[clang] [clang] Speed up Decl::getASTContext() (PR #226612)

[Nico Weber via cfe-commits]

================
@@ -548,7 +548,15 @@ TranslationUnitDecl *Decl::getTranslationUnitDecl() {
 }
 
 ASTContext &Decl::getASTContext() const {
-  return getTranslationUnitDecl()->getASTContext();
+  if (const auto *TUD = dyn_cast<TranslationUnitDecl>(this))
+    return TUD->getASTContext();
----------------
nico wrote:

I could! But checking `this`'s type is very cheap, and then we don't have to `getDeclContext()` for TranslationUnitDecls. Looking up things globally is presumably common and it's nice for this to be fast.

But to be honest, I haven't measured how just

```
ASTContext &Decl::getASTContext() const {
  if (!DC->CachedASTContext)
    DC->CachedASTContext = &getTranslationUnitDecl()->getASTContext();
  return *DC->CachedASTContext;
}
```

would do. I should probably measure it. I'll do so and report back.

https://github.com/llvm/llvm-project/pull/226612