Porting code from Nvidia GPU to AMD : lesson learned

In the past several weeks I have been porting a codebase from Nvidia CUDA platform to AMD HIP. Several critical issues were encountered, some solved, some attributed to compiler bugs, some remaining unfathomable to me.

There are 3 important things I have learned so far from the painstaking debugging process.

  • An unsigned integer with n bits only allows 0~(n-1) times bitwise left shift (<<). Excess shifts lead to undefined behavior. For Nvidia platform, 0 bit will be added, whereas for AMD, 1 bit will be added!!!
  • Currently there is a serious compiler bug: the wavefront vote function __any(pred) , which is supposed to work like __any_sync(__activemask(), pred) in CUDA, yields incorrect result in divergent threads!!!
  • This is very easy to miss: the parameter of wavefront vote functions __any(pred), __all(pred), etc is a 32-bit integer for both Nvidia and AMD platforms. If, however, a 64-bit integer is passed to the function, higher bits will be truncated!!! The solution is to explicitly cast the 64-bit integer to bool, which is then implicitly cast to int.

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