Re: [PATCH] ARC: ARCv2: jump label: implement jump label patching

From: Peter Zijlstra
Date: Thu Jun 20 2019 - 03:20:37 EST

On Wed, Jun 19, 2019 at 11:55:41PM +0000, Vineet Gupta wrote:

> FWIW I tried to avoid all of this by using the 2 byte NOP_S and B_S variants which
> ensures we can never straddle cache line so the alignment issue goes away. There's
> a nice code size reduction too - see [1] . But I get build link errors in
> networking code around DO_ONCE where the unlikely code is too much and offset
> can't be encoded in signed 10 bits which B_S is allowed.

Yeah, so on x86 we have a 2 byte and a 5 byte relative jump and have the
exact same issue. We're currently using 5 byte jumps unconditionally for
the same reason.

Getting it to use the 2 byte one where possible is a 'fun' project for
someone with spare time at some point. It might need a GCC plugin to
pull off, I've not put too much tought into it.