Hi,
Okay & got it, Thanks for detail :)
Jojo R
在 2021年4月29日 +0800 PM5:11，Richard Sandiford <richard.sandif...@arm.com>，写道：
> JojoR via Gcc <gcc@gcc.gnu.org> writes:
> > Hi,
> >
> > I have a little know about for 'Sizes and offsets as runtime invariants’,
> >
> > and need to add vector types like V2SImode as compile-time constants
> > with enabled vector types of runtime invariants.
> >
> > Could I enable two vector types at same time ?
> > I guess it’s not allow :(
> >
> > Could anyone give me some hints ?
>
> Vector modes like V2SI always represent fixed-length vectors.
> Variable-length vector modes have to be defined explicitly in the
> .def file.
>
> A target can have both variable-length and fixed-length vectors.
>
> Probably the best place to look for examples is the aarch64 backend.
> There we have:
>
> - V2SI (64-bit Advanced SIMD)
> - V4SI (128-bit Advanced SIMD)
> - VNx2SI (variable-length SVE vector with 2 32-bit elements per
> 128-bit granule)
> - VNx4SI (variable-length SVE vector with 4 32-bit elements per
> 128-bit granule)
> etc.
>
> In target-independent code, things like GET_MODE_NUNITS are always
> poly_ints. In other words, the number of elements might be constant
> (is_constant()) or might vary at runtime (!is_constant()).
>
> On targets like x86 that don't define variable-length vectors,
> things like GET_MODE_NUNITS are always compile-time constants
> *within config/i386*. This removes the need to use is_constant()
> everywhere.
>
> On targets like aarch64 that do define variable-length vectors,
> things like GET_MODE_NUNITS are poly_ints even in config/aarch64.
> However, GET_MODE_NUNITS is still constant for V2SI and V4SI.
> We just have to use to_constant() to get the constant 2 or 4.
>
> Thanks,
> Richard
