From 1f1c432226cf3db399b2a2a627e3c5720b02b1d6 Mon Sep 17 00:00:00 2001 From: Marek Polacek Date: Tue, 19 Sep 2023 16:31:17 -0400 Subject: c++: implement P2564, consteval needs to propagate up [PR107687] This patch implements P2564, described at , whereby certain functions are promoted to consteval. For example: consteval int id(int i) { return i; } template constexpr int f(T t) { return t + id(t); // id causes f to be promoted to consteval } void g(int i) { f (3); } now compiles. Previously the code was ill-formed: we would complain that 't' in 'f' is not a constant expression. Since 'f' is now consteval, it means that the call to id(t) is in an immediate context, so doesn't have to produce a constant -- this is how we allow consteval functions composition. But making 'f' consteval also means that the call to 'f' in 'g' must yield a constant; failure to do so results in an error. I made the effort to have cc1plus explain to us what's going on. For example, calling f(i) produces this neat diagnostic: w.C:11:11: error: call to consteval function 'f(i)' is not a constant expression 11 | f (i); | ~~^~~ w.C:11:11: error: 'i' is not a constant expression w.C:6:22: note: 'constexpr int f(T) [with T = int]' was promoted to an immediate function because its body contains an immediate-escalating expression 'id(t)' 6 | return t + id(t); // id causes f to be promoted to consteval | ~~^~~ which hopefully makes it clear what's going on. Implementing this proposal has been tricky. One problem was delayed instantiation: instantiating a function can set off a domino effect where one call promotes a function to consteval but that then means that another function should also be promoted, etc. In v1, I addressed the delayed instantiation problem by instantiating trees early, so that we can escalate functions right away. That caused a number of problems, and in certain cases, like consteval-prop3.C, it can't work, because we need to wait till EOF to see the definition of the function anyway. Overeager instantiation tends to cause diagnostic problems too. In v2, I attempted to move the escalation to the gimplifier, at which point all templates have been instantiated. That attempt flopped, however, because once we've gimplified a function, its body is discarded and as a consequence, you can no longer evaluate a call to that function which is required for escalating, which needs to decide if a call is a constant expression or not. Therefore, we have to perform the escalation before gimplifying, but after instantiate_pending_templates. That's not easy because we have no way to walk all the trees. In the v2 patch, I use two vectors: one to store function decls that may become consteval, and another to remember references to immediate-escalating functions. Unfortunately the latter must also stash functions that call immediate-escalating functions. Consider: int g(int i) { f(i); // f is immediate-escalating } where g itself is not immediate-escalating, but we have to make sure that if f gets promoted to consteval, we give an error. A new option, -fno-immediate-escalation, is provided to suppress escalating functions. v2 also adds a new flag, DECL_ESCALATION_CHECKED_P, so that we don't escalate a function multiple times, and so that we can distinguish between explicitly consteval functions and functions that have been promoted to consteval. In v3, I removed one of the new vectors and changed the other one to a hash set. This version also contains numerous cleanups. v4 merges find_escalating_expr_r into cp_fold_immediate_r. It also adds a new optimization in cp_fold_function. v5 greatly simplifies the code. v6 simplifies the code further and removes an ff_ flag. v7 removes maybe_promote_function_to_consteval and further simplifies cp_fold_immediate_r logic. v8 removes maybe_store_immediate_escalating_fn. PR c++/107687 PR c++/110997 gcc/c-family/ChangeLog: * c-cppbuiltin.cc (c_cpp_builtins): Update __cpp_consteval. * c-opts.cc (c_common_post_options): Pre-C++20, unset flag_immediate_escalation. * c.opt (fimmediate-escalation): New option. gcc/cp/ChangeLog: * call.cc (in_immediate_context): No longer static. * constexpr.cc (cxx_eval_call_expression): Adjust assert. * cp-gimplify.cc (deferred_escalating_exprs): New vec. (remember_escalating_expr): New. (enum fold_flags): Remove ff_fold_immediate. (immediate_escalating_function_p): New. (unchecked_immediate_escalating_function_p): New. (promote_function_to_consteval): New. (cp_fold_immediate): Move above. Return non-null if any errors were emitted. (maybe_explain_promoted_consteval): New. (cp_gimplify_expr) : Assert we've handled all immediate invocations. (taking_address_of_imm_fn_error): New. (cp_fold_immediate_r): Merge ADDR_EXPR and PTRMEM_CST cases. Implement P2564 - promoting functions to consteval. : Implement P2564 - promoting functions to consteval. (cp_fold_r): If an expression turns into a CALL_EXPR after cp_fold, call cp_fold_immediate_r on the CALL_EXPR. (cp_fold_function): Set DECL_ESCALATION_CHECKED_P if deferred_escalating_exprs does not contain current_function_decl. (process_and_check_pending_immediate_escalating_fns): New. * cp-tree.h (struct lang_decl_fn): Add escalated_p bit-field. (DECL_ESCALATION_CHECKED_P): New. (immediate_invocation_p): Declare. (process_pending_immediate_escalating_fns): Likewise. * decl2.cc (c_parse_final_cleanups): Set at_eof to 2 after all templates have been instantiated; and to 3 at the end of the function. Call process_pending_immediate_escalating_fns. * error.cc (dump_template_bindings): Check at_eof against an updated value. * module.cc (trees_out::lang_decl_bools): Stream escalated_p. (trees_in::lang_decl_bools): Likewise. * pt.cc (push_tinst_level_loc): Set at_eof to 3, not 2. * typeck.cc (cp_build_addr_expr_1): Don't check DECL_IMMEDIATE_FUNCTION_P. gcc/ChangeLog: * doc/invoke.texi: Document -fno-immediate-escalation. libstdc++-v3/ChangeLog: * testsuite/18_support/comparisons/categories/zero_neg.cc: Add dg-prune-output. * testsuite/std/format/string_neg.cc: Add dg-error. gcc/testsuite/ChangeLog: * g++.dg/cpp23/consteval-if10.C: Remove dg-error. * g++.dg/cpp23/consteval-if2.C: Likewise. * g++.dg/cpp23/feat-cxx2b.C: Adjust expected value of __cpp_consteval. * g++.dg/cpp26/feat-cxx26.C: Likewise. * g++.dg/cpp2a/consteval-memfn1.C: Add dg-error. * g++.dg/cpp2a/consteval11.C: Likewise. * g++.dg/cpp2a/consteval3.C: Adjust dg-error. * g++.dg/cpp2a/consteval34.C: Add dg-error. * g++.dg/cpp2a/consteval36.C: Likewise. * g++.dg/cpp2a/consteval9.C: Likewise. * g++.dg/cpp2a/feat-cxx2a.C: Adjust expected value of __cpp_consteval. * g++.dg/cpp2a/spaceship-synth9.C: Adjust dg-error. * g++.dg/cpp2a/consteval-prop1.C: New test. * g++.dg/cpp2a/consteval-prop10.C: New test. * g++.dg/cpp2a/consteval-prop11.C: New test. * g++.dg/cpp2a/consteval-prop12.C: New test. * g++.dg/cpp2a/consteval-prop13.C: New test. * g++.dg/cpp2a/consteval-prop14.C: New test. * g++.dg/cpp2a/consteval-prop15.C: New test. * g++.dg/cpp2a/consteval-prop16.C: New test. * g++.dg/cpp2a/consteval-prop17.C: New test. * g++.dg/cpp2a/consteval-prop18.C: New test. * g++.dg/cpp2a/consteval-prop19.C: New test. * g++.dg/cpp2a/consteval-prop20.C: New test. * g++.dg/cpp2a/consteval-prop2.C: New test. * g++.dg/cpp2a/consteval-prop3.C: New test. * g++.dg/cpp2a/consteval-prop4.C: New test. * g++.dg/cpp2a/consteval-prop5.C: New test. * g++.dg/cpp2a/consteval-prop6.C: New test. * g++.dg/cpp2a/consteval-prop7.C: New test. * g++.dg/cpp2a/consteval-prop8.C: New test. * g++.dg/cpp2a/consteval-prop9.C: New test. --- gcc/doc/invoke.texi | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) (limited to 'gcc/doc') diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index 2b51ff3..681e3f3 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -219,6 +219,7 @@ in the following sections. -fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords +-fno-immediate-escalation -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines @@ -3386,6 +3387,39 @@ word as an identifier. You can use the keyword @code{__typeof__} instead. This option is implied by the strict ISO C++ dialects: @option{-ansi}, @option{-std=c++98}, @option{-std=c++11}, etc. +@opindex fno-immediate-escalation +@opindex fimmediate-escalation +@item -fno-immediate-escalation +Do not enable immediate function escalation whereby certain functions +can be promoted to consteval, as specified in P2564R3. For example: + +@example +consteval int id(int i) @{ return i; @} + +constexpr int f(auto t) +@{ + return t + id(t); // id causes f to be promoted to consteval +@} + +void g(int i) +@{ + f (3); +@} +@end example + +compiles in C++20: @code{f} is an immediate-escalating function (due to +the @code{auto} it is a function template and is declared @code{constexpr}) +and @code{id(t)} is an immediate-escalating expression, so @code{f} is +promoted to @code{consteval}. Consequently, the call to @code{id(t)} +is in an immediate context, so doesn't have to produce a constant (that +is the mechanism allowing consteval function composition). However, +with @option{-fno-immediate-escalation}, @code{f} is not promoted to +@code{consteval}, and since the call to consteval function @code{id(t)} +is not a constant expression, the compiler rejects the code. + +This option is turned on by default; it is only effective in C++20 mode +or later. + @opindex fimplicit-constexpr @item -fimplicit-constexpr Make inline functions implicitly constexpr, if they satisfy the -- cgit v1.1