[Mesa-dev] TGSI texture offset handling

[Ian Romanick]

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On 08/26/2011 09:26 AM, Dave Airlie wrote:
> I've involuntarily started looking into this when I tried to add TXF
> support, so we need to expand TGSI to add support for texture offsets.
> 
> Requirements:
> the extreme requirement so far is for textureGatherOffsets which can
> accept 4 x ivec2, the normal textureFetchOffset can accept an ivec3.
> The hardware seems to only tolerate -8..+7 ranges, but the interface
> is quite open and as Brian pointed out its queryable.

It's queryable, but nobody supports anything other than [-8, 7].

> So adding extra src regs doesn't seem like the right answer going
> forward (my last TXF patch did that). It really looks like expanding
> the tgsi_instruction_texture token is the best plan, but it really
> doesn't have the space to store 4 * ivec2 in it. Adding a new
> tgsi_instruction_offset token with the values in it doesn't really
> work so well either, since the GLSL compiler gives us ir_constant and
> we parse those into the immediates file.
> 
> My current plan looks like
> 
> struct tgsi_instruction_texture
> {
>    unsigned Texture  : 8;    /* TGSI_TEXTURE_ */
>    unsigned OffsetNum : 4;
>    unsigned OffsetFile : 4;
> };
> 
> struct tgsi_instruction_offset
> {
>    int Index:16;
>    unsigned SwizzleX : 2;  /* TGSI_SWIZZLE_x */
>    unsigned SwizzleY : 2;  /* TGSI_SWIZZLE_x */
>    unsigned SwizzleZ : 2;  /* TGSI_SWIZZLE_x */
> };

What's the advantage of this (combined with
tgsi_instruction_texture::OffsetFile) versus just encoding some number
of pairs of small, signed integers?  Each offset is a signed 4-bit
value, so you can pack all four pairs in a single 32-bit value (it's
almost like the hardware guys planned it that way).

If I had to guess, I'd wager that if the ranges change, they will change
to be arbitrary, non-constant values.  Adding flexibility to have larger
constant values seem unlikely to have future payoff.

> Where OffsetFile will most likely be always the IMMEDIATE file, then
> we just add 4 tgsi_instruction_offset to tgsi_full_instruction and go
> from there.
> 
> Anyone see any problems with this? I'm sure someone will have an
> alternate method that I've tried today and given up on.
> 
> http://people.freedesktop.org/~cbrill/dri-log/index.php?date=2011-08-26&channel=dri-devel&highlight_names=&date=2011-08-26
> has some of the other ideas myself and calim have been back and
> forwarding on today.
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