GLSL software shader compiles, but will not render

[Christoph V]

OSX 10.6.8, MacBook Pro, ATI X1600

I have a problem with a shader.

This (test) code works:

Vertex:
void main(void)
{
 gl_TexCoord[0] = gl_MultiTexCoord0;
 gl_Position = ftransform();
}
 
Fragment:
uniform sampler2DRect tex0;
uniform int alpha;
uniform int red;
uniform float polyx[20];
uniform float polyy[20];
void main (void) {
 vec4 color0 = texture2DRect(tex0,gl_TexCoord[0].st);
 vec2 q = gl_TexCoord[0].st;
 float px1, py1;
 float d = 0.0;
 int alpha2 = 1;
 for ( int j = 0; j < 20; j++ ) {
  px1 = polyx[j];
  py1 = polyy[j];
  d = px1 + py1;
 }
 color0.g += 0.3 + d*0.001;
 gl_FragColor = clamp(color0, 0.0, 1.0);
}

This shader code does not render anything:

Vertex:    
void main(void)
{
 gl_TexCoord[0] = gl_MultiTexCoord0;
 gl_Position = ftransform();
}
 
Fragment:
uniform sampler2DRect tex0;
uniform int alpha;
uniform int red;
uniform float polyx[20];
uniform float polyy[20];
void main (void) {
 vec4 color0 = texture2DRect(tex0,gl_TexCoord[0].st);
 vec2 q = gl_TexCoord[0].st;
 float px1, py1;
 float d = 0.0;
 int alpha2 = 1;
 for ( int j = 0; j < 20; j++ ) {
  px1 = polyx[j];
  py1 = polyy[j];
  d = px1 + py1;
  if ( d == 0.0 ) d += 0.01;
 }
 color0.g += 0.3 + d*0.001;
 gl_FragColor = clamp(color0, 0.0, 1.0);
}

The only difference is this totally legal line:

  if ( d == 0.0 ) d += 0.01;

When I move this line one line down, so that it is outside the for-loop, then the shader works.

Both shaders compile without a problem. But when the second code runs all I get is that the original texture (tex0) is passed through without any changes.

Interestingly both codes work in the OpenGL Shader Builder application.

I am sure that I pass all variables correctly. I also get good values for all variables when I pull the UniformLocation (greater than -1).

Note: The ATIX1600 graphics card does not seem to be able to handle shaders with arrays that are addressed with a variable index (here: j) on a hardware basis. The OpenGL Shader Builder shows me that it switches to "Apple Software Renderer").

Is this a bug in Apple's Software Renderer? Or am I missing something?