In the testcase below the two left fold expressions each expand into a
constant logical expression with 1024 terms, for which potential_const_expr
takes more than a minute to return true. This happens because p_c_e_1
performs trial evaluation of the first operand of a &&/|| in order to
determine whether to consider the potentiality of the second operand.
And because the expanded expression is left-associated, this trial
evaluation causes p_c_e_1 to be quadratic in the number of terms of the
expression.
This patch fixes this quadratic behavior by making p_c_e_1 preemptively
compute potentiality of the second operand of a &&/||, and perform trial
evaluation of the first operand only if the second operand isn't
potentially constant. We must be careful to avoid emitting bogus
diagnostics during the preemptive computation; to that end, we perform
this shortcut only when tf_error is cleared, and when tf_error is set we
now first check potentiality of the whole expression quietly and replay
the check noisily for diagnostics.
Apart from fixing the quadraticness for left-associated logical exprs,
this change also reduces compile time for the libstdc++ testcase
20_util/variant/87619.cc by about 15% even though our <variant> uses
right folds instead of left folds. Likewise for the testcase in the PR,
for which compile time is reduced by 30%. The reason for these speedups
is that p_c_e_1 no longer performs expensive trial evaluation of each term
of large constant logical expressions when determining their potentiality.
PR c++/102780
PR c++/108138
gcc/cp/ChangeLog:
* constexpr.c (potential_constant_expression_1) <case TRUTH_*_EXPR>:
When tf_error isn't set, preemptively check potentiality of the
second operand before performing trial evaluation of the first
operand.
(potential_constant_expression_1): When tf_error is set, first check
potentiality quietly and return true if successful, otherwise
proceed noisily to give errors.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/fold13.C: New test.
(cherry picked from commit
9927ecbb42d5be48fa933adc26f8601fab5007ca)
tmp = boolean_false_node;
truth:
{
- tree op = TREE_OPERAND (t, 0);
- if (!RECUR (op, rval))
+ tree op0 = TREE_OPERAND (t, 0);
+ tree op1 = TREE_OPERAND (t, 1);
+ if (!RECUR (op0, rval))
return false;
+ if (!(flags & tf_error) && RECUR (op1, rval))
+ /* When quiet, try to avoid expensive trial evaluation by first
+ checking potentiality of the second operand. */
+ return true;
if (!processing_template_decl)
- op = cxx_eval_outermost_constant_expr (op, true);
- if (tree_int_cst_equal (op, tmp))
- return RECUR (TREE_OPERAND (t, 1), rval);
+ op0 = cxx_eval_outermost_constant_expr (op0, true);
+ if (tree_int_cst_equal (op0, tmp))
+ return (flags & tf_error) ? RECUR (op1, rval) : false;
else
return true;
}
potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
tsubst_flags_t flags)
{
+ if (flags & tf_error)
+ {
+ /* Check potentiality quietly first, as that could be performed more
+ efficiently in some cases (currently only for TRUTH_*_EXPR). If
+ that fails, replay the check noisily to give errors. */
+ flags &= ~tf_error;
+ if (potential_constant_expression_1 (t, want_rval, strict, now, flags))
+ return true;
+ flags |= tf_error;
+ }
+
tree target = NULL_TREE;
return potential_constant_expression_1 (t, want_rval, strict, now,
flags, &target);
--- /dev/null
+// { dg-do compile { target c++17 } }
+// Verify constexpr evaluation of a large left fold logical expression
+// isn't quadratic in the size of the expanded expression.
+
+template<int> struct S { static constexpr bool value = true; };
+
+template<class T, T...> struct integer_sequence { };
+
+template<class T, T N>
+using make_integer_sequence
+#if __has_builtin(__make_integer_seq)
+ = __make_integer_seq<integer_sequence, T, N>;
+#else
+ = integer_sequence<T, __integer_pack(N)...>;
+#endif
+
+template<int... Is>
+constexpr bool f_impl(integer_sequence<int, Is...>) {
+ return (... && S<Is>::value);
+}
+
+static_assert(f_impl(make_integer_sequence<int, 1024>()));
+
+template<int... Is>
+constexpr bool g_impl(integer_sequence<int, Is...>) {
+ return (... || !S<Is>::value);
+}
+
+static_assert(!g_impl(make_integer_sequence<int, 1024>()));