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

Jeff Law law@redhat.com
Fri Sep 19 17:14:00 GMT 2014


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?


Jeff



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