On 4/2/25 3:07 PM, Patrick Palka wrote:
Bootstrapped and regtested on x86_64-pc-linux, does this look OK for trunk/14?
OK.
-- >8 -- The testcase in this PR uses 2.5x more memory and 6x more compile time ever since r14-5979 which implements P2280R4. This is because our speculative constexpr folding now does a lot more work trying to fold ultimately non-constant calls to constexpr functions, and in turn produces a lot of garbage. Sometimes, we do successfully fold more thanks to P2280R4, but it seems to be trivial stuff like calls to std::array::size or std::addressof. The benefit of P2280 therefore doesn't seem worth the cost during speculative constexpr folding, so this patch restricts the paper to only manifestly-constant evaluation. PR c++/119387 gcc/cp/ChangeLog: * constexpr.cc (p2280_active_p): Define. (cxx_eval_constant_expression) <case VAR_DECL>: Use it to restrict P2280 relaxations. <case PARM_DECL>: Likewise. --- gcc/cp/constexpr.cc | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-) diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc index 4820bcc84aa..ee968bc4630 100644 --- a/gcc/cp/constexpr.cc +++ b/gcc/cp/constexpr.cc @@ -1294,6 +1294,22 @@ struct constexpr_ctx { mce_value manifestly_const_eval; };+/* True if the constexpr relaxations afforded by P2280R4 for unknown+ references and objects are in effect. */ + +static bool +p2280_active_p (const constexpr_ctx *ctx) +{ + if (ctx->manifestly_const_eval != mce_true) + /* Disable these relaxations during speculative constexpr folding, + as it can significantly increase compile time/memory use + (PR119387). */ + return false; + + /* P2280R4 is accepted as a DR back to C++11. */ + return cxx_dialect >= cxx11; +} + /* Remove T from the global values map, checking for attempts to destroy a value that has already finished its lifetime. */@@ -7792,7 +7808,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,r = TARGET_EXPR_INITIAL (r); if (DECL_P (r) /* P2280 allows references to unknown. */ - && !(VAR_P (t) && TYPE_REF_P (TREE_TYPE (t)))) + && !(p2280_active_p (ctx) && VAR_P (t) && TYPE_REF_P (TREE_TYPE (t)))) { if (!ctx->quiet) non_const_var_error (loc, r, /*fundef_p*/false); @@ -7844,9 +7860,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t, r = build_constructor (TREE_TYPE (t), NULL); TREE_CONSTANT (r) = true; } - else if (TYPE_REF_P (TREE_TYPE (t))) + else if (p2280_active_p (ctx) && TYPE_REF_P (TREE_TYPE (t))) /* P2280 allows references to unknown... */; - else if (is_this_parameter (t)) + else if (p2280_active_p (ctx) && is_this_parameter (t)) /* ...as well as the this pointer. */; else {
