glsl glitch experiment ( will not works in better gpu )

shader.frag

#ifdef GL_ES
precision mediump float;
#endif

uniform float time;
uniform vec2 resolution;

#define saturate(i) clamp(i,0.,1.)

mat2 rotate( float _t ){
  return mat2( cos( _t ), sin( _t ), -sin( _t ), cos( _t ) );
}

float distFunc( vec3 _p ){
  vec3 p = _p;
  return length( p ) - 0.5;
  p.xy = rotate( time ) * p.xy;
  p.yz = rotate( time ) * p.yz;
  p.zx = rotate( time ) * p.zx;
  vec3 d = abs( p ) - vec3( 0.4 );
  return min( max( max( d.x, d.y ), d.z ), 0.0 ) + length( max( d, 0.0 ) );
}

vec3 normalFunc( vec3 _p, float _d ){
  vec2 d = vec2( 0.0, 1.0 ) * _d;
  return normalize( vec3(
    distFunc( _p + d.yxx ) - distFunc( _p - d.yxx ),
    distFunc( _p + d.xyx ) - distFunc( _p - d.xyx ),
    distFunc( _p + d.xxy ) - distFunc( _p - d.xxy )
  ) );
}

void main(){
  vec2 uv = gl_FragCoord.xy / resolution;
  vec2 p = ( gl_FragCoord.xy * 2.0 - resolution ) / resolution.x;
  
  vec3 rayBeg = vec3( 0.0, 0.0, 1.0 );
  vec3 rayDir = vec3( p.x * vec3( 1.0, 0.0, 0.0 ) + p.y * vec3( 0.0, 1.0, 0.0 ) + vec3( 0.0, 0.0, -1.0 ) );
  float rayLen = 0.0;
  vec3 rayPos = rayBeg;
  vec3 rayCol = vec3( 0.0, 0.0, 0.0 );
  float dist = 0.0;
  
  for( int i=0; i<64; i++ ){
    dist = distFunc( rayPos );
    rayLen += dist;
    rayPos = rayBeg + rayLen * rayDir;
  }
  
  if( dist < 0.01 ){
    vec3 ligPos = vec3( 0.6, 0.7, 1.0 );
    vec3 ligDir = normalize( rayPos - ligPos );
    vec3 nor = normalFunc( rayPos, 1E-3 );
    vec3 dif = ( saturate( dot( ligDir, -nor ) ) * 0.8 + 0.2 ) * vec3( 1.0, 0.0, 0.3 );
    vec3 spe = pow( saturate( dot( normalize( ligDir + rayDir ), -nor ) ), 40.0 ) * vec3( 0.6 );
    rayCol = dif + spe;
  }else{
    rayCol = gl_FragColor.xyz;
  }
  
  gl_FragColor = vec4( rayCol, 1.0 );
}