[PATCH] D85276: [PGO][CUDA][HIP] Skip generating profile on the device stub and wrong-side functions.

Thu Aug 6 10:38:37 PDT 2020

hliao added inline comments.

================
Comment at: clang/lib/CodeGen/CodeGenPGO.cpp:839-840

+  // Skip host-only functions in the CUDA device compilation and device-only
+  // functions in the host compilation.
+  if (CGM.getLangOpts().CUDA &&
----------------
tra wrote:
> hliao wrote:
> > tra wrote:
> > > We will still have around some functions that may never be used on the host side (HD functions referenced from device code only).  I'm not sure if that's a problem for profiling, though. I wonder if we can somehow tie `skipRegionMappingForDecl` to whether we've actually codegen'ed the function. 
> > Skipping wrong-side functions here just makes the report not confusing as these functions are not emitted at all and are supposed never running on the host/device side. If we still create the mapping for them, e.g., we may report they have 0 runs instead of reporting nothing (just like comments between function.) That looks a little bit confusing.
> > It seems the current PGO adds everything for coverage mapping and late prune them based on checks here. Just try to follow that logic to skip wrong-side functions. If we need to revise the original logic and generate coverage mapping for emitted functions only, the change here is unnecessary.
> I'd add a comment here that this 'filter' is just a rough best-effort approximation that still allows some effectively device-only Decls through.
> The output should still be correct, even though the functions will never be used. Maybe add a TODO to deal with it if/when we know if the Decl was codegen'ed.
> 
Add that comment. But, I tend to not deal that "effectively" host-only/device-only ones as that should be developers' responsibility to handle them. The additional zero coverage mapping may be useful as well. If a function is really device-only but is attributed with HD, the 0 coverage may help developers correcting them.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D85276/new/

https://reviews.llvm.org/D85276