https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106151
Bug ID: 106151
Summary: Inconsistent optimization when defaulting aggregate vs
non-aggregate
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Consider this example:
using size_t = decltype(sizeof(0));
struct string_view {
// string_view() : ptr(nullptr), len(0) { }
string_view() = default;
char const* ptr = nullptr;
size_t len = 0;
};
struct Fields
{
string_view a;
string_view b;
string_view c;
string_view d;
string_view e;
string_view f;
};
struct Foo : Fields
{
void clear();
Foo() = default;
};
void Foo::clear() {
*this = Foo{};
}
struct Bar : Fields
{
// No constructor, not even defaulted.
void clear();
};
void Bar::clear() {
*this = Bar{};
}
On gcc 12.1 -std=c++20 -O2 -mavx2, this emits:
Foo::clear():
mov QWORD PTR [rdi], 0
mov QWORD PTR [rdi+8], 0
mov QWORD PTR [rdi+16], 0
mov QWORD PTR [rdi+24], 0
mov QWORD PTR [rdi+32], 0
mov QWORD PTR [rdi+40], 0
mov QWORD PTR [rdi+48], 0
mov QWORD PTR [rdi+56], 0
mov QWORD PTR [rdi+64], 0
mov QWORD PTR [rdi+72], 0
mov QWORD PTR [rdi+80], 0
mov QWORD PTR [rdi+88], 0
ret
Bar::clear():
mov ecx, 12
xor eax, eax
rep stosq
ret
There are a lot of subtle changes you can make to change the generated code:
* if Fields has 3 members or fewer, it's a bunch of mov's either way
* if Fields has 4 or 5 members, Bar::clear uses vmov insead of mov.
* if Fields has 6+ members, Bar::clear uses stosq
* if Fields has an array of string_view, Foo::clear and Bar::clear are
identical for sizes 2, 3, and >= 9.
If string_view uses the constructor I have commented out (with the member
initializer list, instead of defaulted and using the default member
initializers, then:
* if Fields holds an array of string_view with at least 4 string_views, both
Foo::clear and Bar::clear emit a loop.
* otherwise, it's always a bunch of mov's for both Foo and Bar, regardless of
how many members.
Feels like something is up here.
