[Mesa-dev] [PATCH 1/1] nir: Use a freelist in nir_opt_dce to avoid spamming ralloc

[Eric Anholt]

Thomas Helland <thomashelland90 at gmail.com> writes:

> Also, allocate worklist_elem in groups of 20, to reduce the burden of 
> allocation. Do not use rzalloc, as there is no need. This lets us drop 
> the number of calls to ralloc from aproximately 10% of all calls to 
> ralloc(130 000 calls), down to a mere 2000 calls to ralloc_array_size. 
> This cuts the runtime of shader-db by 1%, while at the same time 
> reducing the number of stalled cycles, executed cycles, and executed 
> instructions by about 1 % as reported by perf. I did a five-run 
> benchmark pre and post and got a statistical variance less than 0.1% pre 
> and post. This was with i965's ir validation polluting the benchmark, so 
> the numbers are even better in release builds.
>
> Performance change as found with perf-diff:
> 4.74%     -0.23%  libc-2.26.so            [.] _int_malloc
> 1.88%     -0.21%  libc-2.26.so            [.] malloc
> 2.27%     +0.16%  libmesa_dri_drivers.so  [.] match_value.part.7
> 2.95%     -0.12%  libc-2.26.so            [.] _int_free
>           +0.11%  libmesa_dri_drivers.so  [.] worklist_push
> 1.22%     -0.08%  libc-2.26.so            [.] malloc_consolidate
> 0.16%     -0.06%  libmesa_dri_drivers.so  [.] mark_live_cb
> 1.21%     +0.06%  libmesa_dri_drivers.so  [.] match_expression.part.6
> 0.75%     -0.05%  libc-2.26.so            [.] cfree at GLIBC_2.2.5
> 0.50%     -0.05%  libmesa_dri_drivers.so  [.] ralloc_size
> 0.57%     +0.04%  libmesa_dri_drivers.so  [.] nir_replace_instr
> 1.29%     -0.04%  libmesa_dri_drivers.so  [.] unsafe_free

I'm curious, since a NIR instruction worklist seems like a generally
useful thing to have:

Could nir_worklist.c keep the implementation of this?

Also, I wonder if it wouldn't be even better to have a u_dynarray of
instructions in the worklist, with push/pop on the end of the array, and
a struct set tracking the instructions in the array to avoid
double-adding.  I actually don't know if that would be better or not, so
I'd be happy with the worklist management just moved to nir_worklist.c.
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