[Mesa-dev] Backends and support for pow-instructions

[Marek Olšák]

All Gallium drivers must support POW, but some drivers like r300-r500
fragment shaders lower it to LG2+MUL+EX2.

Marek

On Sun, Jul 13, 2014 at 7:50 PM, Thomas Helland
<thomashelland90 at gmail.com> wrote:
> Hi all,
>
> I've been looking at some shaders from Portal recently.
> A lot of them seem to hand-roll a pow-function with
> log2, multiply, and exp2, like this:
>
> r3.x = log2( r3.x );
> r3.y = log2( r3.y );
> r3.z = log2( r3.z );
> r3.xyz = r3.xyz * vd2.zzz;
> r3.x = exp2( r3.x );
> r3.y = exp2( r3.y );
> r3.z = exp2( r3.z );
>
> The corresponding output from the shader-db run shows:
>
>    (assign  (xyz) (var_ref r3)  (expression vec3 log2 (swiz xyz (var_ref r3)
> )) )
> 0x000003c8: math log(8)     g27<1>.xyzF     g27<4,4,1>.xyzzF null
> { align16 WE_normal 1Q compacted };
>    (assign  (xz) (var_ref r6)  (swiz yy (array_ref (var_ref vc) (constant
> int (0)) ) ))
> 0x000003d0: mov(8)          g25<1>.xzF      g1<0,4,1>.yF
> { align16 WE_normal 1Q };
>    (assign  (w) (var_ref r0)  (expression float + (expression float dot
> (swiz xyz (array_ref (var_ref vc) (constant int (28)) ) )(expression vec3
> neg (swiz xyz (var_ref r5) )) ) (expression float neg (swiz z (array_ref
> (var_ref vc) (constant int (30)) ) )) ) )
> 0x000003e0: dp3(8)          g105<1>.xF      g5.4<0,4,1>.xyzzF
> -g26<4,4,1>.xyzzF { align16 WE_normal 1Q };
>    (assign  (xyz) (var_ref r0)  (expression vec3 + (expression vec3 * (swiz
> xyz (var_ref r0) )(expression vec3 neg (swiz xxx (var_ref
> inversesqrt_retval) )) ) (expression vec3 neg (swiz xyz (array_ref (var_ref
> vc) (constant int (28)) ) )) ) )
> 0x000003f0: mad(8)          g30<1>.xyzF     g108<4,4,1>F.xyzz
> g30<4,4,1>F.xyzz -g97<4,4,1>F.x { align16 WE_normal 1Q };
>    (assign  (xyz) (var_ref r6)  (expression vec3 * (swiz yxx (var_ref r7)
> )(swiz xyz (var_ref r6) )) )
> 0x00000400: mul(8)          g25<1>.xyzF     g24<4,4,1>.yxxxF
> g25<4,4,1>.xyzzF { align16 WE_normal 1Q };
>    (assign  (xyz) (var_ref r3)  (expression vec3 * (swiz xyz (var_ref r3)
> )(constant vec3 (2.200000 2.200000 2.200000)) ) )
> 0x00000410: mul(8)          g27<1>.xyzF     g27<4,4,1>.xyzzF 2.2F
> { align16 WE_normal 1Q };
>    (assign  (w) (var_ref r0)  (expression float + (expression float dot
> (swiz xyz (array_ref (var_ref vc) (constant int (28)) ) )(expression vec3
> neg (swiz xyz (var_ref r5) )) ) (expression float neg (swiz z (array_ref
> (var_ref vc) (constant int (30)) ) )) ) )
> 0x00000420: add(8)          g107<1>.xF      g105<4,4,1>.xF  -g6.4<0,4,1>.zF
> { align16 WE_normal 1Q };
>    (assign  (xyz) (var_ref r0)  (expression vec3 + (expression vec3 * (swiz
> www (array_ref (var_ref vc) (constant int (27)) ) )(swiz xyz (var_ref r0) ))
> (swiz xyz (var_ref r5) )) )
> 0x00000430: mad(8)          g30<1>.xyzF     g26<4,4,1>F.xyzz g5.3<0,1,0>F.w
> g30<4,4,1>F.xyzz { align16 WE_normal 1Q };
>    (assign  (x) (var_ref dot_retval)  (expression float dot (swiz xyz
> (array_ref (var_ref vc) (constant int (31)) ) )(swiz xyz (var_ref r6) )) )
> 0x00000440: dp3(8)          g18<1>.xF       g7<0,4,1>.xyzzF g25<4,4,1>.xyzzF
> { align16 WE_normal 1Q compacted };
>    (assign  (xyz) (var_ref r3)  (expression vec3 exp2 (swiz xyz (var_ref r3)
> )) )
> 0x00000448: math exp(8)     g27<1>.xyzF     g27<4,4,1>.xyzzF null
> { align16 WE_normal 1Q compacted };
>
>
> As far as I've been able to understand, the i965 hardware
> has a pow() instruction, how is this with other hardware?
>
> I've considered writing an algebraic optimization to convert
> this into an ir_binop_pow. If my understanding is correct the backend
> will then implement this in a similar fashion as above if it does not
> have a native pow() instruction.
>
> If, on the other hand, we have a pow() instruction, my guess is
> we'd see reduced instruction-counts.
>
> Is my understanding correct? Is this something that's worth doing?
>
> regards,
> Thomas Helland
>
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