[LLVMdev] module level assembly optimization

[reed kotler]

On 10/15/2013 01:39 PM, reed kotler wrote:
> On 10/15/2013 01:30 PM, Rafael Espíndola wrote:
>> On 14 October 2013 21:56, reed kotler <rkotler at mips.com> wrote:
>>> I would like to do constant pools over an entire module instead of
>>> just on a
>>> per function basis as constant islands does it now.
>>>
>>> It seems there are two options for this:
>>>
>>> 1) collect the machine functions with their machine instructions for the
>>> whole module, edit them as needed and then
>>> call asmprinter for them one at a time.
>>>
>>> 2) collect all the instruction streams and store them in lists, one per
>>> section and put them all out at the end.
>>>
>>> There are possibly other kinds of module level optimizations that could
>>> benefit from this.
>>>
>>> Some of this might be better done by moving this functionality into
>>> target
>>> independent code but I think I could do it all
>>> without such changes.
>>>
>>> Any thoughts?
>> My experience is here consists of only once debugging an issue on the
>> ARM constant island pass, so take my opinion with a grain of salt
>>
>> When looking at the pass, it is easy to notice how much it looks like
>> an assembler doing relaxations. It needs to know exact offsets, adding
>> data to an island changes offsets, etc. It is also easy to notice how
>> hard it is to test a pass like that that is nested deep inside
>> codegen.
>>
>> My thought at the time was that it would be nice to implement the pass
>> in the assembler itself by having codegen use pseudo instructions.
>> Something like
>>
>> .const_island_set_reg r4, 0x12345678
>>
>> The assembler would then be responsible for creating the islands and
>> producing a load (or using something like movw + movt if it was better
>> in that case.
>>
>> The advantages would be
>> * The assembler already naturally handles exact addresses.
>> * It would be much easier to test.
>> * It would work across functions
>>
>> Then again, it is entirely possible I missed something that mandates
>> this pass being in codegen.
>>
>> Cheers,
>> Rafael
> I agree with you 100%.
>
> I think it should be an assembler function too, as should long branch
> optimization.
> In fact the earliest pdp-11 unix compilers did long branch optimization
> in the assembler.
>
> Moving this to the assembler would require some community buy in.
>
> To do this in the assembler and not create any issues, I think you would
> need to serialize one the asmstreamer interfaces and have a way
> to edit the instruction stream parts.
>
> Then you could play it back through asm printer.
>
> Reed
>

I thought about serializing asmprinter for this purpose but that code 
would really belong in the target independent code and that would 
require a proposal to the community and I don't know what kind of 
response there would be.

Certainly, it would be generally useful.
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