[LLVMdev] Newbie: Target Lowering info.

[Evan Cheng]

On Nov 29, 2007, at 12:48 PM, Sanjiv Gupta wrote:

> thanks Evan,
> I have just started writing td files.
>
> Any ideas how do I describe instructions for an accumulator based  
> machine.

That is tricky. It doesn't really fit the LLVM codegen model. One  
thing you can do is to add pseudo registers to allow "normal" register  
allocation. Then you can add a post pass to convert everything to  
stack operation. That's how we handle x87 floating point math. The  
model is not an exact match to an accumulator based model, but it's an  
idea.

You will probably also need to tweak the instruction scheduler to get  
better code later.

> The other pecularity is that we do not want to have any software  
> stack.
> So the instructions like load and store have no meanings.

Not sure what you mean by load and store have no meanings?

>
> In that case,   how do I lower instructions that operate on stack  
> frame?

LLVM codegen has a frame index representation. They are created during  
lowering pass and during register allocation. The prologue and  
epilogue elimination pass will then translate them using explicit  
stack pointer, etc.

Evan

>
> TIA,
> Sanjiv
>
> On 11/26/07, Evan Cheng <evan.cheng at apple.com> wrote:
>
> On Nov 24, 2007, at 7:28 PM, Sanjiv Gupta wrote:
>
> > Could anybody guide me what information do I need to know about my
> > target in order to provide the target lowering info to the llvm DAG
> > generator? We do not have any fixed registers for argument passing.
> > Everything including the formal and actual arguments will take  
> part in
> > a global interprocedural regalloc.
>
> You don't have to formally specify calling convention. For example,
> ARM backend does not have a ARMCallingConv.td file, all the argument
> passing info are implicitly defined in the lowering code.
>
> To start, you should specify legal register files, legal operations,
> etc. See XXXISelLowering.cpp for examples.
>
> Evan
>
> >
> >
> > Any pointers to learn about this will be a great help.
> > Sanjiv
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