[clang] Reduce the small vector size for DeclTypeInfo.

[hokein]

The Declarator class is large (4584 bytes) and used as a stack-local variable during parsing.

This patch reduces the default size of its DeclTypeInfo member, reducing the overall size down to 3880 bytes. This allows clang handle more deeply nested expressions without exhausting the stack.

Combined with #148726, the nesting threshold for such expressions increases to ~1100.

No performance impact being observed.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

Changes

The Declarator class is large (4584 bytes) and used as a stack-local variable during parsing.

This patch reduces the default size of its DeclTypeInfo member, reducing the overall size down to 3880 bytes. This allows clang handle more deeply nested expressions without exhausting the stack.

Combined with #148726, the nesting threshold for such expressions increases to ~1100.

No performance impact being observed.

---

Full diff: https://github.com/llvm/llvm-project/pull/148788.diff

1 Files Affected:

• (modified) clang/include/clang/Sema/DeclSpec.h (+1-1)

diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
index 567ad2d5934d4..4b7c8d609735f 100644
--- a/clang/include/clang/Sema/DeclSpec.h
+++ b/clang/include/clang/Sema/DeclSpec.h
@@ -1918,7 +1918,7 @@ class Declarator {
   /// parsed.  This is pushed from the identifier out, which means that element
   /// #0 will be the most closely bound to the identifier, and
   /// DeclTypeInfo.back() will be the least closely bound.
-  SmallVector<DeclaratorChunk, 8> DeclTypeInfo;
+  SmallVector<DeclaratorChunk, 4> DeclTypeInfo;
 
   /// InvalidType - Set by Sema::GetTypeForDeclarator().
   LLVM_PREFERRED_TYPE(bool)

[AaronBallman]

LGTM! 8 declarator chunks is actually quite a bit, I suspect 4 is a more sensible default for the average TU. 2 might be a bit too few for what I'd suspect is the average use case.