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 --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index e68202a5b5d9ef93be2d5ccdfeafdfa9283aaf76..6b8f6af007020a6355e29df0f5b230beda817306 100644 (file)
--- 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/cp/cp-tree.h b/gcc/cp/cp-tree.h
index cfdfc4f9abe75ff22b030c2adc8de3df41df8603..19e7b2edede300583830b47a831daef35e418818 100644 (file)
--- 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/testsuite/g++.dg/cpp2a/constexpr-vector1.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-vector1.C
new file mode 100644 (file)
index 0000000..196c6ec
--- /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() {}