[Bf-cycles] RFC: Deprecate and remove ssef/ssei/sseb ?

Sergey Sharybin sergey.vfx at gmail.com
Thu Mar 23 20:11:41 CET 2017

Hey everyone,

This topic is inspired by annoyance of having both float{3,4} and ssef data
types in Cycles. For a long time there was a good reason for that: we did
not have any vectorization on float3/4 operations because that was causing
rendering slowdown. But since Blender 2.78b we've got global SSE
optimization enabled on AVX and AVX2 kernels and to my knowledge we can
enable it for SSE4.1 kernels. This causes some redundancy and causes the
following issues:

- There is now two almost matched code bases: one is vectorization of
flaot3/4 and other one is ssef
- Such duplication is increasing risk of two code bases diverging from each
other: we can fix bug in one of the code paths but not in another.
- it is not really clear now whether someone need to prefer ssef over
float4 for his optimzied code.
- This often causes avoidable duplicated code paths which are ifdef-ed in
the kernel (one using float4 and other one using ssef).

Similar notes applies on ssei and sseb as well.

I think it makes sense to prefer float3/4 nowadays (and their integer and
boolean analogs) nowadays and retire sse{f,i,b} implementations. This will
definitely avoid confusion about what data type to use for the new code and
avoid having vectorization code implemented twice. There is no so many
places where this types are used in the kernel and in most cases it's quite
trivial to replace with float4 directly.

However, there are following downsides:

- We'll need to support some vectorization instructions on float4, for
example, len_squared<>(ssef).

This is quite trivial job, just needs to be done with care. Not so much of
an issue.

- In most cases ssef is passed as constant reference.

This is a bit more tricky. From experiments, passing float4 to a
force-inlined function does not always avoid copy-constructor from being
called. This is giving issues in Pluecker intersection code.

Simplest solution here would be to still have code path if-dfefed and keep
constant references in there. This wouldn't allow us to merge GPU and CPU
code paths easily but will get us free from redundant classes without
performance loss.

Introduction of constant references we'll need to raise anyway. The only
stopper here is OpenCL which does not have those. Crazy approach could be
to have ccl_ref macro, so we can write foo(const float4 ccl_ref bar)
(similar to ccl_restrict). This will allow us to merge some codepaths
between CPU and GPU and avoid unwanted copy-constructor overhead on CPU.

Perhaps this constant reference topic we can save for later and solve
issues one by one.

The mail is getting too long now, so let me ask this: what do you guys
think? Does this ssef to float4 replacement makes sense? Do i miss
something and we still need to have sse types?

With best regards, Sergey Sharybin
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