[LLVMdev] Moving data between regs

[Evan Cheng]

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
>
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