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* [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
@ 2021-02-20 15:47 richardpku at gmail dot com
2021-02-22 9:43 ` [Bug libstdc++/99181] " jakub at gcc dot gnu.org
` (10 more replies)
0 siblings, 11 replies; 12+ messages in thread
From: richardpku at gmail dot com @ 2021-02-20 15:47 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
Bug ID: 99181
Summary: char_traits<char> (and thus string_view) compares
strings differently in constexpr and non-constexpr
contexts
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: richardpku at gmail dot com
Target Milestone: ---
Minimal program to produce bug (run it on a platform where char is a signed
type, such as i386/x86-64):
/tmp % cat a.cpp
#include <string_view>
#include <iostream>
using namespace std;
int main() {
// constexpr
constexpr bool i = ("\xff"sv > "aaa"sv);
cout << i << ",";
// not constexpr
auto a = "\xff"sv, b = "aaa"sv;
cout << (a > b) << endl;
return 0;
}
/tmp % g++ -std=gnu++2a a.cpp && ./a.out
0,1
The expected result is "1,1".
In a non-constexpr context, std::char_traits<char>::compare invokes
__builtin_memcmp, which is required by C standard to interpret characters as
unsigned char.
In a constexpr context, however, std::char_traits<char>::compare invokes
__gnu_cxx::char_traits<char>::compare, which in turn calls
__gnu_cxx::char_traits<char>::lt to compare chars.
__gnu_cxx::char_traits<char>::lt (unlike std::char_traits<char>::lt) is not
specialized to compare chars as unsigned char.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
@ 2021-02-22 9:43 ` jakub at gcc dot gnu.org
2021-02-22 14:27 ` jakub at gcc dot gnu.org
` (9 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-02-22 9:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
static _GLIBCXX_CONSTEXPR bool
lt(const char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
{
// LWG 467.
return (static_cast<unsigned char>(__c1)
< static_cast<unsigned char>(__c2));
}
static _GLIBCXX17_CONSTEXPR int
compare(const char_type* __s1, const char_type* __s2, size_t __n)
{
if (__n == 0)
return 0;
#if __cplusplus >= 201703L
if (__builtin_constant_p(__n)
&& __constant_char_array_p(__s1, __n)
&& __constant_char_array_p(__s2, __n))
return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
#endif
return __builtin_memcmp(__s1, __s2, __n);
}
Seems the std::char_traits<char>::lt specialization implements LWG467, and so
does std::char_traits<char>::compare when not in constant expression
evaluation,
but the C++17 case above will call __gnu_cxx::char_traits<char_type>::compare
which doesn't know about LWG 467.
--- libstdc++-v3/include/bits/char_traits.h.jj 2021-01-04 10:26:03.558953845
+0100
+++ libstdc++-v3/include/bits/char_traits.h 2021-02-22 10:42:08.197199781
+0100
@@ -349,7 +349,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if (__builtin_constant_p(__n)
&& __constant_char_array_p(__s1, __n)
&& __constant_char_array_p(__s2, __n))
- return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
+ {
+ for (std::size_t __i = 0; __i < __n; ++__i)
+ if (lt(__s1[__i], __s2[__i]))
+ return -1;
+ else if (lt(__s2[__i], __s1[__i]))
+ return 1;
+ return 0;
+ }
#endif
return __builtin_memcmp(__s1, __s2, __n);
}
seems to fix this, but not sure if there isn't a better way.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
2021-02-22 9:43 ` [Bug libstdc++/99181] " jakub at gcc dot gnu.org
@ 2021-02-22 14:27 ` jakub at gcc dot gnu.org
2021-02-22 14:42 ` redi at gcc dot gnu.org
` (8 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-02-22 14:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |jakub at gcc dot gnu.org
Last reconfirmed| |2021-02-22
Ever confirmed|0 |1
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 50234
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50234&action=edit
gcc11-pr99181.patch
Full untested patch.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
2021-02-22 9:43 ` [Bug libstdc++/99181] " jakub at gcc dot gnu.org
2021-02-22 14:27 ` jakub at gcc dot gnu.org
@ 2021-02-22 14:42 ` redi at gcc dot gnu.org
2021-02-23 8:37 ` cvs-commit at gcc dot gnu.org
` (7 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: redi at gcc dot gnu.org @ 2021-02-22 14:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We're in namespace std here so size_t doesn't need to be qualified, but
otherwise the patch is fine.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (2 preceding siblings ...)
2021-02-22 14:42 ` redi at gcc dot gnu.org
@ 2021-02-23 8:37 ` cvs-commit at gcc dot gnu.org
2021-02-23 9:12 ` jakub at gcc dot gnu.org
` (6 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-02-23 8:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:efa64fcce12074dd542670feb02eaee53e810a30
commit r11-7340-gefa64fcce12074dd542670feb02eaee53e810a30
Author: Jakub Jelinek <jakub@redhat.com>
Date: Tue Feb 23 09:30:18 2021 +0100
libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]
Because of LWG 467, std::char_traits<char>::lt compares the values
cast to unsigned char rather than char, so even when char is signed
we get unsigned comparision. std::char_traits<char>::compare uses
__builtin_memcmp and that works the same, but during constexpr evaluation
we were calling __gnu_cxx::char_traits<char_type>::compare. As
char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
used __gnu_cxx::char_traits<char_type>::lt rather than
std::char_traits<char>::lt and thus compared chars as signed if char is
signed.
This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
into std::char_traits<char>::compare by hand, so that it calls the right
lt method.
2021-02-23 Jakub Jelinek <jakub@redhat.com>
PR libstdc++/99181
* include/bits/char_traits.h (char_traits<char>::compare): For
constexpr evaluation don't call
__gnu_cxx::char_traits<char_type>::compare but do the comparison
loop
directly.
* testsuite/21_strings/char_traits/requirements/char/99181.cc: New
test.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (3 preceding siblings ...)
2021-02-23 8:37 ` cvs-commit at gcc dot gnu.org
@ 2021-02-23 9:12 ` jakub at gcc dot gnu.org
2021-02-23 10:46 ` redi at gcc dot gnu.org
` (5 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-02-23 9:12 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed on the trunk, probably needs backporting to 10/9/8 branches.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (4 preceding siblings ...)
2021-02-23 9:12 ` jakub at gcc dot gnu.org
@ 2021-02-23 10:46 ` redi at gcc dot gnu.org
2021-03-19 23:29 ` cvs-commit at gcc dot gnu.org
` (4 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: redi at gcc dot gnu.org @ 2021-02-23 10:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #5)
> Fixed on the trunk, probably needs backporting to 10/9/8 branches.
Yes, all three branches. It's been wrong since r249137 on trunk, which I
backported to gcc-7 (for 7.3.0) as r252030
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (5 preceding siblings ...)
2021-02-23 10:46 ` redi at gcc dot gnu.org
@ 2021-03-19 23:29 ` cvs-commit at gcc dot gnu.org
2021-04-20 23:32 ` cvs-commit at gcc dot gnu.org
` (3 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-03-19 23:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:311c57f6d8f285d69e44bf94152c753900cb1a0a
commit r10-9474-g311c57f6d8f285d69e44bf94152c753900cb1a0a
Author: Jakub Jelinek <jakub@redhat.com>
Date: Tue Feb 23 09:30:18 2021 +0100
libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]
Because of LWG 467, std::char_traits<char>::lt compares the values
cast to unsigned char rather than char, so even when char is signed
we get unsigned comparision. std::char_traits<char>::compare uses
__builtin_memcmp and that works the same, but during constexpr evaluation
we were calling __gnu_cxx::char_traits<char_type>::compare. As
char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
used __gnu_cxx::char_traits<char_type>::lt rather than
std::char_traits<char>::lt and thus compared chars as signed if char is
signed.
This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
into std::char_traits<char>::compare by hand, so that it calls the right
lt method.
2021-02-23 Jakub Jelinek <jakub@redhat.com>
PR libstdc++/99181
* include/bits/char_traits.h (char_traits<char>::compare): For
constexpr evaluation don't call
__gnu_cxx::char_traits<char_type>::compare but do the comparison
loop
directly.
* testsuite/21_strings/char_traits/requirements/char/99181.cc: New
test.
(cherry picked from commit efa64fcce12074dd542670feb02eaee53e810a30)
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (6 preceding siblings ...)
2021-03-19 23:29 ` cvs-commit at gcc dot gnu.org
@ 2021-04-20 23:32 ` cvs-commit at gcc dot gnu.org
2021-04-22 16:51 ` cvs-commit at gcc dot gnu.org
` (2 subsequent siblings)
10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-04-20 23:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:03e18a40070c6fe89397a73d85a38a99371cf8a1
commit r9-9424-g03e18a40070c6fe89397a73d85a38a99371cf8a1
Author: Jakub Jelinek <jakub@redhat.com>
Date: Tue Feb 23 09:30:18 2021 +0100
libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]
Because of LWG 467, std::char_traits<char>::lt compares the values
cast to unsigned char rather than char, so even when char is signed
we get unsigned comparision. std::char_traits<char>::compare uses
__builtin_memcmp and that works the same, but during constexpr evaluation
we were calling __gnu_cxx::char_traits<char_type>::compare. As
char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
used __gnu_cxx::char_traits<char_type>::lt rather than
std::char_traits<char>::lt and thus compared chars as signed if char is
signed.
This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
into std::char_traits<char>::compare by hand, so that it calls the right
lt method.
2021-02-23 Jakub Jelinek <jakub@redhat.com>
PR libstdc++/99181
* include/bits/char_traits.h (char_traits<char>::compare): For
constexpr evaluation don't call
__gnu_cxx::char_traits<char_type>::compare but do the comparison
loop
directly.
* testsuite/21_strings/char_traits/requirements/char/99181.cc: New
test.
(cherry picked from commit 311c57f6d8f285d69e44bf94152c753900cb1a0a)
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (7 preceding siblings ...)
2021-04-20 23:32 ` cvs-commit at gcc dot gnu.org
@ 2021-04-22 16:51 ` cvs-commit at gcc dot gnu.org
2021-04-22 17:11 ` jakub at gcc dot gnu.org
2021-09-11 14:23 ` pinskia at gcc dot gnu.org
10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-04-22 16:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-8 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:21a7edf3a0cea656f2de834ea8320870496d5a2f
commit r8-10889-g21a7edf3a0cea656f2de834ea8320870496d5a2f
Author: Jakub Jelinek <jakub@redhat.com>
Date: Tue Feb 23 09:30:18 2021 +0100
libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]
Because of LWG 467, std::char_traits<char>::lt compares the values
cast to unsigned char rather than char, so even when char is signed
we get unsigned comparision. std::char_traits<char>::compare uses
__builtin_memcmp and that works the same, but during constexpr evaluation
we were calling __gnu_cxx::char_traits<char_type>::compare. As
char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
used __gnu_cxx::char_traits<char_type>::lt rather than
std::char_traits<char>::lt and thus compared chars as signed if char is
signed.
This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
into std::char_traits<char>::compare by hand, so that it calls the right
lt method.
2021-02-23 Jakub Jelinek <jakub@redhat.com>
PR libstdc++/99181
* include/bits/char_traits.h (char_traits<char>::compare): For
constexpr evaluation don't call
__gnu_cxx::char_traits<char_type>::compare but do the comparison
loop
directly.
* testsuite/21_strings/char_traits/requirements/char/99181.cc: New
test.
(cherry picked from commit 311c57f6d8f285d69e44bf94152c753900cb1a0a)
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (8 preceding siblings ...)
2021-04-22 16:51 ` cvs-commit at gcc dot gnu.org
@ 2021-04-22 17:11 ` jakub at gcc dot gnu.org
2021-09-11 14:23 ` pinskia at gcc dot gnu.org
10 siblings, 0 replies; 12+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-04-22 17:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution|--- |FIXED
--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.
^ permalink raw reply [flat|nested] 12+ messages in thread
* [Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts
2021-02-20 15:47 [Bug libstdc++/99181] New: char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts richardpku at gmail dot com
` (9 preceding siblings ...)
2021-04-22 17:11 ` jakub at gcc dot gnu.org
@ 2021-09-11 14:23 ` pinskia at gcc dot gnu.org
10 siblings, 0 replies; 12+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-09-11 14:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |8.5
^ permalink raw reply [flat|nested] 12+ messages in thread
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