On Fri, Dec 13, 2024 at 01:15:36PM +0100, Tobias Burnus wrote:
> OpenMP states for C++:
>
> "Directives may not appear in constexpr functions or in constant expressions."
>
> There is some support for this already in GCC, but not for [[omp::decl]]-type
> of directives and it also doesn't work that well. For the example, for the
> newly added testcase, the result with the patch is simple and clear:
>
> error: OpenMP directives may not appear in ‘constexpr’ functions
>
> without the patch:
>
> error: uninitialized variable ‘i’ in ‘constexpr’ function
> error: uninitialized variable ‘i’ in ‘constexpr’ function
> sorry, unimplemented: ‘#pragma omp allocate’ not yet supported
> sorry, unimplemented: ‘#pragma omp allocate’ not yet supported
> error: ‘constexpr int f()’ called in a constant expression
> error: ‘constexpr int g()’ called in a constant expression
>
> Note: I think OpenACC has a similar issue but as the specification
> is silent about it, the patch only handles OpenMP.
>
> * * *
>
> I have not touched the 'case OMP_...:' in constexpr.cc, added in
> previous patches; in principle, those should be now unreachable
> and could be removed.
> I also have not included any OpenACC pragmas, even though they have
> the same issue. (However, contrary to OpenMP, the OpenACC spec is
> silent about constexpr.)
>
> * * *
>
> Comments, suggestions, concerns?
LGTM, though I think the restriction is against the direction of C++
development over the recent years.
Pretty much no constructs are disallowed in constexpr functions,
just with some exceptions what is still disallowed when encountering it.
So, I think the above restriction should be replaced for OpenMP 6.1 or
whatever version is planned next.
Generally, we want to allow something like
constexpr
type foo (...)
{
if consteval {
...
} else {
#pragma omp ...
...
}
}
or even just if (some_cond) { #pragma omp ... } in there,
as long as some_cond doesn't evaluate to true at compile time.
So, similar in how more recent C++ versions handle say asm statements
in constexpr functions (or before C++26 e.g. throwing exceptions).
If you encounter it while evaluating it, it is not constant expression,
if you don't, it is not a bug.
And, I don't see why declarative directives should be a problem in constexpr
functions.
Jakub
