[Bug libstdc++/104223] New: GCC unable to inline trivial functions passed to views::filter and transform unless lifted into types
redbeard0531 at gmail dot com
gcc-bugzilla@gcc.gnu.org
Tue Jan 25 13:11:22 GMT 2022
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104223
Bug ID: 104223
Summary: GCC unable to inline trivial functions passed to
views::filter and transform unless lifted into types
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redbeard0531 at gmail dot com
Target Milestone: ---
I'm not sure if this is an issue with the optimizer or the way that the library
code is written, or some combination of the two, but the end result seems
unfortunate. with() and without() are logically the same function, the only
difference is that with() lifts the function pointers into types using a
templated lambda variable, while without() just passes the function names
directly to the library. It seems interesting that the optimizer knows that
they are "constant enough" to emit direct rather than indirect calls to t() and
f(), however, it isn't constant enough to inline those calls.
https://godbolt.org/z/EqWzzh916
Flags: -std=c++2b -O3
Reproduces on at least 11.2 and trunk
#include <ranges>
namespace views = std::views;
void trace() noexcept;
inline int f(int i) {
trace();
return i;
}
inline bool t(int) { return true; }
// for some reason gcc needs this in order to inline f() and t()
template <auto f>
auto typify = [] (int i) { return f(i); };
void with() {
for (auto&& x : views::single(1) | views::transform(typify<f>) |
views::filter(typify<t>)) {}
}
void without() {
for (auto&& x : views::single(1) | views::transform(f) | views::filter(t))
{}
}
with():
sub rsp, 8
call trace()
add rsp, 8
jmp trace()
without():
sub rsp, 8
mov edi, 1
call f(int)
mov edi, eax
call t(int)
test al, al
jne .L10
add rsp, 8
ret
.L10:
mov edi, 1
add rsp, 8
jmp f(int)
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