[PATCH][IRA] Analysis of register usage of functions for usage by IRA.

Vladimir Makarov vmakarov@redhat.com
Tue Jan 14 19:36:00 GMT 2014

On 12/05/2013 07:47 PM, Tom de Vries wrote:
> On 14-03-13 10:34, Tom de Vries wrote:
>>> I thought about implementing your optimization for LRA by myself.
>>> But it
>>> >is ok if you decide to work on it.  At least, I am not going to start
>>> >this work for a month.
>>>> >>I'm also currently looking at how to use the analysis in LRA.
>>>> >>AFAIU, in lra-constraints.c we do a backward scan over the insns,
>>>> and keep track
>>>> >>of how many calls we've seen (calls_num), and mark insns with
>>>> that number. Then
>>>> >>when looking at a live-range segment consisting of a def or use
>>>> insn a and a
>>>> >>following use insn b, we can compare the number of calls seen for
>>>> each insn, and
>>>> >>if they're not equal there is at least one call between the 2
>>>> insns, and if the
>>>> >>corresponding hard register is clobbered by calls, we spill after
>>>> insn a and
>>>> >>restore before insn b.
>>>> >>
>>>> >>That is too coarse-grained to use with our analysis, since we
>>>> need to know which
>>>> >>calls occur in between insn a and insn b, and more precisely
>>>> which registers
>>>> >>those calls clobbered.
>>> >
>>>> >>I wonder though if we can do something similar: we keep an array
>>>> >>call_clobbers_num[FIRST_PSEUDO_REG], initialized at 0 when we
>>>> start scanning.
>>>> >>When encountering a call, we increase the call_clobbers_num
>>>> entries for the hard
>>>> >>registers clobbered by the call.
>>>> >>When encountering a use, we set the call_clobbers_num field of
>>>> the use to
>>>> >>call_clobbers_num[reg_renumber[original_regno]].
>>>> >>And when looking at a live-range segment, we compare the
>>>> clobbers_num field of
>>>> >>insn a and insn b, and if it is not equal, the hard register was
>>>> clobbered by at
>>>> >>least one call between insn a and insn b.
>>>> >>Would that work? WDYT?
>>>> >>
>>> >As I understand you looked at live-range splitting code in
>>> >lra-constraints.c.  To get necessary info you should look at
>>> ira-lives.c.
>> Unfortunately I haven't been able to find time to work further on the
>> LRA part.
>> So if you're still willing to pick up that part, that would be great.
> Vladimir,
> I gave this a try. The attached patch works for the included test-case
> for x86_64.
> I've bootstrapped and reg-tested the patch (in combination with the
> other patches from the series) on x86_64.
> OK for stage1?
Yes, it is ok for stage1.  Thanks for not forgetting LRA and sorry for
the delay with the answer (it is not a high priority patch for me right

I believe, this patch helps to improve code also because of better
spilling into SSE regs.  Spilling into SSE regs instead of memory has a
rare probability right now as all SSE regs are call clobbered.

Thanks again, Tom.


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