Modern C++ codebases — from browsers to GPU frameworks — rely heavily on templates, and that often means massive abstract syntax trees. Even small inefficiencies in Clang’s AST representation can add up to noticeable compile-time overhead.
This post walks through a set of structural improvements I recently made to Clang’s AST that make type representation smaller, simpler, and faster to create — leading to measurable build-time gains in real-world projects.
A couple of months ago, I landed a large patch in Clang that brought substantial compile-time improvements for heavily templated C++ code.
For example, in stdexec — the reference implementation of the std::execution feature slated for C++26 — the slowest test ( test_on2.cpp ) saw a 7% reduction in build time.
Also the Chromium build showed a 5% improvement (source).
At a high level, the patch makes the Clang AST leaner: it reduces the memory footprint of type representations and lowers the cost of creating and uniquing them.
These improvements will ship with Clang 22, expected in the next few months.
How elaboration and qualified names used to work
Consider this simple snippet:
namespace NS { struct A {}; } using T = struct NS::A;
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