Question

I was told that C and C++ have “undefined behavior”, that is, the same code might behave differently on different platforms, or using different compilers, if I use “certain constructs”.

Does the same apply to OpenCL and/or OpenGL SL?

Answer 1

Yes, according to the OpenCL specification 1.1 which defines undefined behaviour in the front matter as:

The behavior of an OpenCL API call, built-in function used
inside a kernel or execution of a kernel that is explicitly not
defined by OpenCL. A conforming implementation is not required to
specify what occurs when an undefined construct is encountered in
OpenCL

For example:

• Section 5.2.1 defines several:
  • “Reading from a buffer or image object created with CL_MEM_WRITE_ONLY inside a kernel”
  • “Writing to a buffer or image object created with CL_MEM_READ_ONLY inside a kernel”
  • “OpenCL commands that operate on multiple
    buffer objects created with the same host_ptr or
    overlapping host regions”
  • “Concurrent reading from, writing to and copying between overlapping
    sub-buffer objects created with the same buffer object”
• Section 5.2.2 also defines a few:
  • clEnqueueReadBuffer with other commands using the buffer
  • clEnqueueReadBuffer whilst the buffer is mapped

There’s plenty more ways of invoking undefined behaviour from OpenCL though.

For OpenGL SL (specification) there are quite a few easy to find examples:

• “Undefined behavior results from indexing an array with a non-constant expression that’s greater than or equal to the array’s size or less than 0” (Interestingly if it’s a const then it’s a compile time error)
• Likewise for accessing vector elements with array subscript notation
• “Reading a varying variable in a
  vertex shader returns undefined values if it is read before being written”
• “If this pragma [#pragma STDGL invariant(all)] is used after the declaration of any variables or
  functions, then the set of outputs that behave as invariant is undefined”
• “Reading [gl_PointSize or gl_ClipVertex] before writing them results in undefined behavior”
• etc.