[LLVMdev] Preserving accurate stack traces with optimization?

[Philip Reames]

On 10/30/13 9:56 AM, Quentin Colombet wrote:
> 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.
Sure.  Just to note, I *think* your example was exactly what we're 
looking for.  I got a bit confused about your notation, so I'm going to 
start from scratch.

By a "full and accurate stack trace" in the face of inlining, I mean the 
exact stack trace you would get without any inlining (i.e. in a 
unoptimized build.)  To put this another way, I need to be able to 
distinguish the path by which a function was inlined.  Consider the 
following example (in approximate C):

void a() {
   if( randomly_true ) print_stack_trace();
}
void b() {
   a();
}
void c() {
   a();
}
void main() {
   b();
   c();
}

In our environment, we need to be able to distinguish the traces 
"a;b;main" from "a;c;main" reliably.  We need this regardless of what 
decisions the optimizer might make about inlining (or other 
optimizations for that matter).

For another example, "a" might be a routine which requires privileges to 
execute.  "b" might a routine which adds privileges. "c" might be an 
untrusted routine.  Calling "a" from "b" will succeed.  Calling "a" from 
"c" will generate an exception.  (We can handle all the details around 
when to throw exceptions if the information about stack traces is 
accurate and trustworthy.)

Side note: The permission question above can't be addressed statically.  
The call to the privileged routine doesn't have to be direct.  "a" could 
be a series of frames "a1...aN" where "aN" is actually the privileged 
one.  There can also be virtual (or other runtime dispatch) calls in 
that path which prevent static analysis.

Does that help clarify what we're looking for?

Philip


> 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 
> <mailto: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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