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

Vladimir Makarov vmakarov@redhat.com
Wed Feb 13 22:35:00 GMT 2013

On 13-02-07 2:11 PM, Tom de Vries wrote:
> Vladimir,
> On 25/01/13 16:36, Vladimir Makarov wrote:
>> On 01/25/2013 08:05 AM, Tom de Vries wrote:
>>> Vladimir,
>>> this patch adds analysis of register usage of functions for usage by IRA.
>>> The patch:
>>> - adds analysis in pass_final to track which hard registers are set or clobbered
>>>     by the function body, and stores that information in a struct cgraph_node.
>>> - adds a target hook fn_other_hard_reg_usage to list hard registers that are
>>>     set or clobbered by a call to a function, but are not listed as such in the
>>>     function body, such as f.i. registers clobbered by veneers inserted by the
>>>     linker.
>>> - adds a reg-note REG_CALL_DECL, to be able to easily link call_insns to their
>>>     corresponding declaration, even after the calls may have been split into an
>>>     insn (set register to function address) and a call_insn (call register), which
>>>     can happen for f.i. sh, and mips with -mabi-calls.
>>> - uses the register analysis in IRA.
>>> - adds an option -fuse-caller-save to control the optimization, on by default
>>>     at -Os and -O2 and higher.
> <SNIP>
>>> Bootstrapped and reg-tested on x86_64, Ada inclusive. Build and reg-tested on
>>> mips, arm, ppc and sh. No issues found. OK for stage1 trunk?
>> Thanks for the patch.  I'll look at it during the next week.
> Did you get a chance to look at this?
Sorry for the delay with the answer.  I was and am quite busy with other 
more urgent things.  I'll work on it when I have more free time.  In any 
case, I'll do it before stage1 to have your patch ready.
>> Right now I see that the code is based on reload which uses
>> caller-saves.c.  LRA does not use caller-saves.c at all.  Right now we
>> have LRA support only for x86/x86-64 but the next version will probably
>> have a few more targets based on LRA.  Fortunately, LRA modification
>> will be pretty easy with all this machinery.
> I see, thanks for noticing that. Btw I'm now working on a testsuite construct
> dg-size-compare to be able to do
>    dg-size-compare "text" "-fuse-caller-save" "<" "-fno-use-caller-save"
> which I could have used to create a generic testcase, which would have
> demonstrated that the optimization didn't work for x86_64.
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.
>> I am going to use ira-improv branch for some my future work for gcc4.9.
>> And I am going to regularly (about once per month) merge trunk into it.
>> So if you want you could use the branch for your work too.  But this is
>> absolutely up to you.  I don't mind if you put this patch directly to
>> the trunk at stage1 when the review is finished.
> OK, I'd say stage1 then unless during review a reason pops up why it's better to
> use the ira-improv branch.
That is ok.  Stage1 then.

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