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This document contains some examples of disassembled shaders.
For details of the shader ISA see `rnndb/isa.xml`.
Vertex shader examples
========================
Basic vertex shader
--------------------
uniform mat4 modelviewMatrix;
uniform mat4 modelviewprojectionMatrix;
uniform mat3 normalMatrix;
attribute vec4 in_position;
attribute vec3 in_normal;
attribute vec4 in_color;
vec4 lightSource = vec4(2.0, 2.0, 20.0, 0.0);
varying vec4 vVaryingColor;
void main()
{
gl_Position = modelviewprojectionMatrix * in_position;
vec3 vEyeNormal = normalMatrix * in_normal;
vec4 vPosition4 = modelviewMatrix * in_position;
vec3 vPosition3 = vPosition4.xyz / vPosition4.w;
vec3 vLightDir = normalize(lightSource.xyz - vPosition3);
float diff = max(0.0, dot(vEyeNormal, vLightDir));
vVaryingColor = vec4(diff * in_color.rgb, 1.0);
}
Becomes:
MOV t3.xy, void, void, u4.wwww
MOV t3.z, void, void, u5.wwww
MUL t4, u0, t0.xxxx, void
MAD t4, u1, t0.yyyy, t4
MAD t4, u2, t0.zzzz, t4
MAD t4, u3, t0.wwww, t4
MUL t5.xyz, u4.xyzz, t1.xxxx, void
MAD t5.xyz, u5.xyzz, t1.yyyy, t5.xyzz
MAD t1.xyz, u6.xyzz, t1.zzzz, t5.xyzz
MUL t5, u7, t0.xxxx, void
MAD t5, u8, t0.yyyy, t5
MAD t5, u9, t0.zzzz, t5
MAD t0, u10, t0.wwww, t5
RCP t1.w, void, void, t0.wwww
MAD t0.xyz, -t0.xyzz, t1.wwww, t3.xyzz
DP3 t3.xyz, t0.xyzz, t0.xyzz, void
RSQ t3.xyz, void, void, t3.xxxx
MUL t0.xyz, t0.xyzz, t3.xyzz, void
DP3 t0.x, t1.xyzz, t0.xyzz, void
SELECT.LT t0.x, u6.wwww, t0.xxxx, u6.wwww
MUL t0.xyz, t0.xxxx, t2.xyzz, void
MOV t0.w, void, void, u11.xxxx
ADD t4.z, t4.zzzz, void, t4.wwww
MUL t4.z, t4.zzzz, u11.yyyy, void
Uniform mapping:
u0 - u3 modelviewprojectionMatrix
u4.xyz - u6.xyz normalMatrix
u7 - u10 modelviewMatrix
u11.xy 1.0, 0.5 (constants used internally)
u4.w - u6.w 2.0, 20.0, 0.0 (lightSource components, and a zero)
Vertex shader with texture coordinates
---------------------------------------
uniform mat4 modelviewMatrix;
uniform mat4 modelviewprojectionMatrix;
uniform mat3 normalMatrix;
attribute vec4 in_position;
attribute vec3 in_normal;
attribute vec2 in_coord;
vec4 lightSource = vec4(2.0, 2.0, 20.0, 0.0);
varying vec4 vVaryingColor;
varying vec2 coord;
void main()
{
gl_Position = modelviewprojectionMatrix * in_position;
vec3 vEyeNormal = normalMatrix * in_normal;
vec4 vPosition4 = modelviewMatrix * in_position;
vec3 vPosition3 = vPosition4.xyz / vPosition4.w;
vec3 vLightDir = normalize(lightSource.xyz - vPosition3);
float diff = max(0.0, dot(vEyeNormal, vLightDir));
vVaryingColor = vec4(diff * vec3(1.0, 1.0, 1.0), 1.0);
coord = in_coord;
}
Becomes:
MOV t3.xy, void, void, u4.wwww
MOV t3.z, void, void, u5.wwww
MUL t4, u0, t0.xxxx, void
MAD t4, u1, t0.yyyy, t4
MAD t4, u2, t0.zzzz, t4
MAD t4, u3, t0.wwww, t4
MUL t5.xyz, u4.xyzz, t1.xxxx, void
MAD t5.xyz, u5.xyzz, t1.yyyy, t5.xyzz
MAD t1.xyz, u6.xyzz, t1.zzzz, t5.xyzz
MUL t5, u7, t0.xxxx, void
MAD t5, u8, t0.yyyy, t5
MAD t5, u9, t0.zzzz, t5
MAD t0, u10, t0.wwww, t5
RCP t1.w, void, void, t0.wwww
MAD t0.xyz, -t0.xyzz, t1.wwww, t3.xyzz
DP3 t3.xyz, t0.xyzz, t0.xyzz, void
RSQ t3.xyz, void, void, t3.xxxx
MUL t0.xyz, t0.xyzz, t3.xyzz, void
DP3 t0.x, t1.xyzz, t0.xyzz, void
SELECT.LT t0.xyz, u6.wwww, t0.xxxx, u6.wwww
MOV t0.w, void, void, u11.xxxx
MOV t1.xy, void, void, t2.xyyy
ADD t4.z, t4.zzzz, void, t4.wwww
MUL t4.z, t4.zzzz, u11.yyyy, void
Pixel shaders
==============
Empty (passthrough)
--------------------
precision mediump float;
varying vec4 vVaryingColor;
void main()
{
gl_FragColor = vVaryingColor;
}
Becomes:
NOP void, void, void, void
Texture sampling
------------------
precision mediump float;
varying vec4 vVaryingColor;
varying vec2 coord;
uniform sampler2D in_texture;
void main()
{
gl_FragColor = 3.0 * vVaryingColor * texture2D(in_texture, coord);
}
Becomes:
MUL t1, u0.xxxx, t1, void
TEXLD t2, tex0, t2.xyyy, void, void
MUL t1, t1, t2, void
Clip-space z
--------------
Two instructions are always appended to the vertex shader:
ADD t0.__z_, t0.zzzz, void, t0.wwww
MUL t0.__z_, t0.zzzz, u0.xxxx, void ; u0.x=0.5
This adjusts the output position z, based on w. Likely this works around a difference in interpretation between
the hardware and the OpenGL standard.
For the gc2000 in the i.mx6 these two instructions are no longer appended (the only difference in the vertex shader for
smoothed cube between gc600 and gc2000, as generated by the blob driver, is this).
The cutoff point for this is at GC1000. All Vivante GPUs before GC1000 require these two instructions, except
the GC880.
Misc notes
=======================
- The shader engine packs together the registers of concurrently-executing shaders into the register bank.
If you write beyond the number of registers in `VIV_STATE_(PS/VS)_TEMP_REGISTER_CONTROL`, you will overwrite
registers of the next/previous shader instance, resulting in corrupt output.
- t0 at the beginning of the fragment shader has the x/y/z coordinates
x and y are in screen space, z is depth in 0.0 .. 1.0
gl_fragCoord: contains the window-relative coordinates of the current fragment
- In PS, RGROUP 1 register i0.x contains the value of gl_FrontFacing.
i0.y also contains a non-zero value. i0.zw are zero.
- i1..i127 are simply aliases of i0, at least on my GC600.
- The exact same texture sampling instruction, TEXLD, is used to sample 2D as well as Cubemap textures
(and, likely, other types of textures as well on hardware that supports these)
- All GC chips (at least the ones I've encountered, which are up to gc2000) have the same basic shader ISA.
- Register t0 in PS is hardwired to the position. Assigning to this register usually causes the fragment not to be rendered
(I suspect is this used for `gl_FragDepth` out).
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