[llvm-dev] A thought to improve IPRA

[vivek pandya via llvm-dev]

On Sat, Jul 9, 2016 at 12:18 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> On Jul 8, 2016, at 11:41 AM, vivek pandya <vivekvpandya at gmail.com> wrote:
>
>
>
> On Fri, Jul 8, 2016 at 11:46 PM, Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
>>
>> On Jul 8, 2016, at 11:12 AM, vivek pandya <vivekvpandya at gmail.com> wrote:
>>
>> Hello LLVM Developers,
>>
>> I have a thought to improve IPRA and I would like summaries discussion on
>> IRC regarding that so we can develop an idea out of that if it really helps.
>>
>> So idea is to have more callee saved registers at infrequently called
>> leaf procedures and try provide more registers to procedures which are in
>> upper region of the call graph. But as pointed out by Quentin this
>> optimization may help in context of "true" IPRA but in our case we may not
>> require this. But I think that it can improve performance in current IPRA.
>> I explain both arguments ( Quentin's and mine) with following example.
>>
>> Consider following call sequence A->B->C , here C is very less time
>> called leaf procedure while A is called frequently and B may call C based
>> on some condition now while propagating actual register usage information
>> from C to A we almost clobbered most of the registers so in this case as
>> per Quentin's point we does not hurt the performance as we fall back to CC
>> but I think we can improve the performance as follows:
>> If we mark every register preserved by C (i.e having more spill reloads
>> at procedure entry and exit ) and if this can help  at A. Suppose A
>> requires more number of distinct registers than CC can provide and if not
>> provided it will spill variables to memory. Now if we can provide more
>> registers at A by having more spills at C then we can save spill at A which
>> can be beneficial because A is frequently called but C is less frequently
>> called and thus reducing total number of spill/restore in program execution.
>>
>> However again effect of this optimization will be limited by the scope of
>> current IPRA (i.e one Module only) because we can' really propagate the
>> details about more callee saved registers to caller which is defined in
>> other module, but still it may helpful.
>>
>> Any thoughts on this ?
>>
>>
>>
>> I think it is interesting, have you considered:
>>
>> - the code size impact? (C will have a lot of spills)
>>
> Yes, this needs to be address with some heuristics based on call
>  frequency to C and no of clobbers it has. Also can we say that a function
> which does not have any kind of call instruction in it's body will have
> less clobbers ?
>
>
> I am not sure what you mean.
>
A function which may do lots of computation but does not required to call
any other function may  not have too many simultaneous live ranges thus
with very few registers it can be compiled.

>
> - what if C is cold but all (most) of its call sites are located in
>> different modules?
>>
> Can we user Uses to get no of call site in current module and based on
> that we decide to optimize? Again some heuristics .
>
>
> Of course, but what I’m mentioning is exactly what does not work with
> that.
>

>
> - an alternative approach where we would break the CGSCC ordering to
>> codegen B and A before C, so we would be able to spill minimally when
>> performing the codegen for C?
>>
> Do you here mean marking all preserve for C while code gen for B and then
> when we come to C (top-down) we may avoid some spills if C can use regs
> which are not really used by B?
>
>
> Yes, but it may be harder to implement for not much gain after all.
>
> Also this can be applied to a function which is less frequently called and
> which may not be a leaf function. It may help.
>
>
> Sure, you can just refer to this as “PGO driven IPRA”.
>
Ok I will look into this.

Vivek

>
> —
> Mehdi
>
>
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