https://gcc.gnu.org/g:764f02327f7b2dc6ac5abaf89038e51cf0ee6d13
commit r15-9474-g764f02327f7b2dc6ac5abaf89038e51cf0ee6d13
Author: Jason Merrill <ja...@redhat.com>
Date: Sat Apr 12 11:35:18 2025 -0400
c++: shortcut constexpr vector ctor [PR113835]
Since std::vector became usable in constant evaluation in C++20, a vector
variable with static storage duration might be manifestly
constant-evaluated, so we properly try to constant-evaluate its initializer.
But it can never succeed since the result will always refer to the result of
operator new, so trying is a waste of time. Potentially a large waste of
time for a large vector, as in the testcase in the PR.
So, let's recognize this case and skip trying constant-evaluation. I do
this only for the case of an integer argument, as that's the case that's
easy to write but slow to (fail to) evaluate.
In the test, I use dg-timeout-factor to lower the default timeout from 300
seconds to 15; on my laptop, compilation without the patch takes about 20
seconds versus about 2 with the patch.
PR c++/113835
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_outermost_constant_expr): Bail out early
for std::vector(N).
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-vector1.C: New test.
Diff:
---
gcc/cp/constexpr.cc | 9 +++++++++
gcc/testsuite/g++.dg/cpp2a/constexpr-vector1.C | 8 ++++++++
2 files changed, 17 insertions(+)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 7e375823d648..dc59f59aa3f3 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9127,6 +9127,15 @@ cxx_eval_outermost_constant_expr (tree t, bool
allow_non_constant,
tree fndecl = cp_get_callee_fndecl_nofold (x);
if (fndecl && DECL_IMMEDIATE_FUNCTION_P (fndecl))
is_consteval = true;
+ /* Don't try to evaluate a std::vector constructor taking an integer, it
+ will fail in the 'if (heap_var)' block below after doing all the work
+ (c++/113835). This will need adjustment if P3554 is accepted. Note
+ that evaluation of e.g. the vector default constructor can succeed, so
+ we don't shortcut all vector constructors. */
+ if (fndecl && DECL_CONSTRUCTOR_P (fndecl) && allow_non_constant
+ && is_std_class (type, "vector") && call_expr_nargs (x) > 1
+ && TREE_CODE (TREE_TYPE (get_nth_callarg (x, 1))) == INTEGER_TYPE)
+ return t;
}
if (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type))
{
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-vector1.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-vector1.C
new file mode 100644
index 000000000000..196c6ec51fcf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-vector1.C
@@ -0,0 +1,8 @@
+// PR c++/113835
+// { dg-timeout-factor 0.05 }
+// { dg-do compile { target c++20_only } }
+
+#include <vector>
+const std::size_t N = 1'000'000;
+std::vector<int> x(N);
+int main() {}
