https://gcc.gnu.org/g:8dce1aa0579ab86a626e24c0af29455f30305595
commit r14-11691-g8dce1aa0579ab86a626e24c0af29455f30305595
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.
is_std_class comes from r15-4953.
PR c++/113835
gcc/cp/ChangeLog:
* cp-tree.h (is_std_class): Declare.
* constexpr.cc (is_std_class): New function.
(is_std_allocator): Use it.
(cxx_eval_outermost_constant_expr): Bail out early
for std::vector(N).
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-vector1.C: New test.
(cherry picked from commit 764f02327f7b2dc6ac5abaf89038e51cf0ee6d13)
Diff:
---
gcc/cp/cp-tree.h | 1 +
gcc/cp/constexpr.cc | 25 +++++++++++++++++++++----
gcc/testsuite/g++.dg/cpp2a/constexpr-vector1.C | 8 ++++++++
3 files changed, 30 insertions(+), 4 deletions(-)
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index cfdfc4f9abe7..19e7b2edede3 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8683,6 +8683,7 @@ extern bool is_rvalue_constant_expression (tree);
extern bool is_nondependent_constant_expression (tree);
extern bool is_nondependent_static_init_expression (tree);
extern bool is_static_init_expression (tree);
+extern bool is_std_class (tree, const char *);
extern bool is_std_allocator (tree);
extern bool potential_rvalue_constant_expression (tree);
extern bool require_potential_constant_expression (tree);
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index e68202a5b5d9..6b8f6af00702 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -2356,22 +2356,30 @@ is_std_construct_at (const constexpr_call *call)
&& is_std_construct_at (call->fundef->decl));
}
-/* True if CTX is an instance of std::allocator. */
+/* True if CTX is an instance of std::NAME class. */
bool
-is_std_allocator (tree ctx)
+is_std_class (tree ctx, const char *name)
{
if (ctx == NULL_TREE || !CLASS_TYPE_P (ctx) || !TYPE_MAIN_DECL (ctx))
return false;
tree decl = TYPE_MAIN_DECL (ctx);
- tree name = DECL_NAME (decl);
- if (name == NULL_TREE || !id_equal (name, "allocator"))
+ tree dname = DECL_NAME (decl);
+ if (dname == NULL_TREE || !id_equal (dname, name))
return false;
return decl_in_std_namespace_p (decl);
}
+/* True if CTX is an instance of std::allocator. */
+
+bool
+is_std_allocator (tree ctx)
+{
+ return is_std_class (ctx, "allocator");
+}
+
/* Return true if FNDECL is std::allocator<T>::{,de}allocate. */
static inline bool
@@ -8851,6 +8859,15 @@ 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() {}
