[LLVMdev] Multi-Instruction Patterns

[Evan Cheng]

On Sep 24, 2008, at 8:44 AM, Dan Gohman wrote:

>>>
>>>
>>> So given two patterns that match the same thing, what's the
>>> tiebreaker?
>>> I thought it was order in the .td file but that doesn't appear to be
>>> the
>>> case.  I put my pattern first and it isn't selected.  I change the
>>> other
>>> pattern slightly so it won't match anything and then my pattern gets
>>> used (so I know my pattern is valid).
>>>
>>> Also, I really wanted to express this pattern as transforming from  
>>> one
>>> DAG to another, not down to machine instructions.  I saw this in
>>> x86InstSSE.td:
>>>
>>> // FIXME: may not be able to eliminate this movss with coalescing  
>>> the
>>> src and
>>> // dest register classes are different. We really want to write this
>>> pattern
>>> // like this:
>>> // def : Pat<(f32 (vector_extract (v4f32 VR128:$src), (iPTR 0))),
>>> //           (f32 FR32:$src)>;
>>>
>>> (this is actually a very useful and important pattern, I wish it was
>>> available!)
>>
>> Right. It would be nice to be able to eliminate the unnecessary
>> movss.  It hasn't shown up on my radar so I haven't really thought  
>> out
>> the right way to model this. I can see a couple of options:
>>
>> 1. Treat these instructions as cross register class copies. The src
>> and dst classes are different (VR128 and FR32) but "compatible".
>> 2. Model it as extract_subreg which coalescer can eliminate.
>>
>> #2 is conceptually correct. The problem is 128 bit XMM0 is the same
>> register as 32 bit (or 64 bit) XMM0. So it's not possible to define
>> the super-register / sub-register relationship.
>
> I don't understand the problem with subregs here. Is it just a
> naming issue? That can be solved by introducing alternate names,
> like XMM0_32 and XMM0_64, for each of the subregs. They could
> still be printed as "xmm0" in the assembly output of course.

Right. That's a workable solution. However, it still adds complexity:

XMM0_32 = MOVPS2SSrr XMM0

We need to teach the allocator that the two registers are the "same"  
and this is a identity copy.

Evan

>
>
> Dan
>
>
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