message: add runtime_compilation.rst design doc
diff: @@ -396,3 +396,27 @@ specified.
  Currently, callbacks functions are only supported for transforms that
  do not use planar format for input or output.
  +
  +Runtime compilation
  +-------------------
  +
  +rocFFT includes many kernels for common FFT problems. Some plans may
  +require additional kernels aside from what is built in to the
  +library. In these cases, rocFFT will compile optimized kernels for
  +the plan when the plan is created.
  +
  +Compiled kernels are cached to persistent storage where possible, so
  +that they are available to subsequent plans that require the same
  +kernels.
  +
  +The default cache path is ``.cache/rocFFT/rocfft_kernel_cache.db`` in
  +the current user's home directory. If the home directory cannot be
  +determined, or if that location is not writable, the cache file is
  +written to a temporary directory as chosen by the operating system.
  +If that location is also not writable, kernels are only cached in
  +memory for the lifetime of the current process.
  +
  +The cache path may be overridden by setting the
  +``ROCFFT_RTC_CACHE_PATH`` environment variable. rocFFT will create
  +the cache file with the name specified in the environment variable, if
  +it is set.
  