[LLVMdev] Re: Spilling register and frame indices

[Vladimir Prus]

Andrew Lenharth wrote:

> On Tue, 2006-05-23 at 13:04 -0500, Chris Lattner wrote:
>> > That approach sounds suboptimal. By "reserving" one register we can
>> > already cause some values to be spilled, that otherwise would be stored
>> > in register.
>> 
>> Right.
>> 
>> PowerPC has the same problem in certain cases.  For example, vector loads
>> only support reg+reg addressing, which means you have to load the offset
>> from the stack pointer into a register before doing the load.
>> 
>> In the case of PPC, we currently just reserve a register for this
>> purpose. This is suboptimal, but doesn't cause a significant performance
>> issue normally.
> 
> Alpha also has this problem for some offsets.  I haven't looked at what
> PPC does, but alpha has the loads and stores that might need the
> indexing kill an extra register, thus ensuring it is available for the
> inserted instruction.  You don't need to reserve a register (globally)
> only for these cases, just make sure you have it available when you need
> it.

I'm sorry I don't understand your suggestion. Unless I reserve a register
globally, I might not have any register at all to use for addressing.

- Volodya