[LLVMdev] Dynamically alternating between register allocators

[Chris Lattner]

This doesn't necessarily need to be in the passmanager, but I'd like  
to see the code generator grow the ability to codegen an entire  
function multiple times.  Ideally, we'd be able to codegen all the way  
to final machineinstrs in multiple configurations and then pick the  
best based on some metric.  This would allow lots of interesting  
experiments and allow people willing to wait for really good code to  
get it.  Once the "best" codegen for a function is done, the final  
machineinstrs would be sent to the asmprinter or code emitter.

It isn't clear to me how to best fit this into the passmanager model  
though.

-Chris

http://nondot.org/sabre
http://llvm.org

On Sep 5, 2007, at 2:32 PM, Evan Cheng <evan.cheng at apple.com> wrote:

> Hi Fernando,
>
> To do this you would need to change the register allocation pass on
> the fly. I don't think this is currently possible. Someone please
> correct me if I am wrong.
>
> I would like to see hierarchical pass capability built into pass
> manager. This allows us to group live variables, phi lowering, two-
> address, lower subreg, live interval analysis, coalescing, allocation,
> and spilling into a single register allocator pass. The register
> allocator pass is then responsible for controlling its own sub-passes
> (not just the order of passes, but also termination conditions).
> Similarly, branch folding and if-conversion should be lumped together
> into a cfg optimization pass. Devang, hint hint. :-)
>
> Evan
>
> On Sep 5, 2007, at 9:36 AM, Fernando Magno Quintao Pereira wrote:
>
>>
>> Hey all,
>>
>>    is there a simple way to alternate between different register
>> allocators at run time? I would like to decide dynamically which
>> register
>> allocator to use. The decision is based on information that is
>> produced by
>> a pass that executes before register allocation takes place.
>>
>> Is it possible to modify RegisterRegAlloc::Registry to take into
>> consideration this information? Is there any similar example in LLVM?
>>
>> all the best,
>>
>> Fernando
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