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

[Tom de Vries <Tom_deVries at mentor dot com>]

On 13/02/13 23:35, Vladimir Makarov wrote:
> 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.

Vladimir,

do you have an ETA on this review?

>>> 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.

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.

Thanks,
- Tom