[Mesa-dev] V3 Loop unrolling in NIR

[Timothy Arceri]

On Thu, 2016-09-15 at 12:28 +0300, Eero Tamminen wrote:
> Hi,
> 
> Have you any plans for supporting partial unrolling?

Not currently no although it shouldn't be too difficult to add this.

> 
> I.e. if the loop count is too large to be completely unrolled, unroll
> it 
> few times (that still fits into instruction cache) and then loop
> that.
> 
> E.g. for a loop with 51 rounds, Mesa could unroll it 4 rounds, loop
> that 
> 12 times and unroll (or loop) remaining 3 rounds separately.
> 
> 
> 	- Eero
> 
> On 15.09.2016 10:03, Timothy Arceri wrote:
> > 
> > Big thanks to Connor for his feedback on previous versions, and
> > to Jason for answering my all my nir questions.
> > 
> > This series works on ssa defs so for now it's only enabled for
> > the scalar backend on Gen7+.
> > 
> > V3:
> > - So called complex loop unrolling has been implemented.
> > - An instruction limit and rules from the GLSL IR pass to override
> >  the limit for unrolling have been implemented.
> > - Lots of other stuff see individual patches.
> > 
> > total instructions in shared programs: 8488940 -> 8488648 (-0.00%)
> > instructions in affected programs: 48903 -> 48611 (-0.60%)
> > helped: 68
> > HURT: 89
> > 
> > Most of this HURT comes for switching to using
> > nir_lower_indirect_derefs(). See patch 1 for more deals.
> > 
> > total cycles in shared programs: 69787006 -> 69758740 (-0.04%)
> > cycles in affected programs: 2525708 -> 2497442 (-1.12%)
> > helped: 900
> > HURT: 919
> > 
> > total loops in shared programs: 2071 -> 1499 (-27.62%)
> > loops in affected programs: 687 -> 115 (-83.26%)
> > helped: 655
> > HURT: 99
> > 
> > Helped here comes from a number of things. One example is the
> > nir pass is better than the GLSL pass at unrolling loops
> > regardless of which terminator has the lowest limit. We could
> > easily go further and handle unrolling of loops with complex
> > terminators e.g the ifs then or else blocks contain instructions
> > currently we just bail if they are not empty, I still need to
> > check if its worth while.
> > 
> > Another reason could be that I've set the instruction limit too
> > high but that doesn't seem to be the case.
> > 
> > I believe 82/99 of the HURT is from shaders that look something
> > like this:
> > 
> >   vec2 array[const_size_of_array];
> >   for (i = 0; i < const_size_of_array; i++) {
> >     ...  = array[i];
> > 
> >     ... lots of instructions (more that the unroll limit) ...
> >   }
> > 
> > The GLSL IR pass would force this to unroll as long as
> > const_size_of_array
> > wasn't greater than 32. However by the time we get to the nir pass
> > the
> > arrays have been removed, it seems like this may only be happening
> > for
> > vectors but I haven't looked into what is causing it yet.
> > 
> > The other 17 shaders seem to be various corner cases that can be
> > fixed
> > in folow-up patches.
> > 
> > total spills in shared programs: 2212 -> 2212 (0.00%)
> > spills in affected programs: 0 -> 0
> > helped: 0
> > HURT: 0
> > 
> > total fills in shared programs: 1891 -> 1891 (0.00%)
> > fills in affected programs: 0 -> 0
> > helped: 0
> > HURT: 0
> > 
> > LOST:   6
> > GAINED: 32
> > 
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> > 
> 
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