https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79189
Bug ID: 79189
Summary: Poor code generation when using stateless lambda
instead of normal function
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ldionne.2 at gmail dot com
Target Milestone: ---
When doing an indirect call (through a function pointer), GCC generates
different code depending on whether the function pointer was obtained by
converting a stateless lambda or a normal function. The code generated for a
normal function is vastly superior, and in fact the call can sometimes be
elided.
Code:
#if 1
template <typename T>
static auto const increment = [](void* self) { ++*static_cast<T*>(self); };
#else
template <typename T>
void increment(void* self) { ++*static_cast<T*>(self); }
#endif
struct VTable { void (*increment)(void*); };
template <typename T> static VTable const vtable{increment<T>};
struct any_iterator {
template <typename Iterator>
explicit any_iterator(Iterator it)
: vptr_{&vtable<Iterator>}, self_{new Iterator(it)}
{ }
VTable const* vptr_;
void* self_;
};
int main() {
int input[100] = {0};
any_iterator first{&input[0]};
first.vptr_->increment(first.self_);
}
Codegen with the lambda:
increment<int*>::{lambda(void*)#1}::_FUN(void*):
add QWORD PTR [rdi], 4
ret
main:
sub rsp, 408
xor eax, eax
mov ecx, 50
mov rdi, rsp
rep stosq
mov edi, 8
call operator new(unsigned long)
mov QWORD PTR [rax], rsp
mov rdi, rax
call [QWORD PTR vtable<int*>[rip]]
xor eax, eax
add rsp, 408
ret
_GLOBAL__sub_I_main:
mov QWORD PTR vtable<int*>[rip], OFFSET
FLAT:increment<int*>::{lambda(void*)#1}::_FUN(void*)
ret
Codegen with the function:
main:
sub rsp, 8
mov edi, 8
call operator new(unsigned long)
xor eax, eax
add rsp, 8
ret
Note that Clang (trunk) makes a much better job at optimizing this. Also see
[1] for this example on compiler explorer. See [2] for a larger example that
exhibits the same behavior, and where this results in a ~10x speed difference
because the call is done in a loop.
[1]: https://godbolt.org/g/HQb5Y5
[2]: http://melpon.org/wandbox/permlink/Gs3njR3STPLk2Ecr
