[Mesa-dev] Optimizations in mesa shader compiler

[Brian Paul]

On 07/13/2010 05:46 PM, Segovia, Benjamin wrote:
> To be more precise,
> - The shader compiler present in src/mesa/slang is used.
>
> - The glsl shader (randomly taken on the net) is:
> void main(){
>    const float PI = 3.14159265358979323846264;
>    const float angle = 45.0;
>    const float rad_angle = angle*PI/180.0;
>    vec4 a = gl_Vertex;
>    vec4 b = a;
>
>    b.x = a.x*cos(rad_angle) - a.y*sin(rad_angle);
>    b.y = a.y*cos(rad_angle) + a.x*sin(rad_angle);
>    gl_Position = gl_ModelViewProjectionMatrix*b;
> }
>
> - IR Output produced is:
> # Vertex Program/Shader 1
>    0: MOV TEMP[0].x, CONST[0].xxxx;
>    1: MUL TEMP[1].x, CONST[0].yyyy, TEMP[0].xxxx;
>    2: RCP TEMP[1].z, CONST[0].zzzz;
>    3: MUL TEMP[0].w, TEMP[1].xxxx, TEMP[1].zzzz;
>    4: MOV TEMP[0].z, TEMP[0].wwww;
>    5: MOV TEMP[1].xz, INPUT[0];
>    6: MOV TEMP[1].xzw, INPUT[0];
>    7: COS TEMP[0].w, TEMP[0].wwww;
>    8: MUL TEMP[2].x, INPUT[0].xxxx, TEMP[0].wwww;
>    9: SIN TEMP[0].w, TEMP[0].zzzz;
>   10: MUL TEMP[2].y, INPUT[0].yyyy, TEMP[0].wwww;
>   11: SUB TEMP[1].x, TEMP[2].xxxx, TEMP[2].yyyy;
>   12: COS TEMP[2].x, TEMP[0].zzzz;
>   13: SIN TEMP[2].y, TEMP[0].zzzz;
>   14: MUL TEMP[2].z, INPUT[0].xxxx, TEMP[2].yyyy;
>   15: MAD TEMP[2].y, INPUT[0].yyyy, TEMP[2].xxxx, TEMP[2].zzzz;
>   16: MUL TEMP[0], STATE[2], TEMP[2].yyyy;
>   17: MAD TEMP[2], STATE[1], TEMP[1].xxxx, TEMP[0];
>   18: MAD TEMP[0], STATE[3], TEMP[1].zzzz, TEMP[2];
>   19: MAD TEMP[2], STATE[4], TEMP[1].wwww, TEMP[0];
>   20: MOV OUTPUT[0], TEMP[2];
>   21: END
> */
> /* Parameters / constants */
> /*
> dirty state flags: 0x3
> param[0] sz=3 CONST (null) = {3.14, 45, 180, 3.14}
> param[1] sz=4 STATE state.matrix.mvp.transpose.row[0] = {0, 0, 0, 0}
> param[2] sz=4 STATE state.matrix.mvp.transpose.row[1] = {0, 0, 0, 0}
> param[3] sz=4 STATE state.matrix.mvp.transpose.row[2] = {0, 0, 0, 0}
> param[4] sz=4 STATE state.matrix.mvp.transpose.row[3] = {0, 0, 0, 0}
> */
>
> - Finally, Gen code also includes the constant computations in the shader.
>
> Ben
>
> ________________________________________
> From: mesa-dev-bounces+benjamin.segovia=intel.com at lists.freedesktop.org [mesa-dev-bounces+benjamin.segovia=intel.com at lists.freedesktop.org] On Behalf Of Segovia, Benjamin [benjamin.segovia at intel.com]
> Sent: Tuesday, July 13, 2010 4:11 PM
> To: mesa-dev at lists.freedesktop.org
> Subject: [Mesa-dev] Optimizations in mesa shader compiler
>
> Hello all,
>
> I just looked at the (intermediate) code produced by the shader compiler. In the last version, I did not find anything on constant propagation and constant computations. In the Intel current driver, the final gen code produces an ugly sequence of computations of constant values (known at compile time). I would like to spend some time on it but I am wondering if it is still supported or if other shader compilers are going to replace it.


Hi Ben,

The current Mesa GLSL compiler (slang) does some constant expression 
simplification, but not sin, cos, etc.  The code in question is in the 
slang_simplify.c file if you want to take a look.

In the mean time, Eric Anholt and Ian Romanick of Intel are working on 
a new compiler for Mesa.  See the glsl2 branch in git.  Maybe Ian or 
Eric can comment on this area of their compiler.  Or maybe you can 
jump in and help them with the new code.

-Brian