[llvm-dev] [lldb-dev] RFC: libtrace

Tue Jun 26 14:29:00 PDT 2018

> On Jun 26, 2018, at 2:00 PM, Jim Ingham via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> 
>> * unwinding and backtrace generation
> 
> Jason says this will be somewhat tricky to pull out of lldb.  OTOH much of the complexity of unwind is reconstructing all the non-volatile registers, and if you don't care about values, you don't really need that.  So some kind of lightweight pc/sp only backtrace would be more appropriate, and probably faster for your needs.

If it were me & performance were the utmost concern, and I had a restricted platform set that I needed to support where I can assume the presence of eh_frame and that it is trustworthy in prologue/epilogues, then I'd probably just write a simple Unwind/RegisterContext plugin pair that exclusively live off of that.

If it's just stack walking, and we can assume no omit-frame-pointer code and we can assume the 0th function is always stopped in a non-prologue/epilogue location, then even simpler would be the old RegisterContextMacOSXFrameBackchain plugin would get you there.  That's what we used before we had the modern unwind/registercontext plugin that we use today.  It doesn't track spilled registers at all, it just looks for saved pc/framepointer values on the stack.

A general problem with stopping the inferior process and examining things is that it is slow.  Even if you use a NativeHost approach and get debugserver/lldb-server out of the equation, if you stop in a hot location it's very difficult to make this performant.  We've prototyped things like this in the past and it was always far too slow.  I don't know what your use case looks like, but I do worry about having one process controlling an inferior process in general for fast-turnaround data collection/experiments, it doesn't seem like the best way to go about it. 

> 
> Jim
> 
>> 
>> 
>> 
>>> At the same time we think that in doing so we can break things up into more granular pieces, ultimately exposing a larger testing surface and enabling us to create exhaustive tests, giving LLDB more fine grained testing of important subsystems.
>> 
>> Are you thinking of the new utility as something that would naturally live in llvm/tools or as something that would live in the LLDB repository?
>> I would rather put it under LLDB and then link LLDB against certain pieces in cases where that makes sense.
>> 
>> 
>>> 
>>> A good example of this would be LLDB’s DWARF parsing code, which is more featureful than LLVM’s but has kind of evolved in parallel.  Sinking this into LLVM would be one early target of such an effort, although over time there would likely be more.
>> 
>> As you are undoubtedly aware we've been carefully rearchitecting LLVM's DWARF parser over the last few years to eventually become featureful enough so that LLDB could use it, so any help on that front would be most welcome. As long as we are careful to not regress in performance/lazyness, features and fault-tolerance, deduplicating the implementations can only be good for LLVM and LLDB.
>> 
>> Yea, this is the general idea.   Has anyone actively been working on this specific effort recently?  To my knowledge someone started and then never finished, but the efforts also never made it upstream, so my understanding is that it's a goal, but one that nobody has made significant headway on.
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