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* [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied)
@ 2012-11-21 11:31 M8R-ug85cr at mailinator dot com
2012-11-21 12:13 ` [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals) redi at gcc dot gnu.org
` (9 more replies)
0 siblings, 10 replies; 11+ messages in thread
From: M8R-ug85cr at mailinator dot com @ 2012-11-21 11:31 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Bug #: 55425
Summary: constexpr does not work in many situations (both
built-in and user supplied)
Classification: Unclassified
Product: gcc
Version: 4.7.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: M8R-ug85cr@mailinator.com
constexpr expressions/functions do not work in many cases because of compiler
wrongly interpreting the code. Seen in gcc-MinGW 4.7.0 and 4.7.1.
Situation 1: __func__
--------------------------
A return statement is not a return statement if the returned value is __func__
(also true for non-standard identifiers like __PRETTY_FUNCTION__).
// good
//static const char func[] = "function-name";
//constexpr const char* x() { return func; }
// bad
constexpr const char* x() { return __func__;}
int main() { __builtin_puts(x()); return 0; }
Compiler output:
----------------
error: body of constexpr function 'constexpr const char* x()' not a
return-statement
Cross-reference to corresponding MinGW ticket:
http://sourceforge.net/tracker/index.php?func=detail&aid=3471328&group_id=2435&atid=102435
Situation 2: user literals
--------------------------
The (obviously constant) string that the compiler builds from the literal is
not constant according to the compiler:
#include <stdio.h>
constexpr int valid_bin_number(const char* c) { return *c ? ((*c == '1' || *c
== '0') ? valid_bin_number(c+1) : false ) : true; }
unsigned int operator"" _bin(const char* str)
{
static_assert(valid_bin_number(str), "not a binary number");
unsigned int ret = 0;
for(unsigned int i = 0; str[i] != '\0'; ++i)
{
char digit = str[i];
ret = ret * 2 + (digit - '0');
}
return ret;
}
int main()
{
unsigned int a = 10000_bin;
(void) a;
return 0;
}
Compiler output:
----------------
In function 'unsigned int operator"" _bin(const char*)':
error: non-constant condition for static assertion
error: 'str' is not a constant expression
Cross-reference to corresponding MinGW ticket:
http://sourceforge.net/tracker/?func=detail&atid=102435&aid=3582841&group_id=2435
Situation 3: __m128i type
--------------------------
Assigning a literal value to a constexpr __m128 fails because the literal is
not a literal.
#include <emmintrin.h>
constexpr unsigned int a[] = { 5, 3}; // works (of course)
constexpr float b[] = { 1.1, 3.7 }; // works, and no warning?!
__m128i c[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } };
const __m128i d[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } };
static const __m128i e[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } };
namespace { const __m128i f[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } }; }
constexpr __m128i g[] = { { 0x55633cd9, 0x88ca7a96 }, { 0x0ed8c2a8,
0x7795b179 } }; // <---- fails
int main() { return 0; }
Compiler output:
----------------
error: the type 'const __m128i [] {aka const __vector(2) long long int []}' of
constexpr variable 'g' is not literal
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
@ 2012-11-21 12:13 ` redi at gcc dot gnu.org
2012-11-21 12:19 ` redi at gcc dot gnu.org
` (8 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2012-11-21 12:13 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-11-21 12:12:19 UTC ---
(In reply to comment #0)
>
> A return statement is not a return statement if the returned value is __func__
> (also true for non-standard identifiers like __PRETTY_FUNCTION__).
>
> // good
> //static const char func[] = "function-name";
> //constexpr const char* x() { return func; }
>
> // bad
> constexpr const char* x() { return __func__;}
>
> int main() { __builtin_puts(x()); return 0; }
The standard says __func__ is a function-local variable, defined as if by
constexpr const char* x() {
static const char __func__[] = "function-name ";
return __func__;
}
Clearly this is not a valid constexpr function.
Changing this would be an extension.
> Situation 2: user literals
> --------------------------
>
> The (obviously constant) string that the compiler builds from the literal is
> not constant according to the compiler:
>
> #include <stdio.h>
>
> constexpr int valid_bin_number(const char* c) { return *c ? ((*c == '1' || *c
> == '0') ? valid_bin_number(c+1) : false ) : true; }
>
> unsigned int operator"" _bin(const char* str)
> {
> static_assert(valid_bin_number(str), "not a binary number");
'str' is not a constant expression, so 'valid_bin_number(str)' is not a
constant expression either. This is not a bug.
> Situation 3: __m128i type
> --------------------------
>
> Assigning a literal value to a constexpr __m128 fails because the literal is
> not a literal.
No, the error says __m128 is not a literal type, which I assume is true.
Changing that would be an enhancement request.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
2012-11-21 12:13 ` [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals) redi at gcc dot gnu.org
@ 2012-11-21 12:19 ` redi at gcc dot gnu.org
2012-11-21 12:41 ` jakub at gcc dot gnu.org
` (7 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: redi at gcc dot gnu.org @ 2012-11-21 12:19 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-11-21 12:19:06 UTC ---
(In reply to comment #1)
> > Situation 3: __m128i type
> > --------------------------
> >
> > Assigning a literal value to a constexpr __m128 fails because the literal is
> > not a literal.
>
> No, the error says __m128 is not a literal type, which I assume is true.
> Changing that would be an enhancement request.
With G++ 4.8 __m128i is a literal type, so this example works.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
2012-11-21 12:13 ` [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals) redi at gcc dot gnu.org
2012-11-21 12:19 ` redi at gcc dot gnu.org
@ 2012-11-21 12:41 ` jakub at gcc dot gnu.org
2012-11-21 13:51 ` jakub at gcc dot gnu.org
` (6 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: jakub at gcc dot gnu.org @ 2012-11-21 12:41 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-21 12:40:29 UTC ---
1) is not valid constexpr IMHO, as the standard says that __func__ works as if
static const char __func__[] = "function-name";
has been provided, but when it is provided, it is not valid constexpr.
2) I don't see why you think you could use something like this at all.
You are defining a non-constexpr operator "", so str there definitely isn't
pointer to a string literal, it is a standalone function that might be
inlined if the inliner choses so. So it is obvious the static_assert
which must be evaluated before the optimizations, without depending on
whether the function is inlined or not, must fail.
You want something like:
constexpr int
cstrlen (const char *c)
{
return *c ? cstrlen (c + 1) + 1 : 0;
}
constexpr unsigned int
bin_number (const char *c)
{
return *c ? (*c == '1' ? (1U << cstrlen (c + 1)) : *c == '0' ? 0 : throw 0) +
bin_number (c + 1) : 0;
}
constexpr unsigned int
operator"" _bin (const char *c)
{
return bin_number (c);
}
int
main ()
{
constexpr unsigned int a = 10000_bin;
(void) a;
return 0;
}
instead, then you can verify it the same at compile time.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (2 preceding siblings ...)
2012-11-21 12:41 ` jakub at gcc dot gnu.org
@ 2012-11-21 13:51 ` jakub at gcc dot gnu.org
2012-11-21 20:52 ` glisse at gcc dot gnu.org
` (5 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: jakub at gcc dot gnu.org @ 2012-11-21 13:51 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-21 13:50:43 UTC ---
Or more efficiently
constexpr unsigned int
bin_number (const char *c, unsigned int x)
{
return *c ? bin_number (c + 1, (x << 1) + (*c == '0' || *c == '1' ? *c - '0'
: throw 0)) : x;
}
constexpr unsigned int
operator"" _bin (const char *c)
{
return bin_number (c, 0);
}
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (3 preceding siblings ...)
2012-11-21 13:51 ` jakub at gcc dot gnu.org
@ 2012-11-21 20:52 ` glisse at gcc dot gnu.org
2013-07-01 6:14 ` richard-gccbugzilla at metafoo dot co.uk
` (4 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: glisse at gcc dot gnu.org @ 2012-11-21 20:52 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Marc Glisse <glisse at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |glisse at gcc dot gnu.org
--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> 2012-11-21 20:52:13 UTC ---
(In reply to comment #2)
> (In reply to comment #1)
> > No, the error says __m128 is not a literal type, which I assume is true.
> > Changing that would be an enhancement request.
>
> With G++ 4.8 __m128i is a literal type, so this example works.
PR 53094 shows that many other vector things don't work with constexpr though.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (4 preceding siblings ...)
2012-11-21 20:52 ` glisse at gcc dot gnu.org
@ 2013-07-01 6:14 ` richard-gccbugzilla at metafoo dot co.uk
2014-11-19 10:27 ` paolo.carlini at oracle dot com
` (3 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: richard-gccbugzilla at metafoo dot co.uk @ 2013-07-01 6:14 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Richard Smith <richard-gccbugzilla at metafoo dot co.uk> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |richard-gccbugzilla@metafoo
| |.co.uk
--- Comment #6 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
(In reply to Jonathan Wakely from comment #1)
> The standard says __func__ is a function-local variable, defined as if by
>
> constexpr const char* x() {
> static const char __func__[] = "function-name ";
> return __func__;
> }
>
> Clearly this is not a valid constexpr function.
> Changing this would be an extension.
I disagree with this. The standard doesn't say __func__ is only predefined if
it is used, so if this argument held then all constexpr functions would be
invalid.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (5 preceding siblings ...)
2013-07-01 6:14 ` richard-gccbugzilla at metafoo dot co.uk
@ 2014-11-19 10:27 ` paolo.carlini at oracle dot com
2014-11-19 10:37 ` paolo.carlini at oracle dot com
` (2 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: paolo.carlini at oracle dot com @ 2014-11-19 10:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
--- Comment #7 from Paolo Carlini <paolo.carlini at oracle dot com> ---
I think we can close this. The __func__ snippet now works in mainline with
-std=c++14, I'm adding it to the testsuite.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (6 preceding siblings ...)
2014-11-19 10:27 ` paolo.carlini at oracle dot com
@ 2014-11-19 10:37 ` paolo.carlini at oracle dot com
2014-11-19 17:41 ` paolo at gcc dot gnu.org
2014-11-19 17:43 ` paolo.carlini at oracle dot com
9 siblings, 0 replies; 11+ messages in thread
From: paolo.carlini at oracle dot com @ 2014-11-19 10:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
--- Comment #8 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Uhm, in fact probably it should be accepted with -std=c++11 too, let's wait a
second.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (7 preceding siblings ...)
2014-11-19 10:37 ` paolo.carlini at oracle dot com
@ 2014-11-19 17:41 ` paolo at gcc dot gnu.org
2014-11-19 17:43 ` paolo.carlini at oracle dot com
9 siblings, 0 replies; 11+ messages in thread
From: paolo at gcc dot gnu.org @ 2014-11-19 17:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
--- Comment #9 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Wed Nov 19 17:40:42 2014
New Revision: 217788
URL: https://gcc.gnu.org/viewcvs?rev=217788&root=gcc&view=rev
Log:
/cp
2014-11-19 Paolo Carlini <paolo.carlini@oracle.com>
PR c++/55425
* constexpr.c (constexpr_fn_retval): Accept __func__, __FUNCTION__,
and __PRETTY_FUNCTION__.
/testsuite
2014-11-19 Paolo Carlini <paolo.carlini@oracle.com>
PR c++/55425
* g++.dg/cpp0x/constexpr-__func__.C
Added:
trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-__func__.C
Modified:
trunk/gcc/cp/ChangeLog
trunk/gcc/cp/constexpr.c
trunk/gcc/testsuite/ChangeLog
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals)
2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
` (8 preceding siblings ...)
2014-11-19 17:41 ` paolo at gcc dot gnu.org
@ 2014-11-19 17:43 ` paolo.carlini at oracle dot com
9 siblings, 0 replies; 11+ messages in thread
From: paolo.carlini at oracle dot com @ 2014-11-19 17:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55425
Paolo Carlini <paolo.carlini at oracle dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |FIXED
Target Milestone|--- |5.0
--- Comment #10 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Now we can close this.
^ permalink raw reply [flat|nested] 11+ messages in thread
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2012-11-21 11:31 [Bug c++/55425] New: constexpr does not work in many situations (both built-in and user supplied) M8R-ug85cr at mailinator dot com
2012-11-21 12:13 ` [Bug c++/55425] constexpr does not work in many situations (both built-in and user supplied literals) redi at gcc dot gnu.org
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