[Intel-gfx] [PATCH] drm/i915: Optimise VMA lookup slightly

[Tvrtko Ursulin]

On 13/12/2016 12:41, Chris Wilson wrote:
> On Tue, Dec 13, 2016 at 12:22:18PM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> A few details to hopefully make a very hot function a tiny bit
>> more efficient:
>>
>>  1. Cast VM pointers before substraction to save the compiler
>>     doing a smart one which includes multiplication.
>
> Indeed. Not pretty though.
>
> static always_inline __kernel_ptrdiff_t ptrdiff(const void *a, const void *b)
> {
> 	return a - b;
> }
>
> cmp = ptrdiff(vma->vm, vm);
> if (cmp)
> 	return cmp;

Pffifle as you would say. :)

>>  2. Use smaller type for comparison since we only care about
>>     the sign.
>
> Should be a no-op since the compiler also should only care about the
> sign and not be moving the registers about, just the cc and we should be
> inlining... Is gcc not smart enough? :(

I am pretty sure there was a significant difference in the size of 
generated code so apparently it isn't.

>>  3. Prefer the ppgtt lookup branch and inline it, allowing the
>>     compiler to optimise out the second part of i915_vma_compare
>>     and save one call indirection.
>
> This runs counter to a better optimisation that completely avoids
> calling obj_to_vma for ppgtt lookups (i.e. in execbuffer we go straight
> from handle to vma, skipping the handle to obj intermediate lookup).
>
> Primary caller for this function should be ggtt users, with single
> negative lookups before creating the ppgtt vma.

Empirically ppgtt users call this much much more on any benchmarks I've 
tried since with the change all I see is lookup_and_create in the 
profiles and not obj_to_vma.

Regards,

Tvrtko