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* [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression
@ 2012-02-04 1:16 jyasskin at gcc dot gnu.org
2012-02-04 2:13 ` [Bug libstdc++/52119] " richard-gccbugzilla at metafoo dot co.uk
` (11 more replies)
0 siblings, 12 replies; 13+ messages in thread
From: jyasskin at gcc dot gnu.org @ 2012-02-04 1:16 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Bug #: 52119
Summary: numeric_limits::min() is not a constant expression
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: jyasskin@gcc.gnu.org
numeric_limits<T>::min() is defined as (__glibcxx_signed (T) ? (T)1 <<
__glibcxx_digits (T) : (T)0). Unfortunately, shifting into the sign bit is
undefined behavior (C++11[expr.shift]p2), and undefined behavior makes an
expression non-constant.
clang as of (their) r149727 diagnoses this as:
$ echo '#include <limits>' | clang++ -nostdinc++
-Igcc-4.7-svn/include/c++/4.7.0/{,x86_64-unknown-linux-gnu} -std=c++11
-Wsystem-headers -fsyntax-only -x c++ -
gcc-4.7-svn/include/c++/4.7.0/limits:654:7: error: constexpr function never
produces a constant expression
min() _GLIBCXX_USE_NOEXCEPT { return __glibcxx_min (wchar_t); }
^
gcc-4.7-svn/include/c++/4.7.0/limits:654:44: note: value 2147483648 is
outside the range of representable values of type 'int'
min() _GLIBCXX_USE_NOEXCEPT { return __glibcxx_min (wchar_t); }
^
gcc-4.7-svn/include/c++/4.7.0/limits:131:32: note: expanded from macro
'__glibcxx_min'
(__glibcxx_signed (T) ? (T)1 << __glibcxx_digits (T) : (T)0)
^
gcc-4.7-svn/include/c++/4.7.0/limits:784:31: warning: shift count >= width
of type [-Wshift-count-overflow]
min() noexcept { return __glibcxx_min (char32_t); }
^~~~~~~~~~~~~~~~~~~~~~~~
A better definition might be -__glibcxx_max(T)-1.
This bug is also in 4.6.2. 4.5.0 didn't declare min() as constexpr.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
@ 2012-02-04 2:13 ` richard-gccbugzilla at metafoo dot co.uk
2012-02-04 9:59 ` paolo.carlini at oracle dot com
` (10 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: richard-gccbugzilla at metafoo dot co.uk @ 2012-02-04 2:13 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Richard Smith <richard-gccbugzilla at metafoo dot co.uk> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |richard-gccbugzilla at
| |metafoo dot co.uk
--- Comment #1 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> 2012-02-04 02:12:41 UTC ---
In more detail: Under the C++11 FDIS, this is not a (core) constant expression
due to the "— a result that is not mathematically defined or not in the range of
representable values for its type;" bullet in [expr.const]p2. Under DR1313,
that rule is generalized to all undefined behavior. And a constexpr function
which can't produce a constant expression is ill-formed by [dcl.constexpr]p5.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
2012-02-04 2:13 ` [Bug libstdc++/52119] " richard-gccbugzilla at metafoo dot co.uk
@ 2012-02-04 9:59 ` paolo.carlini at oracle dot com
2012-02-04 10:04 ` paolo.carlini at oracle dot com
` (9 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: paolo.carlini at oracle dot com @ 2012-02-04 9:59 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Paolo Carlini <paolo.carlini at oracle dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2012-02-04
Ever Confirmed|0 |1
--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-02-04 09:58:38 UTC ---
The problematic macro is extremely old. Using
#define __glibcxx_min(T) \
(__glibcxx_signed (T) ? -__glibcxx_max(T) - 1 : (T)0)
seems indeed ok to me. Did you actually test it with clang?
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
2012-02-04 2:13 ` [Bug libstdc++/52119] " richard-gccbugzilla at metafoo dot co.uk
2012-02-04 9:59 ` paolo.carlini at oracle dot com
@ 2012-02-04 10:04 ` paolo.carlini at oracle dot com
2012-02-05 2:05 ` jyasskin at gcc dot gnu.org
` (8 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: paolo.carlini at oracle dot com @ 2012-02-04 10:04 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Paolo Carlini <paolo.carlini at oracle dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jason at gcc dot gnu.org
--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-02-04 10:04:09 UTC ---
And of course the interesting issue here is C++ front-end, which doesn't
produce any diagnostics (thus nobody would ever notice the library issue with
GCC only). Let's add Jason in CC for confirmation.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (2 preceding siblings ...)
2012-02-04 10:04 ` paolo.carlini at oracle dot com
@ 2012-02-05 2:05 ` jyasskin at gcc dot gnu.org
2012-02-05 12:59 ` paolo at gcc dot gnu.org
` (7 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: jyasskin at gcc dot gnu.org @ 2012-02-05 2:05 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
--- Comment #4 from Jeffrey Yasskin <jyasskin at gcc dot gnu.org> 2012-02-05 02:04:00 UTC ---
I hadn't tested
#define __glibcxx_min(T) \
(__glibcxx_signed (T) ? -__glibcxx_max(T) - 1 : (T)0)
but now I have, and it works with clang.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (3 preceding siblings ...)
2012-02-05 2:05 ` jyasskin at gcc dot gnu.org
@ 2012-02-05 12:59 ` paolo at gcc dot gnu.org
2012-02-05 13:01 ` [Bug c++/52119] " paolo.carlini at oracle dot com
` (6 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: paolo at gcc dot gnu.org @ 2012-02-05 12:59 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
--- Comment #5 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2012-02-05 12:58:56 UTC ---
Author: paolo
Date: Sun Feb 5 12:58:51 2012
New Revision: 183905
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=183905
Log:
2012-02-05 Jeffrey Yasskin <jyasskin@gcc.gnu.org>
Paolo Carlini <paolo.carlini@oracle.com>
PR libstdc++/52119
* include/std/limits (__glibcxx_min): Fix to avoid undefined behavior.
Modified:
trunk/libstdc++-v3/ChangeLog
trunk/libstdc++-v3/include/std/limits
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (4 preceding siblings ...)
2012-02-05 12:59 ` paolo at gcc dot gnu.org
@ 2012-02-05 13:01 ` paolo.carlini at oracle dot com
2012-02-13 20:22 ` jason at gcc dot gnu.org
` (5 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: paolo.carlini at oracle dot com @ 2012-02-05 13:01 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Paolo Carlini <paolo.carlini at oracle dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |accepts-invalid
Component|libstdc++ |c++
--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-02-05 13:01:17 UTC ---
This is now a C++ front-end issue.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (5 preceding siblings ...)
2012-02-05 13:01 ` [Bug c++/52119] " paolo.carlini at oracle dot com
@ 2012-02-13 20:22 ` jason at gcc dot gnu.org
2012-02-13 22:18 ` richard-gccbugzilla at metafoo dot co.uk
` (4 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: jason at gcc dot gnu.org @ 2012-02-13 20:22 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Jason Merrill <jason at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution| |INVALID
--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> 2012-02-13 20:22:37 UTC ---
At the C++ meeting last week we changed that shift from undefined to
implementation-defined, so there is no front end bug.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] numeric_limits::min() is not a constant expression
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (6 preceding siblings ...)
2012-02-13 20:22 ` jason at gcc dot gnu.org
@ 2012-02-13 22:18 ` richard-gccbugzilla at metafoo dot co.uk
2012-02-14 0:39 ` [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed jason at gcc dot gnu.org
` (3 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: richard-gccbugzilla at metafoo dot co.uk @ 2012-02-13 22:18 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
--- Comment #8 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> 2012-02-13 22:17:27 UTC ---
Signed left shift which overflows the corresponding unsigned type is still
undefined, so g++ should reject this:
constexpr int n = 2 << 31;
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (7 preceding siblings ...)
2012-02-13 22:18 ` richard-gccbugzilla at metafoo dot co.uk
@ 2012-02-14 0:39 ` jason at gcc dot gnu.org
2013-06-05 17:26 ` jason at gcc dot gnu.org
` (2 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: jason at gcc dot gnu.org @ 2012-02-14 0:39 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
Jason Merrill <jason at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |NEW
Resolution|INVALID |
Summary|numeric_limits::min() is |[C++11] overflow in signed
|not a constant expression |left shift isn't diagnosed
--- Comment #9 from Jason Merrill <jason at gcc dot gnu.org> 2012-02-14 00:38:14 UTC ---
Good point.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (8 preceding siblings ...)
2012-02-14 0:39 ` [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed jason at gcc dot gnu.org
@ 2013-06-05 17:26 ` jason at gcc dot gnu.org
2013-06-05 17:41 ` jason at gcc dot gnu.org
2013-06-05 22:37 ` jyasskin at gcc dot gnu.org
11 siblings, 0 replies; 13+ messages in thread
From: jason at gcc dot gnu.org @ 2013-06-05 17:26 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
--- Comment #11 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to ajf from comment #10)
> MY apologies if I am missing something obvious, but what exactly makes
> shifting a 1 into the sign bit undefined behavior?
You're right, it isn't:
5.8/2: ... if E1 has a signed type and non-negative value, and E1 × 2^E2 is
representable in the corresponding unsigned type of the result type, then that
value, converted to the result type, is the resulting value ....
>From gcc-bugs-return-423859-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Jun 05 17:30:55 2013
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From: "jyasskin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed
Date: Wed, 05 Jun 2013 17:30:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idR119
--- Comment #12 from Jeffrey Yasskin <jyasskin at gcc dot gnu.org> ---
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3675.html#1457 changed
this between C++11 and C++14.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (9 preceding siblings ...)
2013-06-05 17:26 ` jason at gcc dot gnu.org
@ 2013-06-05 17:41 ` jason at gcc dot gnu.org
2013-06-05 22:37 ` jyasskin at gcc dot gnu.org
11 siblings, 0 replies; 13+ messages in thread
From: jason at gcc dot gnu.org @ 2013-06-05 17:41 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
--- Comment #13 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Jeffrey Yasskin from comment #12)
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3675.html#1457
> changed this between C++11 and C++14.
True, but the defect report applies to C++11.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed
2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
` (10 preceding siblings ...)
2013-06-05 17:41 ` jason at gcc dot gnu.org
@ 2013-06-05 22:37 ` jyasskin at gcc dot gnu.org
11 siblings, 0 replies; 13+ messages in thread
From: jyasskin at gcc dot gnu.org @ 2013-06-05 22:37 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119
--- Comment #14 from Jeffrey Yasskin <jyasskin at gcc dot gnu.org> ---
Whoops, I missed your comment 7 where you already knew the history. Sorry.
^ permalink raw reply [flat|nested] 13+ messages in thread
end of thread, other threads:[~2013-06-05 22:37 UTC | newest]
Thread overview: 13+ messages (download: mbox.gz / follow: Atom feed)
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2012-02-04 1:16 [Bug libstdc++/52119] New: numeric_limits::min() is not a constant expression jyasskin at gcc dot gnu.org
2012-02-04 2:13 ` [Bug libstdc++/52119] " richard-gccbugzilla at metafoo dot co.uk
2012-02-04 9:59 ` paolo.carlini at oracle dot com
2012-02-04 10:04 ` paolo.carlini at oracle dot com
2012-02-05 2:05 ` jyasskin at gcc dot gnu.org
2012-02-05 12:59 ` paolo at gcc dot gnu.org
2012-02-05 13:01 ` [Bug c++/52119] " paolo.carlini at oracle dot com
2012-02-13 20:22 ` jason at gcc dot gnu.org
2012-02-13 22:18 ` richard-gccbugzilla at metafoo dot co.uk
2012-02-14 0:39 ` [Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed jason at gcc dot gnu.org
2013-06-05 17:26 ` jason at gcc dot gnu.org
2013-06-05 17:41 ` jason at gcc dot gnu.org
2013-06-05 22:37 ` jyasskin at gcc dot gnu.org
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