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* [Bug c++/67333] New: [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types
@ 2015-08-24 4:39 myriachan at gmail dot com
2015-08-24 5:50 ` [Bug c++/67333] " miyuki at gcc dot gnu.org
2015-08-29 22:23 ` miyuki at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: myriachan at gmail dot com @ 2015-08-24 4:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67333
Bug ID: 67333
Summary: [C++11][constexpr] constexpr functions incorrectly
prohibit taking references to volatile types
Product: gcc
Version: 5.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: myriachan at gmail dot com
Target Milestone: ---
GCC 4.7.3 (at least) through GCC 6.0 reject the following due to "meow has
side-effects":
#include <cstddef>
#include <type_traits>
template <typename T, std::size_t S>
constexpr std::size_t lengthof(const volatile T (&)[S])
{
return S;
}
int main()
{
volatile int meow[4];
static_cast<void>(meow); // shut up warning
return static_cast<int>(std::integral_constant<std::size_t,
lengthof(meow)>::value);
}
I believe that this is legal per [expr.const] in the Standard, because the
volatile parameter is never used in an lvalue-to-rvalue conversion, which is
what [expr.const] disallows in constant-expressions for volatile types. Clang
versions that understand this code accept it; Visual C++ 2015 does as well.
Someone who replied to my question on the "std-discussion" mailing list
suggested that this is also technically legal as well:
#include <type_traits>
constexpr int Test(int x)
{
volatile int v = x;
return x;
}
int main()
{
return std::integral_constant<int, Test(2)>::value;
}
GCC also rejects this, but Clang accepts this as well. Any attempt to read v
will fail, though, so Clang is enforcing the rule. I'm not on my Windows
machine as I write this, so I can't check MSVC.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/67333] [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types
2015-08-24 4:39 [Bug c++/67333] New: [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types myriachan at gmail dot com
@ 2015-08-24 5:50 ` miyuki at gcc dot gnu.org
2015-08-29 22:23 ` miyuki at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: miyuki at gcc dot gnu.org @ 2015-08-24 5:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67333
Mikhail Maltsev <miyuki at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |miyuki at gcc dot gnu.org
--- Comment #1 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
EDG accepts the first testcase. The second one is rejected, but presumably due
to incomplete C++14 support.
In potential_constant_expression_1 we have a check:
if (TREE_THIS_VOLATILE (t))
{
if (flags & tf_error)
error ("expression %qE has side-effects", t);
The comment in tree.h says:
/* Nonzero means this expression is volatile in the C sense:
its address should be of type `volatile WHATEVER *'.
In other words, the declared item is volatile qualified.
...
If this bit is set in an expression, so is TREE_SIDE_EFFECTS. */
In our case t is:
(gdb) p t
$10 = <var_decl 0x7ffff66a7cf0 meow>
(gdb) pt
warning: Expression is not an assignment (and might have no effect)
<var_decl 0x7ffff66a7cf0 meow
type <array_type 0x7ffff6800c78
type <integer_type 0x7ffff6800a80 int volatile type_6 SI
size <integer_cst 0x7ffff66bd0a8 constant 32>
unit size <integer_cst 0x7ffff66bd0c0 constant 4>
align 32 symtab 0 alias set -1 canonical type 0x7ffff6800a80
precision 32 min <integer_cst 0x7ffff66bd060 -2147483648> max <integer_cst
0x7ffff66bd078 2147483647>
pointer_to_this <pointer_type 0x7ffff6800d20>>
TI
size <integer_cst 0x7ffff669bea0 constant 128>
unit size <integer_cst 0x7ffff669beb8 constant 16>
align 32 symtab 0 alias set -1 canonical type 0x7ffff6800c78
domain <integer_type 0x7ffff6800b28 type <integer_type 0x7ffff669f1f8
sizetype>
type_6 DI
size <integer_cst 0x7ffff669be58 constant 64>
unit size <integer_cst 0x7ffff669be70 constant 8>
align 64 symtab 0 alias set -1 canonical type 0x7ffff6800b28
precision 64 min <integer_cst 0x7ffff669be88 0> max <integer_cst 0x7ffff680adc8
3>>
pointer_to_this <pointer_type 0x7ffff68133f0>>
side-effects addressable volatile used tree_1 decl_5 TI file
/home/miyuki/gcc/test/meow/meow.cc line 15 col 18 size <integer_cst
0x7ffff669bea0 128> unit size <integer_cst 0x7ffff669beb8 16>
align 32 context <function_decl 0x7ffff67ffe00 main>>
I.e. the condition is probably wrong and we should also check that t is an
expression, not a declaration.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/67333] [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types
2015-08-24 4:39 [Bug c++/67333] New: [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types myriachan at gmail dot com
2015-08-24 5:50 ` [Bug c++/67333] " miyuki at gcc dot gnu.org
@ 2015-08-29 22:23 ` miyuki at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: miyuki at gcc dot gnu.org @ 2015-08-29 22:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67333
Mikhail Maltsev <miyuki at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2015-08-29
Blocks| |55004
Ever confirmed|0 |1
Known to fail| |6.0
--- Comment #2 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
For the record: https://gcc.gnu.org/ml/gcc-patches/2015-08/msg01735.html
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55004
[Bug 55004] [meta-bug] constexpr issues
^ permalink raw reply [flat|nested] 3+ messages in thread
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2015-08-24 4:39 [Bug c++/67333] New: [C++11][constexpr] constexpr functions incorrectly prohibit taking references to volatile types myriachan at gmail dot com
2015-08-24 5:50 ` [Bug c++/67333] " miyuki at gcc dot gnu.org
2015-08-29 22:23 ` miyuki at gcc dot gnu.org
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