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nyanthiss
Adept I

float4 to (float*) casting

Hi,

I started playing with darktable OpenCL code. I noticed this code [1]:

void rgb_to_lch (float *rgb, float *lch) {
   ...


kernel void highlights (...) {
   ...  
   float4 pixel = read_imagef(in, sampleri, (int2)(x, y));
   float4 inc, lchi, lchc, lch;
   ...      
   rgb_to_lch((float *)&pixel, (float *)&lchi);
   ...
}

I found out, the float4 to (float*) cast doesn't work, at least on my system (rhel 6.2, OpenCL 1.2 AMD-APP (938.1), HD 7950). Is this expected behaviour ? Could it be that it works on nvidia ?

1. https://github.com/darktable-org/darktable/blob/master/data/kernels/basic.cl

Tags (2)
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6 Replies
binying
Challenger

Re: float4 to (float*) casting

What do you mean by "it doesn't work"?

It compiles on my desktop though ( Win7, AMD2.7, OpenCL1.2, HD6990).

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nyanthiss
Adept I

Re: float4 to (float*) casting

By "it doesn't work" I mean it compiles, but the results are incorrect.

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binying
Challenger

Re: float4 to (float*) casting

void rgb_to_lch (float *rgb, float *lch) {

  int xPos = (int)get_global_id(0);

          int yPos = (int)get_global_id(1);

          if((xPos == 0) && (yPos==0))

                    printf("%f-%f\n", *rgb, *lch);

kernel void highlights (int in, ...) {
   ...  
   float4 pixel = (float4) in;
   float4 lchi = (float4) 0;
   ...      
   rgb_to_lch((float *)&pixel, (float *)&lchi);
   ...
}

And this can give me the results I expect...

Hope this is helpful for u.

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nyanthiss
Adept I

Re: float4 to (float*) casting

Try this:

void
rgb_to_lch (float *rgb, float *lch)
{
  for (int c=0; c<3; c++) lch = rgb * 1.7;
}

kernel void
test1 (global float *in, global float *out)
{
  const int x = get_global_id(0);
  float4 input = vload4(x, in);
  float4 res = (float4)0;
  rgb_to_lch((float *)&input, (float *)&res);
  vstore4(res, x, out);
}

If you try it with input data 1.0, 2.0, 3.0, 4.0 ... etc then the output will be this:

0 0 0 0 6.8 6.8 6.8 0 13.6 13.6 13.6 0 20.4 20.4 20.4 0 ...

Clearly the cast doesn't work (as expected (by me, at least ) )... So question is, is this *supposed* to work and a bug in compiler, or is that casting illegal ?

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MicahVillmow
Staff
Staff

Re: float4 to (float*) casting

While this is not illegal, it isn't recommended to take the address of a vector for performance reasons.  This code isn't failing on our internal builds, so it most likely has been fixed already.

nyanthiss
Adept I

Re: float4 to (float*) casting

Thanks. I will try it with new drivers/app sdk when they appear, but i think i'll fix the code to use vload / vstore anyway.

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