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Casting between two structs in C (CUDA)

Posted on 2010-08-12
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Last Modified: 2012-05-10
The CUDA library effectively defines its own complex number data type, cuComplex, as follows:

  struct __builtin_align__(8) float2
  {
    float x, y;
  };
  typedef float2 cuFloatComplex;
  typedef cuFloatComplex cuComplex;

I am already using my own complex data type:

  typedef struct  { float re; float im; } complex;

and I wish to be able to cast between the two. Here's an example of what I'm currently trying:

  cuComplex cu_data;
  ...
  complex c_data;
  c_data = (complex)cu_data;

but I'm getting the (nvcc) error:

  'no suitable user-defined conversion from "cuComplex" to "complex" exists'.

So how do I do this? Equivalently: is there a way to do a typedef, but to simply rename the members of that struct? (ie rename 'x' to 're', 'y' to 'im')?

Thanks!
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Question by:InteractiveMind
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13 Comments
 
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Expert Comment

by:jkr
ID: 33421947
You could provide your own operator, e.g.
  struct /*__builtin_align__(8)*/ float2
  {
    float x, y;
  };
  typedef float2 cuFloatComplex;
  typedef cuFloatComplex cuComplex;

 typedef struct  { float re; float im; operator cuComplex();} complex;

 complex::operator cuComplex() {

   cuComplex out;

   out.x = re;
   out.y = im;

   return out;
 }

  int main () {

  cuComplex out;
  complex in;

  out = (cuComplex) in;

  return 0;
  }

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by:
jkr earned 167 total points
ID: 33421993
Ooops, seems that I missed the 'C' part - well, in that case, make that a function:
  struct /*__builtin_align__(8)*/ float2
  {
    float x, y;
  };
  typedef float2 cuFloatComplex;
  typedef cuFloatComplex cuComplex;

 typedef struct  { float re; float im; operator cuComplex();} complex;

 cuComplex to_cuComplex(complex* in) {

   cuComplex out;

   out.x = in->re;
   out.y = in->im;

   return out;
 }

  int main () {

  cuComplex out;
  complex in;

  out = to_cuComplex(&in);

  return 0;
  }

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Expert Comment

by:sridhard
ID: 33422053

Hi,

Try using pointers . This will assign/point the correponding values. Note sure whether this helps

  cuComplex cu_data = (cuComplex  *) malloc(sizeof(cuComplex ));
  ...
  complex *c_data;
  c_data = (complex*)cu_data;

You can delete any one pointer later .
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Author Comment

by:InteractiveMind
ID: 33422155
Thank you, both. I think I should mention that I am in fact casting pointers/arrays of such types in my code (but I get the same problem).

@jkr, would your method work for casting pointers? (I can't test it at the moment). If so, my code needs to be as fast as possible, so given that I'm going to need to do [probably] millions of such casts per second, would it be quicker to just do a typedef:

   typedef cuComplex complex;

and then a global search and replace of '.re' with '.x' etc? (I'm reluctant to do so)

Also, can you think of any reason why what I'm already doing wouldn't work? Is it the __builtin_align__(8) that's causing problems?
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Expert Comment

by:jkr
ID: 33422183
>>@jkr, would your method work for casting pointers?

If you use the indirecton appropriately - yes.
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Assisted Solution

by:Infinity08
Infinity08 earned 167 total points
ID: 33427090
>> given that I'm going to need to do [probably] millions of such casts per second, would it be quicker to just do a typedef:

Yes. Because no conversion would be needed then. Performing no conversion is faster than performing a conversion ;)


>> Also, can you think of any reason why what I'm already doing wouldn't work?

The error says it :

>>   'no suitable user-defined conversion from "cuComplex" to "complex" exists'.

the compiler doesn't know how to perform the conversion from cuComplex to complex, because you didn't tell it how to do that (see jkr's post for that).


>> I should mention that I am in fact casting pointers/arrays of such types in my code (but I get the same problem).

Casting pointers wouldn't give you the same problem. It would give you a different problem in C++ : the type safety system in C++ wouldn't allow a cast from cuComplex* to complex*, because the two types are not related - ie. the cast is not safe.
If you shut the compiler up, and force the cast, then the __builtin_align__(8) can indeed cause a problem. If the packing of the complex type is not the same as that of the cuComplex type, then something will go wrong badly.
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Assisted Solution

by:ikework
ikework earned 166 total points
ID: 33427141
If you use the same alignment for your struct (8 bytes) then the cast shouldn't be a problem.
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Expert Comment

by:ikework
ID: 33427161
The safest strategy is to use only cuda's struct in the first place, also in your code, so there is no cast needed anymore.
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by:ambience
ID: 33429106
>> so given that I'm going to need to do [probably] millions of such casts per second, would it be quicker to just do a typedef:

My vote -> YES
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by:ikework
ID: 33429220
@ambience, did you consider the different alignments as previous posts said?
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by:ikework
ID: 33429254
@ambience, sorry I misunderstood your post, I thought you were suggesting just cast anyway, nevermind :)
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Expert Comment

by:soscpd
ID: 33436784
Hi

Your answer is a function ponter. There (in the function) you read/write cuComplex/complex structs anyway you like.

Rafael
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Author Closing Comment

by:InteractiveMind
ID: 33558606
Thanks
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