[LLVMdev] Preserving accurate stack traces with optimization?

[Quentin Colombet]

Hi Philip,

Could you define what is an accurate stack trace for your project?
In other words, what do you mean by full and accurate stack frame?

Without this definition, this is difficult to give you any feedback. In particular, I do not see what it means when we use inlining.
E.g., what do you expect from code like this:
static void fct1(…) {
  ...
}

static void fct2(…) {
  …
  fct1(…)
  ...
}

void fct3(…) {
  fct1(...)
  …
  fct2(…)
  …
}

Assuming everything is inlined in fct3, you get:
void fct3(…) {
   ….
1.   fct1_inst1… fct1_instN
   ….
2.   fct2_inst1… fct2_instK
3.   fct1_inst1… fct1_instN
4.   fct2_instzK+1… fct2_instN
   ...
}

Does it mean you what something like this each point of interest for you stack frame:
1.
#0 fct1
#1 fct3

2.
#0 fct2
#1 fct3

3.
#0 fct1
#1 fct2
#2 fct3

4.
#0 fct2
#1 fct3

Cheers,
-Quentin

On Oct 28, 2013, at 2:56 PM, Philip Reames <listmail at philipreames.com> 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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