[LLVMdev] rematerialization question

[Ivan Llopard]

Hi Heikki,

Le 15/03/2012 15:31, Heikki Kultala a écrit :
> I am a bit confused how the rematerialization works.
>
> It seems currently in our backend we get lots of code where some stack
> offset address is calculated, but this address is then spilled to stack,
> and loaded from stack later.
>
> This does not make sense, it would be better to just recalculate the
> address later, ie rematerialize the original stack offset calculation.
>
> But marking some instruction rematerializable means all operands have to
> be always available?

Yes, the inline spiller will verify that condition.

> So I cannot make my add operation which is used for the stack offset
> calculations rematerializable because not all operands of all adds are
> always available?

Well, you can mark your add operation as being rematerializable in your 
BE but it will not get remat'd if its operands are not available.

>
> So how can I make llvm to try to rematerialize only those stack offset
> adds, but not all adds?

You can force remat for those adds you want to by patching the spiller 
(see InlineSpiller.cpp). I did it for my target (a dirty patch though) 
but I cannot assure that remat will always be possible :(.

Regards,
Ivan

>
> And also, when I have the stack update code in beginning and at end of
> the functions, these stack+immediate adds/subs also should not be
> rematerialized as the sp has changed?
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