &D : PUDs) {
+    for (const Stmt *S : D.Stmts)
+      AC.registerForcedBlockExpression(S);
+  }
+
+  if (AC.getCFG()) {
+    Analyzed = true;
+    for (const auto &D : PUDs) {
+      bool AllReachable = true;
+      for (const Stmt *St : D.Stmts) {
+        const CFGBlock *block = AC.getBlockForRegisteredExpression(St);
+        CFGReverseBlockReachabilityAnalysis *cra =
+            AC.getCFGReachablityAnalysis();
+        if (block && cra) {
+          // Can this block be reached from the entrance?
+          if (!cra->isReachable(&AC.getCFG()->getEntry(), block)) {
+            AllReachable = false;
+            break;
+          }
+        }
+        // If we cannot map to a basic block, assume the statement is
+        // reachable.
+      }
+
+      if (AllReachable)
+        S.Diag(D.Loc, D.PD);
+    }
+  }
+
+  if (!Analyzed)
+    FlushDiagnostics(PUDs);
+}
+
+void sema::AnalysisBasedWarnings::RegisterVarDeclWarning(
+    VarDecl *VD, clang::sema::PossiblyUnreachableDiag PUD) {
+  VarDeclPossiblyUnreachableDiags[VD].emplace_back(PUD);
+}
+
+void sema::AnalysisBasedWarnings::IssueWarningsForRegisteredVarDecl(
+    VarDecl *VD) {
+  if (VarDeclPossiblyUnreachableDiags.find(VD) ==
+      VarDeclPossiblyUnreachableDiags.end()) {
+    return;
+  }
----------------
zwuis wrote:

```cpp
if (...)
  return;
```

Can we use `llvm::is_contained`?

https://github.com/llvm/llvm-project/pull/163885