Re: [PATCH 1/2] Add new RTX instruction class FILLER_INSN

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Aug 19, 2020 at 11:13:40AM +0200, Andrea Corallo wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > So I wonder if this cannot be done with some kind of NOTE, instead?
> 
> I was having a look into reworking this using an insn note as (IIUC)
> suggested.  The idea is appealing but looking into insn-notes.def I've
> found the following comment:
> 
> "We are slowly removing the concept of insn-chain notes from the
> compiler.  Adding new codes to this file is STRONGLY DISCOURAGED.
> If you think you need one, look for other ways to express what you
> mean, such as register notes or bits in the basic-block structure."

That is from 2004.  Since then 9 note types have been removed, but 7
new types added.  (There are 18 insn note types now).

> Would still be justificated in this case to proceed this way?

Yes, it is a lesser evil imho.

> The other
> option would be to add the information into the basic-block or into
> struct rtx_jump_insn.

Or just look at the insn code, define a "filler-nop" insn, allow those
after BBs?  Any reason that wouldn't work?


Segher