proposal: debug/gosym: expose function start line and inline tree

[brancz]

Expose function start line through an API in the debug/gosym package.

The .gopclntab section includes data for the starting line number of a function, however, the debug/gosym package does not export an API that allows reading this data. I propose adding it to the API surface.

Use case: I happen to work on the Parca open-source continuous-profiling project and we quite frequently run into Go binaries, that don't include DWARF debuginfos (which we discourage but it's out of our control) but of course keep the .gopclntab, so we would like to be able to support those binaries as best as possible. The function starting line number in particular is important for the new PGO features, as noted in the documentation appendix.

[seankhliao]

What's the exact api that would be added for this?

[brancz]

Adding a StartLine field to the Func struct, and load it around here in the go12Funcs() function by adding a new function to funcData struct, called startLine, which like cuOffset would load the startLine (just field 9 instead of field 8). Doing all of this would make it be loaded when instantiating the table in NewTable.

This would only work for binaries that are Go >=1.3 as far as I understand, which would be fine for my case, since I'm targeting >=1.20 binaries in my use case.

[ianlancetaylor]

CC @golang/runtime

[prattmic]

For PGO, profiles need to contain inlined frames as well. Thus, I imagine that you want to add access to the inline tree to debug/gosym as well?

(This came up recently in another context that wants inlining information and we were considering a proposal to add it to debug/gosym, or perhaps an x/debug package).

[brancz]

I was going to open a separate proposal for that, but yes!

[cherrymui]

This would only work for binaries that are Go >=1.3 as far as I understand, which would be fine for my case, since I'm targeting >=1.20 binaries in my use case.

The start line in the pclntab is added last cycle (Go 1.20, specifically for PGO). So this would only work with Go 1.20+ binaries. For older binaries maybe fall back to 0?

[brancz]

The start line in the pclntab is added last cycle (Go 1.20, specifically for PGO). So this would only work with Go 1.20+ binaries. For older binaries maybe fall back to 0?

I think that would make sense.