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Re: [PATCH][MIPS] Implement O32 FPXX ABI (GCC)


Matthew Fortune <Matthew.Fortune@imgtec.com> writes:
> Richard Sandiford <rdsandiford@googlemail.com> writes:
>> Matthew Fortune <Matthew.Fortune@imgtec.com> writes:
>> > I've realised that I may need to do 'something' to prevent GCC from
>> loading or
>> > storing DFmode/DImode values to/from FPRs using pairs of SWC1/LWC1
>> when using
>> > an unaligned address. Initial tests show that when loading from an
>> unaligned
>> > address (4-byte aligned) then GCC loads the two halves of a 64-bit
>> value into
>> > GPRs and then moves across to FPRs. This is good but I don't know if
>> it is
>> > guaranteed.
>> >
>> > From what I can tell the backend doesn't specifically deal with
>> loading
>> > unaligned data but rather the normal load/store patterns are used by
>> the
>> > middle end. As such I'm not sure there is anything to prevent direct
>> loads
>> > to FPRs by parts.
>> >
>> > Do you know one way or the other if unaligned doubles can currently be
>> loaded
>> > via pairs of lwc1 (same for store) and if so can you advise on an
>> approach I
>> > could try to prevent this for FPXX? I will try to figure this out on
>> my own in
>> > the meantime.
>> 
>> The port does handle some widths of unaligned access via the
>> {insv,extv,extzv}misalign patterns.  That's how an unaligned DImode
>> value will be handled on 64-bit targets.
>> 
>> The MIPS *misalign patterns only handle integer modes, so for other
>> types of mode the target-independent code will fall back to using an
>> integer load followed by a subreg (or a subreg followed by an integer
>> store).  IIRC that's how an unaligned DFmode access will be handled on
>> 64-bit targets.
>> 
>> For modes that are larger or smaller than *misalign can handle,
>> the target-independent code has to split the access up into smaller
>> pieces and reassemble them.  And these pieces have to have integer
>> modes.
>> E.g. on 32-bit targets a 4-byte-misaligned load into (reg:DF x) could be
>> done by loading (subreg:SI (reg:DF x) 0) and (subreg:SI (reg:DF x) 4).
>> The thing that prevents these individual loads from using LWC1 is
>> CANNOT_CHANGE_MODE_CLASS, which (among other things) makes it invalid
>> for any target-independent code to reduce a subreg of an FPR pair
>> to an individual FPR.
>> 
>> [FWIW, the reason MIPS doesn't define {insv,extv,extzv}misalign for
>> things
>> like DImode on 32-bit targets is because there's no special architecture
>> feature than can be used.  It's just a case of decomposing the access.
>> Since that's a general technique, we want to make the target-independent
>> code do it as well as possible rather than handle it in a port-specific
>> way.]
>> 
>> So yeah, the combination of (a) STRICT_ALIGNMENT, (b) the lack of
>> unaligned
>> floating-point load/store patterns and (c) CANNOT_CHANGE_MODE_CLASS
>> should
>> guarantee what you want.
>
> Thanks for all the details. I did not know if CANNOT_CHANGE_MODE_CLASS
> would give this guarantee as I am conscious of MIPS 1 having to do pairs
> of LWC1/SWC1 for doubles. However, if I understand the code correctly
> the LWC1/SWC1 pairs are generated by splits for MIPS 1 and not directly
> from target-independent code so CANNOT_CHANGE_MODE_CLASS does not impact
> that explicit splitting logic. Is that right?

Yeah, that's right.  All splits of floating-point values are done in the
MIPS code and the MIPS code is free to ignore CANNOT_CHANGE_MODE_CLASS.

One of the many reasons for defining CANNOT_CHANGE_MODE_CLASS the way it
is now is that the FPRs are always little-endian.  If the target-independent
code ever did try to access one half of a pair, it would access the wrong
one on big-endian targets.  So CCCM is by no means just a technicality at
the moment.  Other targets also rely on CCCM for similarly important reasons.

Thanks,
Richard


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