[llvm-branch-commits] [clang] [Clang] Inline cached AST context lookup for declarations (NFC) (PR #223041)

[Mehdi Amini via llvm-branch-commits]

https://github.com/joker-eph created https://github.com/llvm/llvm-project/pull/223041

Route declaration context queries through the cached DeclContext path. Define the common path inline to avoid walking to the translation unit and remove a hot out-of-line call.

CTMark O0 (3 samples): 29.239400 s -> 28.975433 s (-0.903%).

Impact on significant TUs in MLIR build time:
- `mlir/lib/RegisterAllDialects.cpp`: 2.4845% fewer retired instructions.
- `mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp`: 1.4241% fewer retired instructions.

Assisted-by: Codex

>From 3edcb8c335f3f319ad1eb88f8b5a14aa7d634ec1 Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph at gmail.com>
Date: Thu, 10 Sep 2026 07:21:25 -0700
Subject: [PATCH] Inline cached AST context lookup for declarations

Route declaration context queries through the cached DeclContext path.
Define the common path inline to avoid walking to the translation unit
and remove a hot out-of-line call.

The following measurements used the original relationship-cache parent,
before its cache-invalidation fixes.
CTMark O0 (3 samples, CPU 6): 29.239400 s -> 28.975433 s (-0.903%).

Impact on significant TUs in MLIR build time:
- `mlir/lib/RegisterAllDialects.cpp`: 2.4845% fewer retired
  instructions.
- `mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp`: 1.4241% fewer retired
  instructions.

Assisted-by: Codex
---
 clang/include/clang/AST/DeclBase.h | 8 ++++++++
 clang/lib/AST/DeclBase.cpp         | 4 ----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 1a2b1c72a6159..d34f1c6151b09 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -2847,6 +2847,14 @@ inline bool Decl::isTemplateParameter() const {
          getKind() == TemplateTemplateParm;
 }
 
+inline ASTContext &Decl::getASTContext() const {
+  const DeclContext *DC = getDeclContext();
+  // The translation unit has no parent context and owns the AST context.
+  if (!DC)
+    DC = castToDeclContext(this);
+  return DC->getParentASTContext();
+}
+
 // Specialization selected when ToTy is not a known subclass of DeclContext.
 template <class ToTy,
           bool IsKnownSubtype = ::std::is_base_of<DeclContext, ToTy>::value>
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 59ddc64cf30e0..33c924a7fcc9f 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -547,10 +547,6 @@ TranslationUnitDecl *Decl::getTranslationUnitDecl() {
   return cast<TranslationUnitDecl>(DC);
 }
 
-ASTContext &Decl::getASTContext() const {
-  return getTranslationUnitDecl()->getASTContext();
-}
-
 /// Helper to get the language options from the ASTContext.
 /// Defined out of line to avoid depending on ASTContext.h.
 const LangOptions &Decl::getLangOpts() const {