https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125332
Bug ID: 125332
Summary: `flatten` attribute has no effect at -O0 optimization
level
Product: gcc
Version: 16.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: vittorio.romeo at outlook dot com
Target Milestone: ---
Code:
template<int N>
[[gnu::flatten]]
inline int f() {
if constexpr (N == 0) {
return 42;
} else {
return f<N-1>();
}
}
[[gnu::flatten]]
int main() {
return f<100>();
}
With either g++ trunk or g++ 16.1, using `-std=c++26 -O0`, no inlining happens
whatsoever. The generated assembly contains ~100 function calls:
"main":
push rbp
mov rbp, rsp
call "int f<100>()"
nop
pop rbp
ret
"int f<100>()":
push rbp
mov rbp, rsp
call "int f<99>()"
pop rbp
ret
"int f<99>()":
push rbp
mov rbp, rsp
call "int f<98>()"
pop rbp
ret
# etc...
On Compiler Explorer:
https://godbolt.org/z/xPW8nq6jE
If this is intended behavior, the documentation of `flatten` could be improved
that the attribute has no effect unless optimizations are enabled.
