[Mesa-dev] [RFC PATCH] nir: Add byte_extract operation.

Eric Anholt eric at anholt.net
Wed Dec 2 16:40:49 PST 2015

Matt Turner <mattst88 at gmail.com> writes:

> Two shaders that appear in Unigine benchmarks (Heaven and Valley) unpack
> three bytes from an integer and convert each into a float:
>    float((val >> 16u) & 0xffu)
>    float((val >>  8u) & 0xffu)
>    float((val >>  0u) & 0xffu)
> Instead of shifting, masking, and type converting like this:
>    shr(8)          g15<1>UD        g25<8,8,1>UD    0x00000010UD
>    and(8)          g16<1>UD        g15<8,8,1>UD    0x000000ffUD
>    mov(8)          g17<1>F         g16<8,8,1>UD
>    shr(8)          g18<1>UD        g25<8,8,1>UD    0x00000008UD
>    and(8)          g19<1>UD        g18<8,8,1>UD    0x000000ffUD
>    mov(8)          g20<1>F         g19<8,8,1>UD
>    and(8)          g21<1>UD        g25<8,8,1>UD    0x000000ffUD
>    mov(8)          g22<1>F         g21<8,8,1>UD
> i965 can simply extract a byte and convert to float in a single
> instruction:
>    mov(8)          g17<1>F         g25.2<16,4,4>UB
>    mov(8)          g20<1>F         g25.1<16,4,4>UB
>    mov(8)          g22<1>F         g25.0<16,4,4>UB
> Decreases the number of instructions and cycles in the two programs by:
>  #1706: 3728 -> 3363 instructions (-9.79%), 9594 -> 9180 cycles (-4.32%)
>  #1721: 4027 -> 3662 instructions (-9.06%), 10264 -> 9572 cycles (-6.74%)

I've been handling my byte extracts by special cases in ibfe/ubfe.
Would it make sense to recognize these shifts and turn them into bfes,
then special case ibfe/ubfe in the driver to emit simpler things when

extract_byte is one case of extraction I need to do all the time, and
the other is aligned *signed* 16-bit fields.

If we do want extract_byte, then I'll just need vc4_program.c's
translation updated at the same time as this lands.  I think it should
be a pretty obvious variant from your i965 code, combined with
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