[LLVMdev] clobbering other physical registers in storeRegtoStackSlot.

[sanjiv gupta]

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.

>  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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