[Bug c++/110403] New: constant expression inside vector_size __attribute__ does not compile

[Bug c++/110403] New: constant expression inside vector_size __attribute__ does not compile

[janezz55 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110403

            Bug ID: 110403
           Summary: constant expression inside vector_size __attribute__
                    does not compile
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janezz55 at gmail dot com
  Target Milestone: ---

trying to compile:

template <typename T, std::size_t N>
using array_t __attribute__ ((vector_size(std::bit_ceil(N * sizeof(T))))) = T;

produces the following compile error:

error: 'vector_size' attribute argument value 'std::bit_ceil<long unsigned
int>((8 * sizeof (unsigned int)))' is not an integer constant

but without bit_ceil(), the following error may result:

error: number of vector components 3 not a power of two.

IMO, this is a bug, since N * sizeof(T) compiles without issue, but a call to a
constexpr function does not.

I have prepared an example:
https://wandbox.org/permlink/lGvFnhDCLJ05j1FM

The idea is to use a SIMD register instead of an array on the stack for
temporary storage.

[Bug c++/110403] dependent function constexpr inside vector_size __attribute__ does not compile

[janezz55 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110403

--- Comment #1 from Janez Zemva <janezz55 at gmail dot com> ---
Here is a possible workaround:
#define S__(x) ((x) | (x) >> 1 | (x) >> 2 | (x) >> 3 | (x) >> 4)
#define BITCEIL(x) ((x) & (x) - 1 ? (S__(x) & ~(S__(x) >> 1)) << 1 : (x))
template <typename T, std::size_t N>
using array_t __attribute__ ((vector_size(BITCEIL(N * sizeof(T))))) = T;

[Bug c++/110403] dependent function constexpr inside vector_size __attribute__ does not compile

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110403

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=105233
     Ever confirmed|0                           |1
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2023-06-26
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, closely related to PR105233 which was the same issue for the aligned
attribute (and alignas), and was fixed by performing manifestly constant
evaluation of their argument.  I guess the same could be done for vector_size?