Clang 23’s bytecode interpreter trades allocations for richer compile-time evaluation

Clang’s experimental bytecode interpreter is moving from basic compatibility work toward a more complete and memory-conscious implementation of C and C++ constant evaluation. In a Red Hat Developer engineering update, compiler contributor Timm Baeder says roughly 640 commits have landed since the previous report, adding language coverage, testing controls and new internal data representations.

What changed

The interpreter now supports virtual bases, expansion statements in constant expressions, broader dynamic_cast cases, destruction evaluation and more x86 SIMD intrinsics. Clang can also use tail calls while interpreting bytecode. Teams building Clang 23 from source can make the new interpreter the default with -DCLANG_USE_EXPERIMENTAL_CONST_INTERP=ON, then fall back with -fno-experimental-new-constant-interpreter when comparing behavior against the existing implementation.

The most consequential internal change is the move away from “dummy pointers.” Previously, Clang could allocate an interpreter block—including metadata—for a variable whose value could never be read at compile time. That was wasteful for large arrays and global objects. The new opaque-pointer representation keeps a declaration or expression as its base and allocates a path only when field, array or base operations must be tracked. The LLVM change that re-landed the declaration-based implementation says this removes many of those data allocations; it also records that an earlier attempt was reverted because pointer comparison touched uninitialized padding bytes.

String literals now get a separate string-pointer representation backed by the existing abstract syntax tree node instead of a copied interpreter allocation. An identifier records distinct evaluations of the same literal, preserving C++ rules for pointer comparison without attaching that information to every block pointer.

The trade-off

Not every change reduces memory. Supporting pointer-to-integer casts, offsets and address-label differences requires more provenance than a target-sized integer can hold. Baeder writes that integer values of at least 16 bits now occupy 24 bytes inside the interpreter so they can retain the information needed to reconstruct or reject pointer casts correctly.

That cost illustrates the project’s current phase: semantic correctness increasingly depends on representing details that ordinary runtime integers do not carry. The update reports that the remaining Clang test-suite failures have nearly halved, with few regressions across the Clang and libc++ suites, but the interpreter remains explicitly experimental.

What developers should do

Compiler and library teams with large C++ codebases can now build Clang 23 with the experimental interpreter enabled by default and compare compile-time behavior, memory use and diagnostics against the established evaluator. Opaque and string pointers should matter most for code that performs heavy constant evaluation over large objects or repeated literals.

This is not a migration notice for ordinary Red Hat Enterprise Linux users. It is an upstream testing opportunity for developers who build Clang or maintain code that stresses constexpr evaluation. The next major area named in the update is array-filler support, which has not landed yet.