[LLVMdev] PHI Elimination in Register Allocation Pass

[Quentin Colombet]

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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