[LLVMdev] clobbering other physical registers in storeRegtoStackSlot.

[Evan Cheng]

On Oct 28, 2008, at 3:57 AM, sanjiv gupta wrote:

> On Thu, 2008-10-23 at 11:17 -0700, Evan Cheng wrote:
>>
>> On Oct 22, 2008, at 11:09 AM, Sanjiv.Gupta at microchip.com wrote:
>>
>>> In our case, storeRegToStackSlot, loadRegFromStackSlot clobbers some
>>> other physical register which may be holding a live value used
>>> somewhere else. How do I make regalloc aware so that it saves the
>>> value before storeRegToStackSlot and make it available again at the
>>> point of earlier use?
>>>
>>>
>>
>>
>> Typical solution is to either reserve a register for these purposes  
>> so
>> the backend does not have to worry about its value being clobbered.
> We have only one register (BSR) which we need to set correctly before
> every load/store.  Setting up this register is done via an operation
> called 'banksel'.
>
> The problem is that spill/reload gets in between a banksel and
> load/store operation; and these spill/reload also modify BSR. Regalloc
> fails to recognize this and does not ReMat 'banksel' again before the
> load/store.

Without having access to your backend nor an example, I can't really  
provide an intelligent solution. It does sound like you have to extend  
these target hooks and their clients.

Evan

>
>
>> A better solution would be to teach these routines to make use of the
>> register scavenger (or rather the callers should use the scavenger  
>> and
>> pass the handle to these routines) to find a scratch register. It  
>> will
>> require some changes to the target independent portion of codegen
>> though.
>>
>>
>> Evan
>>>
>>>
>>> TIA,
>>> Sanjiv
>>>
>>>
>>>
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>>
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