[LLVMdev] Named Register Implementation

[Evan Cheng]

> On Mar 29, 2014, at 7:27 AM, Renato Golin <renato.golin at linaro.org> wrote:
> 
>> On 29 March 2014 13:38, Joerg Sonnenberger <joerg at britannica.bec.de> wrote:
>> I disagree. It is the *easy* part to get many known users to work. This
>> includes a bunch of kernels, Lisp implementations etc. The rest can be
>> implemented on top by hand using inline asm, so this is the crucial
>> part.
> 
> Let me re-phrase my opinion...
> 
> From all discussions on the LLVM list, the one that has shown more
> consistently as a problem was the reservation of allocatable
> registers.
> 
> Some background...
> 
> My original intent was to be able to support the kernel unwinding
> code. There was a discussion, in which we agree that for that a
> building stack_pointer would do. But the GCC community has voiced two
> concerns: there is already a feature for that widely in use, and that
> feature is more generic.
> 
> Back to the LLVM list, people were concerned with the implementation
> and semantics of reserving allocatable registers (yes, we do reserve
> some of them specially here and there, but this is a generic
> user-driven module-local reservation mechanism, which can put an
> unknown strain on the register allocator. Folks agreed, then that a
> two step implementation would make sense, given the original
> objectives: first for non-allocatable registers, then a register
> allocation mechanism could be introduced.
> 
> I agree that, to make it work like the GCC, reserving registers would
> be the first thing to implement, but we're not going to support the
> whole shebang from start, so we can start small. For specifically the
> non-allocatable ones, named register is as good as the builtin, and
> that would mean no changes to code that already use it would be
> necessary, which is always a win.
> 
> The plan...
> 
> 0. Map all uses of named register in the kernel: seem to be
> non-allocatable registers only.
> 1. Implement named registers for non-allocatable registers

I'm not sure if this would work even for non-allocatable registers. I fear this may codegen to a copy from the register and subsequent reads are all of the gpr. 

Evan 

> 2. Map uses of allocatable registers (libs, dynamic languages,
> kernels) and make sure there is no other way to do that.
> 2.a. If there is, do that.
> 2.b. If not, implement register reservation and expand named register
> for those classes (this is contentious and why I'm avoiding it for
> now)
> 
> cheers,
> --renato
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