Hello folks this is a quick draft I threw together to sketch out providing a unified interface for branch histories like LBR on Intel and AMD, and BRBE on ARM. I looped in everyone that's working on a relevant patch.

The basic idea is that we introduce a new callback inside the add_sample path to allow for the branch payload to be filled in. This will use the multipart functions (so I need to update their scope). You can either fill in the payload into the callstack buffer and call pmc_multipart_add() or pmc_multipart_copydata() to copy a buffer into the stack frame. I did it this way because the branch history data is fairly large and we can't copy it on the stack.

From the user's perspective they just use the -B flag. E.g. pmcstat -B -S instructions -O samples.out.

(Please ignore that I didn't fill in all the getopt stuff again just a draft to facilitate discussion)

Thanks for sketching this out. The interface looks good to me: a generic PMC_F_BRANCHHIST flag, a backend callback from the sampling path, and a multipart payload for the branch records.

@afscoelho_gmail.com This is the callback interface I mentioned we discussed you rebasing on top of.

@mhorne Can you take a look I wanted to create this as a unified interface for all the branch history flags, use the multipart header to embed it in the stack for any given counter, and introduce a callback interface to prevent overrunning our stack.

Can we get reviews I'm going to do final testing to make sure it doesn't cause any issues.