[LLVMdev] PHI Elimination in Register Allocation Pass

[Natanael Ramos]

Thank you Quentin.

It is an interesting fact that the liveness Analisys be maintained in SSA,
even after the phase of PHI Destruction. It facilitates the implementation
of the register allocator.

Cheers,
Natanael.
Em 09/07/2015 17:28, "Quentin Colombet" <qcolombet at apple.com> escreveu:

> Hi Natanael,
>
> On Jul 9, 2015, at 1:06 PM, Natanael Ramos <naelr8 at gmail.com> wrote:
>
> Good Afternoon.
>
> I am a Computer Science undergraduate student in Brazil and as completion
> of course work, I am developing an register allocator, using the
> infrastructure of the LLVM.
>
> To accomplish this task, I have based my implementation in allocators
> already implemented in LLVM. But a question came to me while I was
> researching in books and articles of compiler theory and own documentation
> of LLVM, I noticed an important step in the optimization phase, the
> deconstruction of form SSA (*PHI Elimination*), from what I've read, this
> step has great impact on the code generation and can be done before or
> after the register allocation phase. And I intend to implement the
> allocator registers using the resources of SSA representation, making this
> step (*PHI Elimination*) vital for development.
>
> Looking at the source code of allocators of LLVM, I see no explicit call
> of PHI Elimination (*AU.addRequiredID (PHIEliminationID)*), my question
> is whether if this step is called, occurs when and where such a call?
>
>
> Yes, this pass is always called, but before register allocation. (Though
> the liveness information still uses the SSA form.)
>
> The pass is added in the pipeline in lib/CodeGen/Passes.cpp, see for
> instance: TargetPassConfig::addOptimizedRegAlloc
>
> Cheers,
> -Quentin
>
>
> --
> Natanael Ramos
> Membro do corpo discente de Ciência da Computação pelo Instituto Federal
> de
> Minas Gerais - Campus Formiga
>
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>
>
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