Enable trial constexpr folding for INIT_EXPR in C++.

C++ compiler now trial-folds INIT_EXPR for constexpr evaluation, fixing test failures.

By Patrick Palka August 14, 2026 committed

This patch enables trial constexpr folding for INIT_EXPR in the C++ front-end, mirroring existing support for TARGET_EXPR. This change addresses test failures in is_constant_evaluated3{a,b}.C by correctly handling local copy-initialization, which is represented as an INIT_EXPR.

In Details

The cp_fold_r function in cp-gimplify.cc now includes logic for case INIT_EXPR to perform trial maybe_constant_init folding, similar to how TARGET_EXPR is handled. This is intended to fix g++.dg/opt/is_constant_evaluated3{a,b}.C testcases by allowing constexpr evaluation of initializers for local variables that use copy-initialization.

For Context

constexpr: A C++ keyword that denotes that a variable or function's value can be determined at compile time. This enables compile-time evaluation, improving performance.
INIT_EXPR: An expression in GCC's internal representation (GIMPLE) that represents initialization of a variable or object.
TARGET_EXPR: An expression in GCC's internal representation (GIMPLE) that represents initialization of a target object, often used in contexts like __builtin_memcpy or aggregate initialization.
constant evaluation: The process by which the compiler determines the value of an expression at compile time rather than at runtime.
GIMPLE: GCC's internal representation (IR) used between the front-end and the optimization passes. It's a three-address code form.

Filed Under: c++constexproptimization

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