OpenMP Optimization Remarks

The OpenMP-Aware optimization pass is able to generate compiler remarks for performed and missed optimisations. To emit them, pass -Rpass=openmp-opt, -Rpass-analysis=openmp-opt, and -Rpass-missed=openmp-opt to the Clang invocation. For more information and features of the remark system the clang documentation should be consulted:

Some OpenMP remarks start with a “tag”, like [OMP100], which indicates that there is further information about them on this page. To directly jump to the respective entry, navigate to https://openmp.llvm.org/docs/remarks/OptimizationRemarks.html#ompXXX where XXX is the three digit code shown in the tag.


[OMP100] Potentially unknown OpenMP target region caller

A function remark that indicates the function, when compiled for a GPU, is potentially called from outside the translation unit. Note that a remark is only issued if we tried to perform an optimization which would require us to know all callers on the GPU.

To facilitate OpenMP semantics on GPUs we provide a runtime mechanism through which the code that makes up the body of a parallel region is shared with the threads in the team. Generally we use the address of the outlined parallel region to identify the code that needs to be executed. If we know all target regions that reach the parallel region we can avoid this function pointer passing scheme and often improve the register usage on the GPU. However, If a parallel region on the GPU is in a function with external linkage we may not know all callers statically. If there are outside callers within target regions, this remark is to be ignored. If there are no such callers, users can modify the linkage and thereby help optimization with a static or __attribute__((internal)) function annotation. If changing the linkage is impossible, e.g., because there are outside callers on the host, one can split the function into an external visible interface which is not compiled for the target and an internal implementation which is compiled for the target and should be called from within the target region.