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


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?

> 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'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?

> 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.

Thanks,
- Tom


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