Re: RFC: Make builtin types only valid for some target features

[Andrew Pinski <pinskia@gmail.com>]

On Sun, Dec 4, 2022 at 11:33 PM Richard Sandiford via Gcc
<gcc@gcc.gnu.org> wrote:
>
> "Kewen.Lin" <linkw@linux.ibm.com> writes:
> > Hi,
> >
> > I'm working to find one solution for PR106736, which requires us to
> > make some built-in types only valid for some target features, and
> > emit error messages for the types when the condition isn't satisfied.
> > A straightforward idea is to guard the registry of built-in type under
> > the corresponding target feature.  But as Peter pointed out in the
> > PR, it doesn't work, as these built-in types are used by some built-in
> > functions which are required to be initialized always regardless of
> > target features, in order to support target pragma/attribute.  For
> > the validity checking on the built-in functions, it happens during
> > expanding the built-in functions calls, since till then we already
> > know the exact information on specific target feature.
> >
> > One idea is to support built-in type checking in a similar way, to
> > check if all used type_decl (built-in type) are valid or not somewhere.
> > I hacked to simply check currently expanding gimple stmt is gassign
> > and further check the types of its operands, it did work but checking
> > gassign isn't enough.  Maybe I missed something, there seems not an
> > efficient way for a full check IMHO.
> >
> > So I tried another direction, which was inspired by the existing
> > attribute altivec, to introduce an artificial type attribute and the
> > corresponding macro definition, during attribute handling it can check
> > target option node about target feature for validity.  The advantage
> > is that the checking happens in FE, so it reports error early, and it
> > doesn't need a later full checking on types.  But with some prototyping
> > work, I found one issue that it doesn't support param decl well, since
> > the handling on attributes of function decl happens after that on
> > attributes of param decl, so we aren't able to get exact target feature
> > information when handling the attributes on param decl.  It requires
> > front-end needs to change the parsing order, I guess it's not acceptable?
> > So I planed to give up and return to the previous direction.
> >
> > Does the former idea sound good?  Any comments/suggestions, and other
> > ideas?
> >
> > Thanks a lot in advance!
>
> FWIW, the aarch64 fp move patterns emit the error directly.  They then
> expand an integer-mode move, to provide some error recovery.  (The
> alternative would be to make the error fatal.)
>
> (define_expand "mov<mode>"
>   [(set (match_operand:GPF_TF_F16_MOV 0 "nonimmediate_operand")
>         (match_operand:GPF_TF_F16_MOV 1 "general_operand"))]
>   ""
>   {
>     if (!TARGET_FLOAT)
>       {
>         aarch64_err_no_fpadvsimd (<MODE>mode);
>         machine_mode intmode
>           = int_mode_for_size (GET_MODE_BITSIZE (<MODE>mode), 0).require ();
>         emit_move_insn (gen_lowpart (intmode, operands[0]),
>                         gen_lowpart (intmode, operands[1]));
>         DONE;
>       }
>
> This isn't as user-friendly as catching the error directly in the FE,
> but I think in practice it's going to be very hard to trap all invalid
> uses of a type there.  Also, the user error in these situations is likely
> to be forgetting to enable the right architecture feature, rather than
> accidentally using the wrong type.  So an error about missing architecture
> features is probably good enough in most cases.

I did have a patch which improved the situation for the SVE types to
provide an error message at compile time when SVE is not enabled
but I didn't get any feedback from either the C or C++ front-end folks.
https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583786.html

I suspect if that patch gets reviewed by the front-end folks, Kewen
could use the same infrastructure to error out on the types for rs6000
backend.


Thanks,
Andrew Pinski

>
> Thanks,
> Richard