[clang] [Clang] Cache AST context lookup for declarations (NFC) (PR #223047)

[Eli Friedman via cfe-commits]

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@@ -1488,6 +1484,18 @@ DeclContext *DeclContext::getNonTransparentContext() {
   return DC;
 }
 
+ASTContext &DeclContext::getParentASTContextSlow() const {
+  const DeclContext *DC = this;
+  while (!DC->isTranslationUnit()) {
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efriedma-quic wrote:

We might as well just past the ASTContext into the DeclContext constructor; should be simple to implement, and makes things slightly faster if we can skip doing a null check.

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My primary concern with this patch is that this is sort of an irreversible decision; if we start assuming the ASTContext is always available cheaply from a DeclContext, we'll never be able go the other way.  But I guess we might already be at that point.

https://github.com/llvm/llvm-project/pull/223047