Re: [024/nnn] poly_int: ira subreg liveness tracking

[Richard Sandiford <richard dot sandiford at linaro dot org>]

Jeff Law <law@redhat.com> writes:
> On 10/23/2017 11:09 AM, Richard Sandiford wrote:
>> Normmaly the IRA-reload interface tries to track the liveness of
>> individual bytes of an allocno if the allocno is sometimes written
>> to as a SUBREG.  This isn't possible for variable-sized allocnos,
>> but it doesn't matter because targets with variable-sized registers
>> should use LRA instead.
>> 
>> This patch adds a get_subreg_tracking_sizes function for deciding
>> whether it is possible to model a partial read or write.  Later
>> patches make it return false if anything is variable.
>> 
>> 
>> 2017-10-23  Richard Sandiford  <richard.sandiford@linaro.org>
>> 	    Alan Hayward  <alan.hayward@arm.com>
>> 	    David Sherwood  <david.sherwood@arm.com>
>> 
>> gcc/
>> 	* ira.c (get_subreg_tracking_sizes): New function.
>> 	(init_live_subregs): Take an integer size rather than a register.
>> 	(build_insn_chain): Use get_subreg_tracking_sizes.  Update calls
>> 	to init_live_subregs.
> OK.
>
> Note this is starting to get close to the discussion around CLOBBER_HIGH
> vs using a self set with a low subreg that we're having with Alan on
> another thread in that liveness tracking of subregs of SVE regs could
> potentially use some improvements.
>
> When I quickly looked at the subreg handling in the df infrstructure my
> first thought was that it might need some updating for SVE.  I can't
> immediately call bits for poly_int/SVE in the patches to-date.  Have you
> dug in there at all for the poly_int/SVE work?

Yeah, although the subreg tracking in this patch is specific to reload,
I thought we had something similar for LRA.  I couldn't find anything
though, and the static type checking of poly_ints would have forced
the issue.

There is the DF_WORD_LR code, which tracks the liveness of words in a
double-word pseudo.  We didn't extend that to variable-length registers
for two reasons: (1) if we did need it, we'd want it for pseudos
that map to 3 or 4 registers, not just 2, so that LD[234] and ST[234]
are handled consistently; and (2) it's only used for DCE at the moment,
and it's rare for LD[234]/ST[234]s to be dead code.

Thanks,
Richard