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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]
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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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