[LLVMdev] Preserving accurate stack traces with optimization?

[Philip Reames]

I ended up writing up a blog post summarizing this discussion and a few 
others that happened at about the same time.  I reframed it slightly in 
terms of debugging of code compiled by Clang, but other than that, 
there's nothing really new.  This won't really be useful to anyone who 
followed the original discussions in detail, but if you didn't follow 
the original threads and are interested, you might find it interesting.

http://www.philipreames.com/Blog/2013/12/21/accurate-stack-traces-compiler-optimization-and-llvmclang/

Once I get some time, I may write up the last section as a proposal for 
the list.  That's probably a couple months out though.

Philip

On 10/28/2013 02:56 PM, Philip Reames wrote:
> Is there a known way to preserve a full and accurate stack trace while
> utilizing most of LLVM's optimization abilities?
>
> We are investigating using LLVM as a JIT for a language which requires
> the ability to generate an accurate stack trace from any arbitrary
> point(1) during the execution.  I know that we can make this work by
> doing inlining externally, manually recording virtual frames, and
> disabling optimizations such as tail call optimizations. To me, this
> seems like an unpleasant hack that would likely inhibit much of LLVM's
> built in optimizing ability.  I suspect that if we ended up having to
> pursue this strategy, it would likely greatly diminish the benefit we
> could get by moving to an LLVM backend. (2)
>
> Currently, I am aware of two lines of related work.  First, I know that
> there has been some work into enabling full speed debug builds (-g -O3)
> for Clang which may be related.  Second, I know that the various
> sanitizer tools include stack traces in their reporting.
>
> What I have not been able to establish is the intended semantics of
> these approaches.  Is the intent that a stack trace will always be
> preserved?  Or simply that a best effort will be made to preserve the
> stack trace? Since for us the need to preserve a full stack trace is a
> matter of correctness, we couldn't use a mechanism which only provided
> best effort semantics.
>
> Are there other lines of related work that I have missed?  Are there any
> other language implementations out there that have already solved this
> problem?  I would welcome references to existing implementations or
> suggestions on how to approach this problem.
>
> Philip
>
> p.s. I know that there are a number of possible approaches to
> identifying when a bit of code doesn't actually need a full stack trace
> and optimizing these more aggressively.  We're considering a number of
> these approaches, but I am mostly interested in identifying a reasonable
> high performance base implementation at this time.  (Feel free to
> comment if you think this is the wrong approach.)
>
> (1) Technically, the semantics are slightly more limited then I've
> described.  The primary usage is for exceptions, security checking, and
> a couple of rarely used routines in the standard library.
> (2) I haven't actually measured this yet.  If anyone feels my intuition
> is likely off here, let me know and I'll invest the time to actually do so.
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