[Bug libstdc++/68133] New: constexpr basic_string_view(const _CharT* __str)

[Bug libstdc++/68133] New: constexpr basic_string_view(const _CharT* __str)

[d.v.a at ngs dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68133

            Bug ID: 68133
           Summary: constexpr basic_string_view(const _CharT* __str)
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d.v.a at ngs dot ru
  Target Milestone: ---

Compilation of this code:

#include<experimental/string_view>

int main()
{
    int arr[std::experimental::string_view("s").length()];
}

produces error:

error: ISO C++ forbids variable length array 'arr' [-Wvla]


Constructor's implementation:

      constexpr basic_string_view(const _CharT* __str)
      : _M_len{__str == nullptr ? 0 : traits_type::length(__str)},
        _M_str{__str}
      { }

But traits_type::length() isn't constexpr

[Bug libstdc++/68133] constexpr basic_string_view(const _CharT* __str)

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68133

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler@googlemail.
                   |                            |com

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
The implementation matches the required semantics (see
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4529.html). Please
note that std::char_traits::length is not constexpr because it is specified
that way and this is a known library issue, see

http://cplusplus.github.io/LWG/lwg-active.html#2232

The reason why the discussed string view constructor is nonetheless declared as
constexpr is due to the fact that a user *could* instantiate it with a
user-defined char trait class (not necessarily a specialization of
std::char_traits) that *has* a constexpr length function.

Based on this argumentation this issue looks invalid to me.
>From gcc-bugs-return-500852-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Oct 28 21:12:51 2015
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Subject: [Bug target/68124] [6 Regression] Many i386 test failures
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68124

--- Comment #8 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Richard Henderson from comment #5)
> Works for me at r229501.
> 
> spawn -ignore SIGHUP /home/rth/gcc/bld/gcc/xgcc -B/home/rth/gcc/bld/gcc/
> /home/rth/gcc/git-master/gcc/testsuite/gcc.dg/vect/pr64421.c -mtune=corei7
> -fno-diagnostics-show-caret -fdiagnostics-color=never -msse2
> -ftree-vectorize -fno-vect-cost-model -fno-common -O2
> -fdump-tree-vect-details -fopenmp-simd -mavx -lm -o ./pr64421.exe
> PASS: gcc.dg/vect/pr64421.c (test for excess errors)

This particular test only fails with -m64:

[hjl@gnu-6 gcc]$ ./xgcc -B./   -S 
/export/gnu/import/git/sources/gcc/gcc/testsuite/gcc.dg/vect/pr64421.c 
-mtune=corei7 -fno-diagnostics-show-caret -fdiagnostics-color=never -msse2
-ftree-vectorize -fno-vect-cost-model -fno-common -O2 -fdump-tree-vect-details
-fopenmp-simd -mavx -lm -m32
[hjl@gnu-6 gcc]$ ./xgcc -B./   -S 
/export/gnu/import/git/sources/gcc/gcc/testsuite/gcc.dg/vect/pr64421.c 
-mtune=corei7 -fno-diagnostics-show-caret -fdiagnostics-color=never -msse2
-ftree-vectorize -fno-vect-cost-model -fno-common -O2 -fdump-tree-vect-details
-fopenmp-simd -mavx -lm -m64
/export/gnu/import/git/sources/gcc/gcc/testsuite/gcc.dg/vect/pr64421.c: In
function ‘*bar.simdclone.2’:
/export/gnu/import/git/sources/gcc/gcc/testsuite/gcc.dg/vect/pr64421.c:16:1:
internal compiler error: in gen_reg_rtx, at emit-rtl.c:1031
0x9a9877 gen_reg_rtx(machine_mode)
        /export/gnu/import/git/sources/gcc/gcc/emit-rtl.c:1031
0x9c9946 copy_to_reg(rtx_def*)
        /export/gnu/import/git/sources/gcc/gcc/explow.c:570
0xd3b5b2 gen_lowpart_general(machine_mode, rtx_def*)
        /export/gnu/import/git/sources/gcc/gcc/rtlhooks.c:62
0x1412d91 gen_split_392(rtx_insn*, rtx_def**)
        /export/gnu/import/git/sources/gcc/gcc/config/i386/sse.md:17100
0x15c0531 split_23
        /export/gnu/import/git/sources/gcc/gcc/config/i386/sse.md:17098
0x15c12ea split_24
        /export/gnu/import/git/sources/gcc/gcc/config/i386/i386.md:4803
0x15c4e52 split_insns(rtx_def*, rtx_insn*)
        /export/gnu/import/git/sources/gcc/gcc/config/i386/i386.md:15867
0x9afb29 try_split(rtx_def*, rtx_insn*, int)
        /export/gnu/import/git/sources/gcc/gcc/emit-rtl.c:3664
0xcd15d7 split_insn
        /export/gnu/import/git/sources/gcc/gcc/recog.c:2874
0xcd18af split_all_insns()
        /export/gnu/import/git/sources/gcc/gcc/recog.c:2964
0xcd36d3 rest_of_handle_split_after_reload
        /export/gnu/import/git/sources/gcc/gcc/recog.c:3900
0xcd3724 execute
        /export/gnu/import/git/sources/gcc/gcc/recog.c:3929
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.
[hjl@gnu-6 gcc]$
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idh124

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

[Bug libstdc++/68133] constexpr basic_string_view(const _CharT* __str)

[d.v.a at ngs dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68133

--- Comment #2 from __vic <d.v.a at ngs dot ru> ---
Too much black magic in the library already...
I think this is defect in specification/design of the class. But it's easier
for you guys to reach the committee than for me

[Bug libstdc++/68133] constexpr basic_string_view(const _CharT* __str)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68133

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If you want that function to be constexpr you need to provide a custom traits
class.