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* [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr
@ 2012-07-18 19:52 luto at mit dot edu
2012-07-19 6:53 ` [Bug c++/54021] " jakub at gcc dot gnu.org
` (8 more replies)
0 siblings, 9 replies; 10+ messages in thread
From: luto at mit dot edu @ 2012-07-18 19:52 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
Bug #: 54021
Summary: [c++0x] __builtin_constant_p should be constexpr
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: luto@mit.edu
It's hard to tell how __builtin_constant_p works right now, due to PR54020.
// Preliminaries.
extern int nonconst_func(int);
constexpr int identity(int x) { return x; }
constexpr int zero() { return identity(0); }
constexpr int one() { return identity(1); }
// These are the same. Only the latter is accepted, though.
// I suspect that the acceptance of the latter is due to the bug above.
constexpr int rejected_const_4(int x) { return __builtin_constant_p(x) ? 4 :
nonconst_func(x); }
constexpr int accepted_const_4(int x) { return
identity(__builtin_constant_p(x)) ? 4 : nonconst_func(x); }
// This is rejected. I would like it to work.
constexpr int four = accepted_const_4(1);
The ability to use the construction __builtin_constant_p(x) ? const_func(x) :
nonconst_func(x) in constexpr context would be quite powerful.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
@ 2012-07-19 6:53 ` jakub at gcc dot gnu.org
2012-07-19 7:12 ` jakub at gcc dot gnu.org
` (7 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: jakub at gcc dot gnu.org @ 2012-07-19 6:53 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org,
| |jason at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-07-19 06:53:11 UTC ---
This is rejected only at -O0 and quite matches how __builtin_constant_p
normally behaves at -O0 - you really need a constant right in the
__builtin_constant_p argument, everything else results in 0. With -O1 and
above you get BUILTIN_CONSTANT_P not folded right away to 0 if not constant and
whether it in the end folds to 0 or 1 depends on optimization, whether a
constant is propagated to it or not.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
2012-07-19 6:53 ` [Bug c++/54021] " jakub at gcc dot gnu.org
@ 2012-07-19 7:12 ` jakub at gcc dot gnu.org
2012-07-19 20:02 ` jason at gcc dot gnu.org
` (6 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: jakub at gcc dot gnu.org @ 2012-07-19 7:12 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-07-19 07:12:18 UTC ---
Perhaps the C++ FE could when parsing a constexpr function during
finish_call_expr of __builtin_constant_p just temporarily force optimize = 1
if it is zero to prevent folding it to 0 (or alternatively, if it folds to 0,
build it non-folded), it would be an extension over how this builtin behaves
right now, on the other side as constexpr is "optimized" even at -O0 it would
match the intent of the builtin. The question is if/when it will be actually
folded to 0 afterwards if not in constexpr context.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
2012-07-19 6:53 ` [Bug c++/54021] " jakub at gcc dot gnu.org
2012-07-19 7:12 ` jakub at gcc dot gnu.org
@ 2012-07-19 20:02 ` jason at gcc dot gnu.org
2012-07-19 20:57 ` jason at gcc dot gnu.org
` (5 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: jason at gcc dot gnu.org @ 2012-07-19 20:02 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> 2012-07-19 20:02:13 UTC ---
Author: jason
Date: Thu Jul 19 20:02:08 2012
New Revision: 189677
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=189677
Log:
PR c++/54021
* call.c (build_cxx_call): Set optimize when folding
__builtin_constant_p in a constexpr function.
Added:
trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-builtin2.C
Modified:
trunk/gcc/cp/ChangeLog
trunk/gcc/cp/call.c
trunk/gcc/testsuite/ChangeLog
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
` (2 preceding siblings ...)
2012-07-19 20:02 ` jason at gcc dot gnu.org
@ 2012-07-19 20:57 ` jason at gcc dot gnu.org
2012-09-08 21:36 ` david at doublewise dot net
` (4 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: jason at gcc dot gnu.org @ 2012-07-19 20:57 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
Jason Merrill <jason at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |FIXED
Target Milestone|--- |4.8.0
--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2012-07-19 20:57:21 UTC ---
I'm not sure when exactly it gets folded later, but it does seem to happen
appropriately.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
` (3 preceding siblings ...)
2012-07-19 20:57 ` jason at gcc dot gnu.org
@ 2012-09-08 21:36 ` david at doublewise dot net
2012-09-08 22:29 ` luto at mit dot edu
` (3 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: david at doublewise dot net @ 2012-09-08 21:36 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
David Stone <david at doublewise dot net> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |david at doublewise dot net
--- Comment #5 from David Stone <david at doublewise dot net> 2012-09-08 21:36:10 UTC ---
I'm running into some issues with this bug, and it's much broader than the test
cases suggest. On gcc 4.7.0, this is what happens:
int main() {
int x = 0;
// This assigns false to a:
bool const a = __builtin_constant_p(x);
// This assigns true to b:
bool const b = __builtin_constant_p(__builtin_constant_p(x));
// This causes "error: the value of 'x' is not usable in a constant
expression"
constexpr bool c = __builtin_constant_p(x);
}
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
` (4 preceding siblings ...)
2012-09-08 21:36 ` david at doublewise dot net
@ 2012-09-08 22:29 ` luto at mit dot edu
2012-09-09 6:00 ` david at doublewise dot net
` (2 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: luto at mit dot edu @ 2012-09-08 22:29 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
--- Comment #6 from Andy Lutomirski <luto at mit dot edu> 2012-09-08 22:29:17 UTC ---
I think that's correct. x isn't a standards-mandated constant expression, so
__builtin_constant_p depends on optimization level and probably shouldn't be
allowed as a constexpr.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
` (5 preceding siblings ...)
2012-09-08 22:29 ` luto at mit dot edu
@ 2012-09-09 6:00 ` david at doublewise dot net
2012-09-09 6:05 ` luto at mit dot edu
2012-09-09 16:48 ` jason at gcc dot gnu.org
8 siblings, 0 replies; 10+ messages in thread
From: david at doublewise dot net @ 2012-09-09 6:00 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
--- Comment #7 from David Stone <david at doublewise dot net> 2012-09-09 06:00:37 UTC ---
That seems to me like saying that `constexpr bool d = sizeof(x);` should be
disallowed because it uses a non-constexpr. You're not using the value of x,
just a property about it. Whether a value is constexpr is guaranteed to be
known at compile time, and so should be usable as a constexpr.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
` (6 preceding siblings ...)
2012-09-09 6:00 ` david at doublewise dot net
@ 2012-09-09 6:05 ` luto at mit dot edu
2012-09-09 16:48 ` jason at gcc dot gnu.org
8 siblings, 0 replies; 10+ messages in thread
From: luto at mit dot edu @ 2012-09-09 6:05 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
--- Comment #8 from Andy Lutomirski <luto at mit dot edu> 2012-09-09 06:05:34 UTC ---
Did you mean "constexpr bool a" instead of "book const a"? If so, I agree.
But consider:
bool const a = something complicated
Is a a constant?
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr
2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
` (7 preceding siblings ...)
2012-09-09 6:05 ` luto at mit dot edu
@ 2012-09-09 16:48 ` jason at gcc dot gnu.org
8 siblings, 0 replies; 10+ messages in thread
From: jason at gcc dot gnu.org @ 2012-09-09 16:48 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021
--- Comment #9 from Jason Merrill <jason at gcc dot gnu.org> 2012-09-09 16:48:46 UTC ---
(In reply to comment #5)
> // This causes "error: the value of 'x' is not usable in a constant expression"
> constexpr bool c = __builtin_constant_p(x);
> }
This is also fixed by my patch.
^ permalink raw reply [flat|nested] 10+ messages in thread
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2012-07-18 19:52 [Bug c++/54021] New: [c++0x] __builtin_constant_p should be constexpr luto at mit dot edu
2012-07-19 6:53 ` [Bug c++/54021] " jakub at gcc dot gnu.org
2012-07-19 7:12 ` jakub at gcc dot gnu.org
2012-07-19 20:02 ` jason at gcc dot gnu.org
2012-07-19 20:57 ` jason at gcc dot gnu.org
2012-09-08 21:36 ` david at doublewise dot net
2012-09-08 22:29 ` luto at mit dot edu
2012-09-09 6:00 ` david at doublewise dot net
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