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* [Bug libstdc++/109631] New: Simple std::optional types returned on stack, not registers @ 2023-04-26 7:22 david at westcontrol dot com 2023-04-26 9:44 ` [Bug libstdc++/109631] " pinskia at gcc dot gnu.org ` (2 more replies) 0 siblings, 3 replies; 4+ messages in thread From: david at westcontrol dot com @ 2023-04-26 7:22 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109631 Bug ID: 109631 Summary: Simple std::optional types returned on stack, not registers Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: david at westcontrol dot com Target Milestone: --- A std::optional<T> is fundamentally like a std::pair<T, bool>, but with nice access features, type safety, etc. But for simple functions it is significantly less efficient, because in gcc it is returned on the stack even when T fits in a single register. <https://godbolt.org/z/6hx6vxaG7> On targets such as x86-64 and AARCH64, simple structs and pairs that fit in two registers, are returned in two registers - there is no need for the caller to reserve stack space and pass a hidden pointer to the callee function. On clang with its standard library, this also applies to std::optional<T> for suitable types T such as "int", but on gcc and its standard C++ library, the stack is used for returning the std::optional<int>. Since both clang and gcc follow the same calling conventions, this must (I believe) be a result of different implementations of std::optional<> in the C++ libraries. (I note that code for std::pair<bool, T> is more efficient here than using std::pair<T, bool>. But perhaps the details of std::optional<> require that the contained element comes first.) #include <optional> #include <utility> using A = std::optional<int>; using B = std::pair<int, bool>; using C = std::pair<bool, int>; A foo_A(int x) { return x; } B foo_b(int x) { B y { x, true }; return y; } C foo_c(int x) { C y { true, x }; return y; } ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/109631] Simple std::optional types returned on stack, not registers 2023-04-26 7:22 [Bug libstdc++/109631] New: Simple std::optional types returned on stack, not registers david at westcontrol dot com @ 2023-04-26 9:44 ` pinskia at gcc dot gnu.org 2023-04-26 9:46 ` pinskia at gcc dot gnu.org 2023-04-26 9:48 ` [Bug middle-end/109631] " pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-04-26 9:44 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109631 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Severity|normal |enhancement ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/109631] Simple std::optional types returned on stack, not registers 2023-04-26 7:22 [Bug libstdc++/109631] New: Simple std::optional types returned on stack, not registers david at westcontrol dot com 2023-04-26 9:44 ` [Bug libstdc++/109631] " pinskia at gcc dot gnu.org @ 2023-04-26 9:46 ` pinskia at gcc dot gnu.org 2023-04-26 9:48 ` [Bug middle-end/109631] " pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-04-26 9:46 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109631 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |DUPLICATE --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Dup of bug 101326. *** This bug has been marked as a duplicate of bug 101326 *** ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug middle-end/109631] Simple std::optional types returned on stack, not registers 2023-04-26 7:22 [Bug libstdc++/109631] New: Simple std::optional types returned on stack, not registers david at westcontrol dot com 2023-04-26 9:44 ` [Bug libstdc++/109631] " pinskia at gcc dot gnu.org 2023-04-26 9:46 ` pinskia at gcc dot gnu.org @ 2023-04-26 9:48 ` pinskia at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-04-26 9:48 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109631 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Component|libstdc++ |middle-end Keywords|ABI |missed-optimization --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Note this is not a libstdc++ issue as your clang invocation is even still using libstdc++ :). Anyways this is a dup of the other bug has some analysis to it. ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2023-04-26 9:48 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-04-26 7:22 [Bug libstdc++/109631] New: Simple std::optional types returned on stack, not registers david at westcontrol dot com 2023-04-26 9:44 ` [Bug libstdc++/109631] " pinskia at gcc dot gnu.org 2023-04-26 9:46 ` pinskia at gcc dot gnu.org 2023-04-26 9:48 ` [Bug middle-end/109631] " pinskia at gcc dot gnu.org
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