> > IMO, what ought to happen here is that the RA should spill
> > the inner register to memory and load the V4SI back from there.
> > (Or vice versa, for an lvalue.) Obviously that's not very efficient,
> > and so a patch like the above might be useful as an optimisation.[*]
> > But it shouldn't be needed for correctness. The target-independent
> > code should already have the information it needs to realise that
> > it can't predict the register index at compile time (at least for SVE).
>
> Or actually, for that case:
>
> /* For pseudo registers, we want most of the same checks. Namely:
>
> Assume that the pseudo register will be allocated to hard registers
> that can hold REGSIZE bytes each. If OSIZE is not a multiple of REGSIZE,
> the remainder must correspond to the lowpart of the containing hard
> register. If BYTES_BIG_ENDIAN, the lowpart is at the highest offset,
> otherwise it is at the lowest offset.
>
> Given that we've already checked the mode and offset alignment,
> we only have to check subblock subregs here. */
> if (maybe_lt (osize, regsize)
> && ! (lra_in_progress && (FLOAT_MODE_P (imode) || FLOAT_MODE_P
> (omode))))
> {
> /* It is invalid for the target to pick a register size for a mode
> that isn't ordered wrt to the size of that mode. */
> poly_uint64 block_size = ordered_min (isize, regsize);
> unsigned int start_reg;
> poly_uint64 offset_within_reg;
> if (!can_div_trunc_p (offset, block_size, &start_reg,
> &offset_within_reg)
> ...
>
Like aarch64 we set REGMODE_NATURAL_SIZE for fixed-size modes to
UNITS_PER_WORD. Isn't that part of the problem?
In extract_bit_field_as_subreg we check lowpart_bit_field_p (= true because
128 is a multiple of UNITS_PER_WORD). This leads to the subreg expression.
If I have REGMODE_NATURAL_SIZE return a VLA number this fails and we extract
via memory - but that of course breaks almost everything else :)
When you say the target-independent code should already have all information it
needs, what are you referring to? Something else than REGMODE_NATURAL_SIZE?
Thanks.
--
Regards
Robin
