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

[Andrea Corallo <andrea.corallo@arm.com>]

Segher Boessenkool <segher@kernel.crashing.org> writes:

> Hi Andrea,
>
> On Wed, Jul 22, 2020 at 12:02:33PM +0200, Andrea Corallo wrote:
>> This first patch implements the addition of a new RTX instruction class
>> FILLER_INSN, which has been white listed to allow placement of NOPs
>> outside of a basic block.  This is to allow padding after unconditional
>> branches.  This is favorable so that any performance gained from
>> diluting branches is not paid straight back via excessive eating of
>> nops.
>
>> It was deemed that a new RTX class was less invasive than modifying
>> behavior in regards to standard UNSPEC nops.
>
> So I wonder if this cannot be done with some kind of NOTE, instead?
>

Hi Segher,

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."

Would still be justificated in this case to proceed this way?  The other
option would be to add the information into the basic-block or into
struct rtx_jump_insn.

My GCC experience is far from sufficient for having a formed opinion on
this, I'd probably bet on struct rtx_jump_insn as the better option.

Any thoughts?

Thanks!

  Andrea