Re: [RFC 3/3] drm/i915: Micro-optimize i915_gem_obj_to_vma

[Dave Gordon <david.s.gordon@intel.com>]

On 21/04/16 13:05, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>
> i915_gem_obj_to_vma is one of the most expensive functions in
> our profiles. Could avoiding some branching by replacing it
> with arithmetic be beneficial? Some benchmarks suggest it
> slightly might.
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 0549dea683e1..243bfb922eb3 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4642,11 +4642,21 @@ struct i915_vma *i915_gem_obj_to_vma(struct drm_i915_gem_object *obj,
>   				     struct i915_address_space *vm)
>   {
>   	struct i915_vma *vma;
> +
> +	BUILD_BUG_ON(I915_GGTT_VIEW_NORMAL != 0);
> +
>   	list_for_each_entry(vma, &obj->vma_list, obj_link) {
> -		if (vma->ggtt_view.type == I915_GGTT_VIEW_NORMAL &&
> -		    vma->vm == vm)
> +		/*
> +		 * Below is just a branching avoiding way of saying:
> +		 * vma_ggtt_view.type == I915_GGTT_VIEW_NORMAL && vma->vm == vm,
> +		 * which relies on the fact I915_GGTT_VIEW_NORMAL has to be
> +		 * zero.
> +		 */
> +		if (!((unsigned long)vma->ggtt_view.type |
> +		    ((unsigned long)vma->vm ^ (unsigned long)vm)))
>   			return vma;
>   	}
> +
>   	return NULL;
>   }

Other alternatives might include splitting the vma_list, so that we have 
one list for the most-frequently searched-for entries (GGTT view NORMAL) 
and for everything else, so the above would just need a single test for 
equality.

Or, slightly less effectively, add GGTT/NORMAL entries at the head of 
the list and others at the tail (and search backwards if you *don't* 
want a GGTT/NORMAL entry). That would still need the comparisons, but 
would likely hit an early match.

.Dave.
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