[llvm-dev] A thought to improve IPRA

vivek pandya via llvm-dev
Thu, 28 Jul 2016 19:59:02 -0000

I have been working on PGO driven IPRA and I want to measure if this help
to reduce execution time. So as mentioned earlier the idea is to make cold
function register usage free i.e saving and restoring all used register by
such cold function so caller of that function will have more free
registers. So here I am changing standard callee saved registers set to a
set which will be decided dynamically based on the actual register usage.

I am facing few problems to get this working:
1 ) While generating CFI for  such function it requires to map Dwarf
register to LLVM register and even if we force LLVM to use Dwarf register
number for CFI then also it will be wrong for some register for which
currently we don't have such mapping for example R8D register on X86 (when
dealing with actual register usage info we may have such case where R8D is
being used)
To fix this I tried to filter the functions which will be optimized by
putting a constraints that it should have attribute NoUnwind but that does
not help. Is it possible to disable CFI generation?

2) R8D is a 48 bit register but pushing and popping such register is not
allowed and current implementation for CalleeSaved Register also uses
either 64 bit or 32 bit version of X86 instruction according to target. So
here I think it may be good to push/pop R8 for R8D (i.e I don't want to
change current implementation which inserts MI for CSR) for that I need to
find biggest register for which given register is alias like R8 has R8D as
alias. How can I find that?
I tried to use getMatchingSuperReg(unsigned Reg, unsigned SubIdx, const
TargetRegisterClass *RC) but here I don't know what will be SubIdx for
given Reg in given RC.

So for example if a function which should be optimized for above
optimization is having following set of clobbered registers:
R8D,R8, ECX, EAX, RAX, ESI It should push/pop R8, RCX, RAX, RSI.

Please help!
- Vivek


On Sat, Jul 9, 2016 at 12:26 AM, vivek pandya <vivekvpandya at gmail.com>
wrote:

>
>
> 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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