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

Richard Sandiford richard.sandiford@arm.com
Mon Sep 22 07:23:00 GMT 2014

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.


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