Re: Help modifying an existing port.

[Jim Wilson <wilson at specifixinc dot com>]

kernel_learner wrote:
> For a certain project I want to modify the compiler
> for this architecture so that it generates code which
> always contains two independent instructions next to
> each other. If the compiler does not find independent
> instructions it should insert a no-op instead. 

Gcc doesn't have any easy way to do this.  Usually, it is done in the 
TARGET_MACHINE_DEPENDENT_REORG hook, which is an optional machine 
dependent optimization pass at the end.  You can write your own code to 
look for insns that aren't paired, and insert the necessary nops.  Try 
looking at existing ones for ideas.

> 1. Creating a new rtx for a NOP instruction

Put a pattern called "nop" in the md file, and then call gen_nop().

> 2. How do I add this new NOP instruction into the
> existing RTX list.

emit_insn (gen_nop()) will emit it at the current place.

emit_insn_before (gen_nop (), x) will emit it before insn X.

> Any help would be really appreciated.. I would also
> like comments on whether my approach of modifying the
> function schedule_block() is right or not

There are other optimization passes that occur after sched, and they 
likely won't preserve the instruction pairs that you create in the 
scheduler.  This probably only works if you do it at the very end, which 
takes you back to TARGET_MACHINE_DEPENDENT_REORG hook.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com