Re: How to implement conditional execution

[Paul Brook <paul at codesourcery dot com>]

On Friday 27 June 2008, Julian Brown wrote:
> On Fri, 27 Jun 2008 15:52:22 +0530
>
> "Mohamed Shafi" <shafitvm@gmail.com> wrote:
> > If the condition in the 'if' instruction is satisfied the processor
> > will execute the next instruction or it will replace with a nop. So
> > this means that i can instructions similar to:
> >
> > if eq Rx, Ry
> >   add Rx, Ry
> > add Rx, 2
> >
> > Will it be possible to implement this in the Gcc backend ?
> > Does any other targets have similar instructions?
>
> This is very much like (a simpler version of) the ARM Thumb-2 IT
> instruction. Look how config/arm/thumb2.md handles that. I think the
> basic idea should be that you should define conditional instruction
> patterns which emit assembly for both instructions simultaneously.

Not quite.  For Thumb-2 we describe most instruction as having conditional 
variants using define_cond_exec. Then arrange for the necessary IfThen 
instructions to be generated via ASM_OUTPUT_OPCODE. Multiple assembly 
instructions from the same pattern are either a historical leftover, or when 
it's inconvenient to describe the operation with proper cond_exec RTL.

Thumb-2 is a bit different to the machine described above because because the 
conditional execution is based on the condition code register, with the 
comparison occurring as a separate instruction, and the IT instruction can 
skip multiple subsequent instructions. I'd expect the same technique to be 
applicable though.

Paul