[PATCH 0/5] Fix handling of word subregs of wide registers
Fri Sep 19 17:14:00 GMT 2014
On 09/19/14 01:23, Richard Sandiford wrote:
> Jeff Law <firstname.lastname@example.org> writes:
>> On 09/18/14 04:07, Richard Sandiford wrote:
>>> This series is a cleaned-up version of:
>>> 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
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