Re: Generated files in libgfortran for Fortran intrinsic procedures (was: Updated Sourceware infrastructure plans)

[Martin Uecker <muecker@gwdg.de>]

Am Donnerstag, dem 18.04.2024 um 14:01 +0200 schrieb Tobias Burnus:
> Hi Janne,
> 
> Janne Blomqvist wrote:
> > back when I was active I did think about this
> > issue. IMHO the best of my ideas was to convert these into C++
> > templates.

I haven't looked at libgfortran but I didn't find it problematic
at all to use C in similar numerical code and this helps
with portability. 

Either I use macros, which I keep short and then do not find
inferior to templates (having used C++ for years previously) or 
- if there is really a lot of code that needs to be specialized 
for a type - simply by using includes:

#define matmul_type double
#include "matmul_impl.c"

Martin


> 
> I think this will work – but we have to be super careful:
> 
> With C++, there is the problem that we definitely do not want to add 
> dependency on libstdc++ nor to use some features which require special 
> hardware support (like exceptions [always bad], symbol aliases, ...). — 
> On some systems, a full C++ support might be not available, like 
> embedded systems (including some odd embedded OS) or offloading devices.
> 
> The libstdc++ dependency would be detected by linking as we currently 
> do. For in-language features, we have to ensure the appropriate flags 
> -fno-exceptions (and probably a few more). And it should be clear what 
> language features to use.
> 
> If we do, I think that would surely be an option.
> 
> > What we're essentially doing with the M4 stuff and the
> > proposed in-house Python reimplementation is to make up for lack of
> > monomorphization in plain old C. Rather than doing some DIY templates,
> > switch the implementation language to something which has that feature
> > built-in, in this case C++.  No need to convert the entire libgfortran
> > to C++ if you don't want to, just those objects that are generated
> > from the M4 templates. Something like
> > 
> > template<typename T>
> > void matmul(T* a, T* b, T* c, ...)
> > {
> >     // actual matmul code here
> > }
> > 
> > extern "C" {
> >    // Instantiate template for every type and export the symbol
> >    void matmul_r4(gfc_array_r4* a, gfc_array_r4* b, gfc_array_r4* c, ...)
> >    {
> >      matmul(a, b, c, ...);
> >    }
> >    // And so on for other types
> > }
> 
> Cheers,
> 
> Tobias