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

Andrea Corallo andrea.corallo@arm.com
Wed Aug 19 17:47:29 GMT 2020


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

> [ Please don't post new patch series as replies to old ]
>
> 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.
>
> Deemed, by whom?  There are several people very against it, too.  You
> need to modify only one simple behaviour (maybe in a handful of places),
> making a new RTX class for that is excessive.

Hi Segher,

That's understood and agreed.  I haven't posted any new patch on this,
the quoted mail is an old one.  I just wanted to discuss how to proceede
this way with my mail of today.

Thanks for your feedback!

  Andrea


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