[Mesa-dev] [PATCH 1/2] nvc0/ir: detect AND/SHR pairs and convert into EXTBF

[Ilia Mirkin]

On Tue, Aug 18, 2015 at 9:57 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Tue, Aug 18, 2015 at 6:49 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> Some shaders appear to extract bits using shift/and combos. Detect
>> (some) of those and convert to EXTBF instead.
>
> What is EXTBF? Extract byte to float?

Extract Bitfield.

>
> I ask because Unigine Heaven has shaders that pack 3x byte-integers
> into one component of a vec4 and extracts them with shifts/ands and
> converts them to floats, and i965 could do the extraction and
> conversion in a single instruction. I'm curious if this is the same
> thing you're optimizing.
>
> I thought about adding an extract_byte(src, byte_num) operation, but
> i965's copy propagation caused me some headache and I shelved it.

Yes, I think it's the same shader... it's doing a texelFetch() and
then grabbing bytes 0, 1, 2 off that.

The generated shader code after the second patch does:

        /*05d0*/                   TLD.LL.P R0, R24, 0x0, 2D, 0x3;
        /*05d8*/                   TEXDEPBAR 0x0;
        /*05e0*/                   I2F.F32.U8 R2, R1;
        /*05e8*/                   FFMA.FTZ R2, R2, R15, R19;
        /*05f0*/                   I2F.F32.U8 R8, R1.B1;
        /*05f8*/                   FFMA.FTZ R8, R8, R15, R19;
        /*0608*/                   I2F.F32.U8 R1, R1.B2;

I'll let you guess what these things mean. TLD = texelfetch :)

  -ilia