[clang] 319e731 - [clang] Shrink AutoTypes map key to reduce peak memory (#218278)

[via cfe-commits]

Author: Anonmiraj
Date: 2026-08-26T21:49:58+03:00
New Revision: 319e7316b63a41830b0c9cef7371ef83ea257c0d

URL: https://github.com/llvm/llvm-project/commit/319e7316b63a41830b0c9cef7371ef83ea257c0d
DIFF: https://github.com/llvm/llvm-project/commit/319e7316b63a41830b0c9cef7371ef83ea257c0d.diff

LOG: [clang] Shrink AutoTypes map key to reduce peak memory (#218278)

While looking for memory regressions, I found that #118288 caused
[CTRE](https://github.com/hanickadot/compile-time-regular-expressions)
to regress by ~9.5% in peak memory (+758 MB).

The fix is simple: use a reference to the FoldingSetNodeID in the map.

| | Max RSS |
| --- | --- |
| trunk | 8928 MB |
| this PR | **8347 MB (−6.5%)** |

[compile-time-tracker](

https://llvm-compile-time-tracker.com/compare.php?from=49de424f45389cb757c3cc8c50daf38d024e2314&to=0cf6a242f9b5a211f9dffd6b8f52aeeb3e91e508&stat=instructions)

Added: 
    

Modified: 
    clang/include/clang/AST/ASTContext.h
    clang/lib/AST/ASTContext.cpp

Removed: 
    


################################################################################
diff  --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index 013bc68a00a06..dd67c5d0410f8 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -284,7 +284,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   // arguments. Since both dependent and dependency are on the same set,
   // we can end up in an infinite recursion when looking for a node if we used
   // a `FoldingSet`, since both could end up in the same bucket.
-  mutable llvm::DenseMap<llvm::FoldingSetNodeID, AutoType *> AutoTypes;
+  // Keyed by an interned FoldingSetNodeIDRef rather than a FoldingSetNodeID to
+  // avoid its large inline SmallVector in every bucket.
+  mutable llvm::DenseMap<llvm::FoldingSetNodeIDRef, AutoType *> AutoTypes;
   mutable llvm::FoldingSet<DeducedTemplateSpecializationType>
     DeducedTemplateSpecializationTypes;
   mutable llvm::FoldingSet<AtomicType> AtomicTypes;
@@ -4084,5 +4086,19 @@ template <> struct llvm::DenseMapInfo<llvm::FoldingSetNodeID> {
     return LHS == RHS;
   }
 };
+template <> struct llvm::DenseMapInfo<llvm::FoldingSetNodeIDRef> {
+  static unsigned getHashValue(FoldingSetNodeIDRef Val) {
+    return Val.ComputeHash();
+  }
+  static bool isEqual(FoldingSetNodeIDRef LHS, FoldingSetNodeIDRef RHS) {
+    return LHS == RHS;
+  }
+  static unsigned getHashValue(const FoldingSetNodeID &Val) {
+    return Val.ComputeHash();
+  }
+  static bool isEqual(const FoldingSetNodeID &LHS, FoldingSetNodeIDRef RHS) {
+    return LHS == RHS;
+  }
+};
 
 #endif // LLVM_CLANG_AST_ASTCONTEXT_H

diff  --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 335d58ff2c9c8..b502c4436de49 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -6926,7 +6926,7 @@ ASTContext::getAutoType(DeducedKind DK, QualType DeducedAsType,
   llvm::FoldingSetNodeID ID;
   AutoType::Profile(ID, *this, DK, DeducedAsType, Keyword,
                     TypeConstraintConcept, TypeConstraintArgs);
-  if (auto const AT_iter = AutoTypes.find(ID); AT_iter != AutoTypes.end())
+  if (auto const AT_iter = AutoTypes.find_as(ID); AT_iter != AutoTypes.end())
     return QualType(AT_iter->getSecond(), 0);
 
   if (DK == DeducedKind::Deduced) {
@@ -6956,7 +6956,7 @@ ASTContext::getAutoType(DeducedKind DK, QualType DeducedAsType,
   assert(InsertedID == ID && "ID does not match");
 #endif
   Types.push_back(AT);
-  AutoTypes.try_emplace(ID, AT);
+  AutoTypes.try_emplace(ID.Intern(BumpAlloc), AT);
   return QualType(AT, 0);
 }