https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115581
Bug ID: 115581
Summary: missed argument forwarding in lambda
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: pobrn at protonmail dot com
Target Milestone: ---
Consider the following piece of C++ code:
```
struct thing {
int x[64];
};
void g(const thing &);
void f1(thing x)
{
g(x);
}
auto *f2 = +[](thing x) {
g(x);
};
struct f3 {
void operator()(thing);
};
void f3::operator()(thing x) {
g(x);
}
```
Checking the generated code on an x86-64 linux system at -O2, note that `f1()`
and `f3::operator()()` simply "forward" their by-value arguments to `g()`:
```
f1(thing):
sub rsp, 8
lea rdi, [rsp+16]
call g(thing const&)
add rsp, 8
ret
```
Notice that this is not done in the case of `f2`, the generated function from
the lambda makes a copy of `x`:
```
f2::{lambda(thing)#1}::_FUN(thing):
sub rsp, 264
movdqu xmm0, XMMWORD PTR [rsp+272]
mov rdi, rsp
movaps XMMWORD PTR [rsp], xmm0
movdqu xmm0, XMMWORD PTR [rsp+288]
[...]
movdqu xmm0, XMMWORD PTR [rsp+512]
movaps XMMWORD PTR [rsp+240], xmm0
call g(thing const&)
add rsp, 264
ret
```
See https://gcc.godbolt.org/z/n43a6MPjY, clang does the same copy, so maybe
there is a rule in C++ that prevents this optimization and that I failed to
consider?
