Re: [patch 1/3] move WARN_ON() out of line

[Jeremy Fitzhardinge]

Arjan van de Ven wrote:
> This patch moves WARN_ON() out of line entirely. I've considered keeping
> the test inline and moving only the slowpath out of line, but I decided
> against that: an out of line test reduces the pressure on the CPUs
> branch predictor logic and gives smaller code, while a function call
> to a fixed location is quite fast. Likewise I've considered doing
> something
> similar to BUG() (eg use a trapping instruction) but that's not really
> better (it needs the test inline again and recovering from an invalid
> instruction isn't quite fun).

Power implements WARN_ON this way, and all the machinery is in place to
generically implement WARN_ON that way if you want.  It does generate
denser code than the call (since its just a single trapping instruction
with no need for argument setup), and the performance cost of the trap
shouldn't matter if warnings are rare (which one would hope).

    J
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