v8 bt skips some frames

[brendangregg]

Maybe there's a good reason for this, but notice we're skipping frames 1-4:

(lldb) v8 bt
 * thread #1: tid = 13, 0x00007fda23874d44 libpthread.so.0`__GI___pthread_mutex_lock + 4, name = 'node', stop reason = signal SIGSEGV
  * frame #0: 0x00007fda23874d44 libpthread.so.0`__GI___pthread_mutex_lock + 4
    frame #5: 0x00007fda20ffe212 node`v8::internal::FunctionCallbackArguments::Call(void (*)(v8::FunctionCallbackInfo<v8::Value> const&)) + 290
[...]

They do show up with the normal bt:

(lldb) bt
* thread #1, name = 'node', stop reason = signal SIGSEGV
  * frame #0: 0x00007fda23874d44 libpthread.so.0`__GI___pthread_mutex_lock + 4
    frame #1: 0x00007fda20b0ed19 node
    frame #2: 0x00007fda20ac5e39 node
    frame #3: 0x00007fda20a9dd4b node
    frame #4: 0x00007fda20a9c9f6 node
    frame #5: 0x00007fda20ffe212 node`v8::internal::FunctionCallbackArguments::Call(void (*)(v8::FunctionCallbackInfo<v8::Value> const&)) + 290
[...]

Should "v8 bt" be skipping them?

[bnoordhuis]

I've been looking at this. llnode skips frames it can't link to either a C++ symbol or a JS stack frame.

It's easy to change but then stack frames for V8 builtins also show up (because llnode does not know how to identify them) and there are often a great number of them.

I added a heuristic to skip frames whose PC is inside a WX memory segment on the assumption that it's almost certainly jitted code. The only problem is that it depends on SBProcess::GetMemoryRegions() so I don't think it's going to work with older lldb versions.

Before I move on with this, does anyone have a better idea?

[rnchamberlain]

We had a brief discussion on skipped frames last year, and there was interest in seeing all frames, even if the information available was minimal, eg V8 transition frames.

Re dependency on GetMemoryRegions(), as that API is in lldb 3.9, which is the stable release now, I don't think that should be a worry.

[bnoordhuis]

Sounds good. I've opened #104, it prints them as <builtin>.