c++: improve constexpr call caching [PR115639]

For the testcase from this PR, checking

  static_assert(0 == big_calc());

takes twice as much time as

  constexpr int ret = big_calc();
  static_assert(0 == ret);

ultimately because in the former, we first constant evaluate big_calc()
with mce_unknown (as part of warning-dependent folding from
cp_build_binary_op).  We then constant evaluate it a second time, with
mce_true, during finish_static_assert.  The result of the first
evaluation isn't reused because of the different mce_value, which in
general can give a different result.

But big_calc() here doesn't depend on mce_value at all (i.e. there's no if
consteval or __builtin_is_constant_evaluated calls, nested or otherwise)
so we should be able to reuse the result in such cases.  Specifically if a
constexpr call with mce_unknown succeeds, we can safely reuse the result
during a subsequent mce_true or mce_false evaluation.

This patch implements this by also caching a successful mce_unknown call
result into the corresponding mce_true and mce_false slots, so that such
a subsequent evaluation effectively reuses the mce_unknown result.  To
make it more convenient to access the cache slot for the same call with
different mce_value, this patch gives each constexpr_call entry three
result slots, one per mce_value, instead of having a distinct
constexpr_call entry for each mce_value.  And we can no longer use
NULL_TREE to denote the call is in progress; instead use unknown_type_node.

After this patch compile time for the above two fragments is the same.

	PR c++/115639

gcc/cp/ChangeLog:

	* constexpr.cc (struct constexpr_call): Add NSDMIs to each
	field.  Replace 'result' data member with 3-element 'results'
	array and a 'result' accessor function.  Remove
	'manifestly_const_eval' data member.
	(constexpr_call_hasher::equal): Adjust after constexpr_call
	layout change.
	(cxx_eval_call_expression): Likewise.  Define some local
	variables closer to their first use.  Use unknown_type_node
	instead of NULL_TREE as the "in progress" result.  After
	successully evaluating a call with mce_unknown, also cache the
	result in the corresponding mce_true and mce_false slots.

Reviewed-by: Jason Merrill <jason@redhat.com>

0 files changed, 0 insertions, 0 deletions