[LLVMdev] pseudo lowering

[Andrew Trick]

On Feb 17, 2013, at 1:11 PM, Reed Kotler <rkotler at mips.com> wrote:

> On 02/17/2013 01:08 PM, Andrew Trick wrote:
>> 
>> On Feb 17, 2013, at 1:01 PM, Reed Kotler <rkotler at mips.com> wrote:
>> 
>>> On 02/17/2013 12:48 PM, Andrew Trick wrote:
>>>> On Feb 16, 2013, at 1:31 PM, Cameron Zwarich <zwarich at apple.com> wrote:
>>>> 
>>>>> That's exactly the right place.
>>>> Really? You don't want the expansion to be optimized? You want to specify a machine model for the pseudo's as if they're real instructions? You don't want to schedule or register allocate the real instructions?
>>>> 
>>>> -Andy
>>> 
>>> So then maybe my code should be called during instruction selection?
>>> 
>>> The very original MIPs port was for Mips I and Mips I has a need for lots of psuedos because it's primitive.
>>> That code has mostly been removed now because we don't support Mips I which is just an historical processor at this time.
>>> 
>>> I approach level one implementations of things from the point of view of correctness and then make things better as I understand the problem better. 
>>> 
>>> So my base scheme for all of this was to create multi line assembler expansion in pseudos.
>>> 
>>> Now that I'm passing mostly all of test-suite, I'm starting improve things.
>>> 
>>> At this time, I'm starting to place expansion in 
>>> expandPostRAPseudo
>>> 
>>> Would it be possible to call this same code during instruction selection?
>>> 
>>> When I emitting the code for certain formats, I would need to call some emitter which basically the same kind of I'm calling from  expandPostRAPseudo now.
>> 
>> expandISelPseudos is the place to cleanup after ISEL hacks, and can create BBs. But, as Cameron explained well in the other thread, if you want a macro assembler for some sequences, do it late in expandPostRAPseudos.
>> -Andy
>> 
> I feel like I should expand things as soon as I have all the information because then other passes can help make things better.
> 
> So maybe I do this earlier in expandIselPseudos.
> 
> Are there any cons ?

If the instructions must be adjacent for correctness or cannot be spilled within the sequence, then you have a problem. Sorry to be vague, but I'll again refer you to Cameron's detailed response.

-Andy
>>>>> On Feb 16, 2013, at 1:08 PM, Reed Kotler <rkotler at mips.com> wrote:
>>>>> 
>>>>>> I have a bunch of pseudos that I want to lower right after instruction selection.
>>>>>> 
>>>>>> Where is the best place to do that?
>>>>>> 
>>>>>> I was planning to use expandPostRAPseudo.
>>>>>> 
>>>>>> Is there a better place?
>>>>>> 
>>>>>> TIA.
>>>>>> 
>>>>>> Reed
>>>>>> 
>>>>>> 
>>>>>> 
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>> 
> 

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