Re: [PATCH 0/5] Fix handling of word subregs of wide registers

[Richard Sandiford <richard dot sandiford at arm dot com>]

Jeff Law <law@redhat.com> writes:
> On 09/19/14 01:23, Richard Sandiford wrote:
>> Jeff Law <law@redhat.com> writes:
>>> On 09/18/14 04:07, Richard Sandiford wrote:
>>>> This series is a cleaned-up version of:
>>>>
>>>>       https://gcc.gnu.org/ml/gcc/2014-03/msg00163.html
>>>>
>>>> The underlying problem is that the semantics of subregs depend on the
>>>> word size.  You can't have a subreg for byte 2 of a 4-byte word, say,
>>>> but you can have a subreg for word 2 of a 4-word value (as well as lowpart
>>>> subregs of that word, etc.).  This causes problems when an architecture has
>>>> wider-than-word registers, since the addressability of a word can
>>>> then depend
>>>> on which register class is used.
>>>>
>>>> The register allocators need to fix up cases where a subreg turns out to
>>>> be invalid for a particular class.  This is really an extension of what
>>>> we need to do for CANNOT_CHANGE_MODE_CLASS.
>>>>
>>>> Tested on x86_64-linux-gnu, powerpc64-linux-gnu and aarch64_be-elf.
>>> I thought we fixed these problems long ago with the change to subreg_byte?!?
>>
>> No, that was fixing something else.  (I'm just about old enough to remember
>> that too!)  The problem here is that (say):
>>
>>      (subreg:SI (reg:DI X) 4)
>>
>> is independently addressable on little-endian AArch32 if X assigned
>> to a GPR, but not if X is assigned to a vector register.  We need
>> to allow these kinds of subreg on pseudos in order to decompose multiword
>> arithmetic.  It's then up to the RA to realise that a reload would be
>> needed if X were assigned to a vector register, since the upper half
>> of a vector register cannot be independently accessed.
>>
>> Note that you could write this example even with the old word-style offsets
>> and IIRC the effect would have been the same.
> OK.  So I kept thinking in terms of the byte offset stuff.  But what 
> you're tackling is related to the mess around the mode of the subreg 
> having a different meaning if its smaller than a word vs word-sized or 
> greater.
>
> Right?

Yeah, that's right.  Addressability is based on words, which is inconvenient
when your registers are bigger than a word.

Thanks,
Richard