c++: implement P2564, consteval needs to propagate up [PR107687]
This patch implements P2564, described at <wg21.link/p2564>, whereby
certain functions are promoted to consteval. For example:
consteval int id(int i) { return i; }
template <typename T>
constexpr int f(T t)
{
return t + id(t); // id causes f<int> 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<int>' 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<int>(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<int> 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<int>(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.
* 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) <case CALL_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.
<case CALL_EXPR>: 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.
projects / thirdparty / gcc.git / commit
