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* [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer
@ 2022-06-16 7:13 de34 at live dot cn
2022-06-16 7:19 ` [Bug libstdc++/105995] " pinskia at gcc dot gnu.org
` (7 more replies)
0 siblings, 8 replies; 9+ messages in thread
From: de34 at live dot cn @ 2022-06-16 7:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
Bug ID: 105995
Summary: QoI: constexpr basic_string variable must use all of
its SSO buffer
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
The current implementation in libstdc++ only supports constexpr
std::basic_string variable of static storage duration which uses all of its SSO
buffer:
#include <string>
constexpr std::string str15{"0123456789abcde"};
// constexpr std::string str14{"0123456789abcd"}; // error: should we support
this?
// constexpr std::string str16{"0123456789abcdef"}; // error: unsupportable
constexpr std::u16string u16str7{u"0123456"};
constexpr std::u32string u32str3{U"012"};
int main()
{
// constexpr std::string strdyn{"0123456789abcde"}; // error: unsupportable
}
Compiler Explorer links:
https://gcc.godbolt.org/z/cvbY1hK3G
https://gcc.godbolt.org/z/634888Y96
I think constexpr string variable that requires heap allocation is currently
unimplementable, and constexpr string variable of dynamic storage duration
can't be supported by libstdc++-v3 without ABI breakage now.
However, should we allow shorter strings to be stored into constexpr
basic_string variables? We can do this by always fully initializing the SSO
buffer (if it's used) during constant evluation.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
@ 2022-06-16 7:19 ` pinskia at gcc dot gnu.org
2022-06-16 7:41 ` de34 at live dot cn
` (6 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-06-16 7:19 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
C++20 supports dynamic allocation for constexpr.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
2022-06-16 7:19 ` [Bug libstdc++/105995] " pinskia at gcc dot gnu.org
@ 2022-06-16 7:41 ` de34 at live dot cn
2022-06-16 11:08 ` redi at gcc dot gnu.org
` (5 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: de34 at live dot cn @ 2022-06-16 7:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
--- Comment #2 from Jiang An <de34 at live dot cn> ---
(In reply to Andrew Pinski from comment #1)
> C++20 supports dynamic allocation for constexpr.
Yeah, but a constexpr variable can't hold dynamically allocated memory.
Dynamically allocated memory must be deallocated within its initialization.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
2022-06-16 7:19 ` [Bug libstdc++/105995] " pinskia at gcc dot gnu.org
2022-06-16 7:41 ` de34 at live dot cn
@ 2022-06-16 11:08 ` redi at gcc dot gnu.org
2022-06-16 11:14 ` redi at gcc dot gnu.org
` (4 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2022-06-16 11:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Right, you can only use std::string as a local variable in a constexpr
function. I don't think it's expected to work as a constexpr variable.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
` (2 preceding siblings ...)
2022-06-16 11:08 ` redi at gcc dot gnu.org
@ 2022-06-16 11:14 ` redi at gcc dot gnu.org
2022-06-16 19:20 ` cvs-commit at gcc dot gnu.org
` (3 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2022-06-16 11:14 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2022-06-16
Severity|normal |enhancement
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This makes it work:
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -352,7 +352,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
#if __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
- _M_local_buf[0] = _CharT();
+ for (_CharT& __c : _M_local_buf)
+ __c = _CharT();
#endif
return _M_local_data();
}
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
` (3 preceding siblings ...)
2022-06-16 11:14 ` redi at gcc dot gnu.org
@ 2022-06-16 19:20 ` cvs-commit at gcc dot gnu.org
2022-06-16 19:21 ` redi at gcc dot gnu.org
` (2 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2022-06-16 19:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:
https://gcc.gnu.org/g:98a0d72a610a87e8e383d366e50253ddcc9a51dd
commit r13-1139-g98a0d72a610a87e8e383d366e50253ddcc9a51dd
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Thu Jun 16 14:57:32 2022 +0100
libstdc++: Support constexpr global std::string for size < 15 [PR105995]
I don't think this is required by the standard, but it's easy to
support.
libstdc++-v3/ChangeLog:
PR libstdc++/105995
* include/bits/basic_string.h (_M_use_local_data): Initialize
the entire SSO buffer.
* testsuite/21_strings/basic_string/cons/char/105995.cc: New test.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
` (4 preceding siblings ...)
2022-06-16 19:20 ` cvs-commit at gcc dot gnu.org
@ 2022-06-16 19:21 ` redi at gcc dot gnu.org
2022-08-03 13:47 ` cvs-commit at gcc dot gnu.org
2022-08-03 13:49 ` redi at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2022-06-16 19:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |13.0
Resolution|--- |FIXED
Status|NEW |RESOLVED
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 13, thanks for the report.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
` (5 preceding siblings ...)
2022-06-16 19:21 ` redi at gcc dot gnu.org
@ 2022-08-03 13:47 ` cvs-commit at gcc dot gnu.org
2022-08-03 13:49 ` redi at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2022-08-03 13:47 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:
https://gcc.gnu.org/g:e562236851e06091256593aa0d3fbda60a28e45b
commit r12-8657-ge562236851e06091256593aa0d3fbda60a28e45b
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Thu Jun 16 14:57:32 2022 +0100
libstdc++: Support constexpr global std::string for size < 15 [PR105995]
I don't think this is required by the standard, but it's easy to
support.
libstdc++-v3/ChangeLog:
PR libstdc++/105995
* include/bits/basic_string.h (_M_use_local_data): Initialize
the entire SSO buffer.
* testsuite/21_strings/basic_string/cons/char/105995.cc: New test.
(cherry picked from commit 98a0d72a610a87e8e383d366e50253ddcc9a51dd)
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug libstdc++/105995] QoI: constexpr basic_string variable must use all of its SSO buffer
2022-06-16 7:13 [Bug libstdc++/105995] New: QoI: constexpr basic_string variable must use all of its SSO buffer de34 at live dot cn
` (6 preceding siblings ...)
2022-08-03 13:47 ` cvs-commit at gcc dot gnu.org
@ 2022-08-03 13:49 ` redi at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2022-08-03 13:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|13.0 |12.2
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Backported for 12.2
^ permalink raw reply [flat|nested] 9+ messages in thread
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