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

[Duncan P. N. Exon Smith]

On Mar 24, 2014, at 10:08 AM, Robinson, Paul <Paul_Robinson at playstation.sony.com> wrote:

>> 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.

Format 1 (extension .profraw since r204676) should be run immediately through llvm-profdata to generate format 2 (extension .profdata).  The only profiles that should be kept around are format 2.

> 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

There’s a hash for each function based on the layout of the counters assigned to it.  If the hash from the data doesn’t match the current frontend, the data is ignored.  Currently, the hash is extremely naive:  the number of counters.