[LLVMdev] Moving data between regs

[Evan Cheng]

I consider this an orthogonal issue. While this particular example  
make use of sub-register info, that's not always the case. You can  
conceivably see a target where it's cheap(er) to move from an integer  
register to a floating point one, etc.

Evan
On Jun 14, 2007, at 12:40 PM, Christopher Lamb wrote:

> Does this depend on vreg subreg support? It sounds as if it could  
> be vaguely related, but I'm not sure it's necessary.
>
> --
> Chris
>
> On Jun 14, 2007, at 12:27 PM, Evan Cheng wrote:
>
>> The short answer is this is not currently done. LLVM register
>> allocator does not currently use a move to a different register class
>> in lieu of spills. This is definitely something we should do in the
>> future. Can you file a bugzilla enhancement request?
>>
>> Thanks,
>>
>> Evan
>>
>> On Jun 13, 2007, at 11:09 AM, Chris Lattner wrote:
>>
>>> On Tue, 12 Jun 2007, Fernando Magno Quintao Pereira wrote:
>>>>     Imagine that I have a virtual v stored in register AL, and I
>>>> am about
>>>> to cross a function call, that effectively overwrites AL. Well, I
>>>> have,
>>>> say, register ESI free, but as it happens, ESI is bigger than  
>>>> AL. It
>>>> should be technically possible to move the contents of AL into
>>>> ESI, to
>>>> avoid spilling 'v', but I don't know if that is possible in LLVM.
>>>> Basically, ESI is not in the class of 'v', and it has no 8 bit
>>>> alias. Is
>>>> this a problem (the move can't be done), or am I missing something?
>>>
>>> Very interesting idea.  Yes, this is doable.  On X86 you would copy
>>> EAX
>>> to ESI, which implicitly saves AL because it's a superclass of AL.
>>> MRegisterInfo has information about super and subclass registers,
>>> Evan can
>>> tell you more.
>>>
>>> -Chris
>>>
>>> -- 
>>> http://nondot.org/sabre/
>>> http://llvm.org/
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>
> --
> Christopher Lamb
>
>
>
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