[llvm-dev] GlobalISel round table follow up: register bank select

Thu Oct 8 05:03:35 PDT 2020

Hi Quentin,

thanks for picking up the conversation!

 > I think we should step back and check what we want before investing 
any time in some rewrite.

That is a very fair point and I might have been getting ahead of myself 
in my last email.
What I would like to see from RegBankSelect is to produce the mapping 
with the overall lowest cost. Keeping track of all different 
combinations of mappings will certainly be non-trivial however, so I 
wonder if there is a smart way to do this without spending too much 
compilation time.

Ideally for instructions with no operands (like G_CONSTANT) it could 
also check whether a cross-bank copy is actually worth it or if it would 
be more beneficial to simply rematerialize the instruction on the 
required bank. For such instructions this information should already be 
available as part of the cost-modelling in RegBankSelect: we could 
simply compare the cost of a mapping on the required bank vs. the cost 
of a cross-bank copy.

Would you see this as a valid direction for RegBankSelect?

Best regards,

Dominik

Am 07.10.20 um 19:47 schrieb Quentin Colombet:
> Hi Dominik,
>
> Thanks for sending this!
>
>> On Oct 7, 2020, at 5:21 AM, Dominik Montada via llvm-dev 
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hi all,
>>
>> this is the second email for the round table follow-up, this time 
>> regarding the issues around the greedy RegBankSelect and alternative 
>> mappings.
>>
>> The issue I brought up was that because RegBankSelect goes top-down, 
>> it never looks at all available mappings for the operands when 
>> considering which of the mappings to apply to the current 
>> instruction. In our architecture we have one register bank dedicated 
>> to pointers and another one for anything else. We often see code 
>> where we have a G_PTR_ADD with a constant. Since the constant is not 
>> a pointer, we put it on the other register bank. We could put it on 
>> the address regbank and do provide alternative mappings for that, but 
>> since the greedy algorithm doesn't actually check the usage of the 
>> constant, it is always put on the other bank.
>
> The intent behind the greedy algorithm was for the mapping to look at 
> X instructions ahead, depending on the optimization level, when 
> assigning one instruction.
> Right now the window is simply 1 instruction and the code is not 
> structured to allow to have more than one.
>
> As we gain more insights on what we want RegBankSelect to do, it makes 
> sense to redesign it.
>
>>
>> When RegBankSelect then sees the G_PTR_ADD and sees that one of its 
>> inputs is on the other register bank already, it then inserts a 
>> costly cross-bank copy instead of checking if that operand has any 
>> alternative mappings which would make the overall mapping for the 
>> current instruction cheaper.
>
> The idea is when a mapping is done, that means it was the best one at 
> the time of the decision (greedy), thus we don’t challenge that.
> “Reverting” an already set mapping is not that simple because yes for 
> this particular use we are inserting costly cross copies, but there 
> are no guarantees that replacing the mapping of the definition will 
> not insert even more costly copies for the other uses.
>
> E.g., consider:
> ```
> A = def <— RBS starts here
> = useFP A
> = useInt A
> = useInt A
> ```
>
> Let’s assume that greedy works the way it is intended. I.e., it 
> assigns A to the int register bank because there are 2 such uses vs. 
> only 1 fp bank use:
> ```
> A<int> = def
> = useFP A<int> <— Next, RBS looks at this one
> = useInt A<int>
> = useInt A<int>
> ```
>
> Now the regbank for useFP is not right and has to be repaired. Right 
> now, we will insert the costly cross copy for that use:
> ```
> A<int> = def
> AFP<fp> = cross_copy A<int>
> = useFP AFP<fp>
> = useInt A<int>
> = useInt A<int>
> ```
>
> Now, if we were to change the definition of A to avoid this copy we 
> would create two costly copy for the useInt. Actually, another 
> question is what would we do when we look at the first useInt? Is this 
> use allowed to change the definition of the instruction again? Do we 
> duplicate the definition? Etc..
> ```
> A<fp> = def <— reassign
> = useFP AFP<fp>
> AInt1<int> = cross_copy A<fp>
> = useInt AInt1<int>
> AInt2<int> = cross_copy A<fp>
> = useInt AInt2<int>
> ```
>
> Bottom line, if we allow to modify the assignments of the definition, 
> the decision making is not local anymore and in particular may require 
> to add repairing code all over the place. As a result the cost model 
> becomes much more complicated.
>
>>
>> Matt suggested that RegBankSelect should probably go bottom-up 
>> instead and I agree with him. I don't think there is a particular 
>> reason why RegBankSelect necessarily has to go top-down.
>
> The rationale for going top-down is that when you reach an 
> instruction, all the operands are assigned so you know what would be 
> the cost of repairing.
> You could said that the problem is the same for definitions when going 
> bottom-up. However, this is likely to be more problematic because 
> usually you have fewer definitions than arguments on each individual 
> instruction, therefore there is more guess work going on (e.g., 
> top-down, you assume a cost for 1 definition, going bottom-up you have 
> to assume a cost for 2 arguments or more precisely you would have to 
> track a window of X instructions for 2 arguments instead of 1 definition.)
>
>>
>> I'm not too familiar with the implementation of RegBankSelect. Would 
>> it be a big effort to make it work bottom-up instead? I'm guessing 
>> one of the biggest areas would be the check whether a cross-bank copy 
>> is needed as well as calculating the overall cost for alternative 
>> mappings as now all usages of the current instruction would have to 
>> be checked instead of the much more limited operands. How big of an 
>> impact would this have?
>
> It’s been a while since I looked at the implementation but I would 
> expect this to be significant.
>
> I think we should step back and check what we want before investing 
> any time in some rewrite. For instance, I don’t see what bottom-up 
> fundamentally gives us. It seems like a workaround to me.
> That said, it would work either way!
>
> Cheers,
> -Quentin
>
>>
>> Cheers,
>>
>> Dominik
>>
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>
-- 
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