Jeff Law <l...@redhat.com> writes: > On 09/19/14 01:23, Richard Sandiford wrote: >> Jeff Law <l...@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
