[Mesa-dev] [PATCH 00/10] glsl: Implement varying packing.

Paul Berry stereotype441 at gmail.com
Thu Dec 13 12:40:37 PST 2012

On 13 December 2012 03:44, Ian Romanick <idr at freedesktop.org> wrote:

> Patches 1 through 4 are
> Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>
> I'll try to make it through the rest tomorrow.  I have skimmed them, and
> it looks mostly okay.  I thing a good follow-up patch will be to pull a
> bunch of the new stuff out to a new file link_varyings.cpp or something.
>  linker.cpp is getting a bit out of control.  ~2700 lines in one file...

Yeah, that seems reasonable, especially since there's additional varying
packing work to do this series.  I'll do that after the series lands.

> On 12/11/2012 03:09 PM, Paul Berry wrote:
>> This patch series adds varying packing to Mesa, so that we can handle
>> varyings composed of things other than vec4's without using up extra
>> varying components.
>> For the initial implementation I've chosen a strategy that operates
>> exclusively at the GLSL IR level, so that it doesn't require the
>> cooperation of the driver back-ends.  This means that varying packing
>> should be immediately useful for all drivers.  However, there are some
>> types of varying packing that can't be done using GLSL IR alone (for
>> example, packing a "noperspective" varying and a "smooth" varying
>> together), but should be possible on some drivers with a small amount
>> of back-end work.  I'm deferring that work for a later patch series.
>> Also, packing of floats and ints together into the same "flat varying"
>> should be possible for drivers that implement
>> ARB_shader_bit_encoding--I'm also deferring that for a later patch
>> series.
>> The strategy is as follows:
>> - Before assigning locations to varyings, we sort them into "packing
>>    classes" based on base type and interpolation mode (this is to
>>    ensure that we don't try to pack floats with ints, or smooth with
>>    flat, for example).
>> - Within each packing class, we sort the varyings based on the number
>>    of vector elements.  Vec4's (as well as matrices and arrays composed
>>    of vec4's) are packed first, then vec2's, then scalars, since this
>>    allows us to align them all to their natural alignment boundary, so
>>    we avoid the performance penalty of "double parking" a varying
>>    across two varying slots.  Vec3's are packed last, double parking
>>    them if necessary.
>> - For any varying slot that doesn't contain exactly one vec4, we
>>    generate GLSL IR to manually pack/unpack the varying in the shader.
>>    For instance, the following fragment shader:
>>    varying vec2 a;
>>    varying vec2 b;
>>    varying vec3 c;
>>    varying vec3 d;
>>    main()
>>    {
>>      ...
>>    }
>>    would get rewritten as follows:
>>    varying vec4 packed0;
>>    varying vec4 packed1;
>>    varying vec4 packed2;
>>    vec2 a;
>>    vec2 b;
>>    vec3 c;
>>    vec3 d;
>>    main()
>>    {
>>      a = packed0.xy;
>>      b = packed0.zw;
>>      c = packed1.xyz;
>>      d.x = packed1.w; // d is "double parked" across slots 1 and 2
>>      d.yz = packed2.xy;
>>      ...
>>    }
>>    This GLSL IR is generated by a lowering pass, so that in the future
>>    we will have the option of disabling it for driver back-ends that
>>    are capable of natively understanding the packed varying format.
>> - Finally, the linker code to handle transform feedback is modified to
>>    account for varying packing (e.g. by feeding back just a subset of
>>    the components of a varying slot rather than the entire varying
>>    slot).  Fortunately transform feedback already has the
>>    infrastructure necessary to do this, since it was needed in order to
>>    implement glClipDistance.
>> I believe this is enough to be useful for the vast majority of
>> programs, and to get us passing the GLES3 conformance tests.
>> Additional improvements, which I'm planning to defer to later patch
>> series, include:
>> - Allow uints and ints to be packed together in the same varying slot.
>>    This should be possible on all back-ends, since ints and uints may
>>    be interconverted without losing information.
>> - On back-ends that support ARB_shader_bit_encoding, allow floats and
>>    ints to be packed together in the same varying slot, since
>>    ARB_shader_bit_encoding allows floating-point values to be encoded
>>    into ints without losing information.
>> - On back-ends that can mix interpolation modes within a single
>>    varying slot, allow additional packing, with help from the driver
>>    back-end.  For instance, i965 gen6 and above can in principle mix
>>    together all interpolation modes except for "flat" within a single
>>    varying slot, if we do a hopefully small amount of back-end work.
>> - Allow a driver back-end to advertise a larger number of varying
>>    components to the linker than it advertises to the client
>>    program--this will allow us to ensure that varying packing *never*
>>    fails.  For example, on i965 gen6 and above, after the above
>>    improvements are made, we should be able to pack any possible
>>    combination of varyings with a maximum waste of 3 varying
>>    components.  That means, for example, that if the i965 driver
>>    advertises 17 varying slots to the linker (== 68 varying
>>    components), but advertises only 64 varying components to the the
>>    client program, then varying packing will always succeed.
>> Note: I also have a new piglit test that exercises this code; I'll be
>> publishing that to the Piglit list ASAP.
>> [PATCH 01/10] glsl/lower_clip_distance: Update symbol table.
>> [PATCH 02/10] glsl/linker: Always invalidate shader ins/outs, even in
>> corner cases.
>> [PATCH 03/10] glsl/linker: Make separate ir_variable field to mean
>> "unmatched".
>> [PATCH 04/10] glsl: Create a field to store fractional varying locations.
>> [PATCH 05/10] glsl/linker: Defer recording transform feedback locations.
>> [PATCH 06/10] glsl/linker: Subdivide the first phase of varying
>> assignment.
>> [PATCH 07/10] glsl/linker: Sort varyings by packing class, then vector
>> size.
>> [PATCH 08/10] glsl: Add a lowering pass for packing varyings.
>> [PATCH 09/10] glsl/linker: Pack within compound varyings.
>> [PATCH 10/10] glsl/linker: Pack between varyings.
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