[LLVMdev] RFC: Binary format for instrumentation based profiling data

[Robinson, Paul]

> We seem to have some agreement that two formats for instrumentation
> based profiling is worthwhile. These are that emitted by compiler-rt in
> the instrumented program at runtime (format 1), and that which is
> consumed by clang when compiling the program with PGO (format 2).
> 
> Format 1
> --------
> 
> This format should be efficient to write, since the instrumented program
> should run with as little overhead as possible. This also doesn't need
> to be stable, and we can assume the same version of LLVM that was used
> to instrument the program will read the counter data. As such, the file
> format is versioned (so we can easily reject versions we don't
> understand) and consists basically of a memory dump of the relevant
> profiling counters.

The "same version" assertion isn't completely true, at a previous job
we had clients who preferred not to regenerate profile data unless they
actually had to (because it was a big pain and took a long time).  But
as long as the versioning is based on actual format changes, not just
repurposing the current LLVM version number (making the previous data
unusable for no technical reason), that's okay.

As long as I'm bothering to say something, is there some way that the
tools will figure out that you're trying to apply old data to new files
that have changed in ways that make the old data inapplicable?  Sorry
if this has been brought up elsewhere and I just missed it.
--paulr

> 
> Format 2
> --------
> 
> This format should be efficient to read and preferably reasonably
> compact. We'll convert from format 1 to format 2 using llvm-profdata,
> and clang will use format 2 for PGO.
> 
> Since the only particularly important operation in this use case is fast
> lookup, I propose using the on disk hash table that's currently used in
> clang for AST serialization/PTH/etc with a small amount of metadata in a
> header.
> 
> The hash table implementation currently lives in include/clang/Basic and
> consists of a single header. Moving it to llvm and updating the clients
> in clang should be easy. I'll send a brief RFC separately to see if
> anyone's opposed to moving it.
> 
> 
> Thoughts?
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