Re: [PATCH v4 00/10] Optimize buffer_is_zero

[Richard Henderson]

On 2/14/24 22:57, Alexander Monakov wrote:
> On Wed, 14 Feb 2024, Richard Henderson wrote:
>
> > v3: 20240206204809.9859-1-amonakov@ispras.ru/">https://patchew.org/QEMU/20240206204809.9859-1-amonakov@ispras.ru/
> >
> > Changes for v4:
> >    - Keep separate >= 256 entry point, but only keep constant length
> >      check inline.  This allows the indirect function call to be hidden
> >      and optimized away when the pointer is constant.
>
> Sorry, I don't understand this. Most of the improvement (at least in our
> testing) comes from inlining the byte checks, which often fail and eliminate
> call overhead entirely. Moving them out-of-line seems to lose most of the
> speedup the patchset was bringing, doesn't it? Is there some concern I am
> not seeing?

What is your benchmarking method?

It was my guess that most of the improvement came from performing those early byte checks 
*at all*, and that the overhead of a function call to a small out of line wrapper would be 
negligible.

By not exposing the function pointer outside the bufferiszero translation unit, the 
compiler can see when the pointer is never modified for a given host, and then transform 
the indirect branch to a direct branch.


r~