Re: [Lightning] About work on possible mips port

[Paolo Bonzini]

On 10/06/2010 06:31 AM, Paulo César Pereira de Andrade wrote:
>    At first I am thinking about using the approach I used, but reverted to
> not make default, for function calls, that is to patch a stack pointer
> adjustment on demand, i.e. as result of jit_allocai or function calls
> that require arguments on stack. There is no push/pop opcodes
> anyway, but would need to assert that no more than 32767 bytes
> are allocated, to make patching possible.

I initially said I don't like the idea of patching the prolog on 
jit_allocai or function calls, however I now noticed that the PowerPC 
port is doing that for jit_allocai.  So, for that case I now think 
patching the prolog is good.

For function calls, however, it would be much better (and portable) to 
adjust the stack pollution support so that instead of

         push %eax
         push %ebx
         call f1           ; jit_calli
         push %eax
         push %ebx
         call f2           ; jit_finish
         add $16, %esp

it generates

         push %eax
         push %ebx
         call f1
         mov %eax, 4(%esp)
         mov %ebx, (%esp)
         call f2
         add $8, %esp

> 1. move pointer to register and to indirect jump/call

That's what jit_calli does for PowerPC.

>    Conditional branches must be limited to 18 bits distance, or, add
> a jump over with an inverse condition and implement an unconditional
> one...

I think this is fine.  It's 256 KB after all.

>    Still also need to find out how to properly implement carry
> primitives, as there is only add/sub without side effects, and
> signed add/sub that generate a trap on overflow.

Something like this:

        add destlo, src1lo, src2lo
        sltu aux, destlo, src1lo        ; sgtu for subtraction

        add desthi, src1hi, src2hi
        add desthi, desthi, aux

Paolo