[Mesa-dev] Suboptimal code generation

[Henri Verbeet]

On 14 November 2014 18:50, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> I can't speak for the radeon guys, but I know I sure would love to see
> any reports of poor code being generated by nouveau in response to
> legitimate-seeming TGSI (or GLSL). In some cases, a simple
> optimization can be added to take care of it, and I'd definitely
> appreciate the extra pair of eyeballs on driver-generated code :)
>
> The report can be as simple as "here is the TGSI snippet, take a look
> at how crappy the code it generates is". At least for nouveau, I can
> feed that directly into a compiler that can target any of the relevant
> backends.
>
> [Note, r600g didn't have an optimizer enabled until ~1y ago; not sure
> if your analysis was with or without sb.]
>
It was with sb, but probably before TGSI got FSLT/FSGE/etc.

For reference, what currently happens for r600g is something like this:

D3D:
    cnd r[0], r[0].w, c[1], c[2]

GLSL:
    R0.xyzw = (R0.w > 0.5 ? ps_c[1].xyzw : ps_c[2].xyzw);

TGSI:
    FSLT TEMP[0].x, IMM[0].xxxx, TEMP[0].xxxx
    UIF TEMP[0].xxxx :0
      MOV TEMP[0], CONST[1]
    ELSE :0
      MOV TEMP[0], CONST[2]
    ENDIF

R600:
    SETGE_DX10         T0.x,  0.5, T0.x
    CNDE_INT           R0.x,  T0.x, KC0[1].x, KC0[2].x
    CNDE_INT           R0.y,  T0.x, KC0[1].y, KC0[2].y
    CNDE_INT           R0.z,  T0.x, KC0[1].z, KC0[2].z
    CNDE_INT           R0.w,  T0.x, KC0[1].w, KC0[2].w

While ideally that would just be 4 CNDGE's, that's better than what I
remember. IIRC there used to be a bunch of int/float conversions as
well.