[LLVMdev] Preserving accurate stack traces with optimization?
Philip Reames
listmail at philipreames.com
Wed Oct 30 19:09:51 PDT 2013
David, Quentin - Thanks for the feedback. Responses inline.
On 10/30/13 11:21 AM, David Blaikie wrote:
> Actually CCing Eric.
>
>
> On Wed, Oct 30, 2013 at 11:00 AM, Quentin Colombet
> <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>
> Philip,
>
> Thanks for the clarification.
>
> As far as I can tell, there is currently no way to preserve a full
> and accurate stack trace while utilizing most of LLVM’s
> optimization abilities.
>
> The work on debug information may help you get the information you
> need, but I do not think we will provide information on stack
> frames that have been removed via inlining or tail call.
>
>
> In theory, at -gmlt we should emit enough debug info to give you
> accurate stack traces including inlined frames. Tail calls I assume we
> can't do anything about.
Tail calls I'm not too worried about. I'm reasonably sure that our
existing optimizer doesn't do any tail call optimizations. Given that,
turning them off doesn't worry me too much performance wise.
First, thank for you for mentioning the -gmit option. I had been
completely unaware of that. I'll have to dig into the implementation
and usage a bit. Can you point me to any documentation? A quick google
search didn't turn up anything.
Can you clarify two things for me? First, is the intent that -gmit
*always* provide accurate stack traces? (modulo bugs of course) If so,
what is your subjective opinion on how close it comes to meeting that
goal today? (i.e. how much help would we need to contribute to get it
to a solid state?)
Out of curiosity, how does line information fair in this? I don't think
we strictly need full line information for our purposes, but I'm curious
to know where it stands. Being able to rely on line info might ease the
implementation of a couple of other parts.
>
> Moreover, if at some point you also need the values of the
> arguments of a removed stack frame, this seems heroic to be able
> to provide such information.
>
>
> Also, in theory, we should be able to describe the locations of
> function arguments/parameters to that inlined call, except for those
> optimized away entirely - but that's probably not quite as well
> tested/implemented at this point.
This is much less important for us at the moment. I don't believe we
need to support reporting of argument values. This might be a nice to
have debugging wise, but is not a must have.
> This is my understanding of what we have currently, folks working
> on the debug support may give you more inputs on that (CC’ed Eric).
> As for the sanitizer, I have no idea what stack trace they are
> reporting, I let them comment on that.
>
>
> I believe they use llvm-symbolizer which uses the debug info to get
> inlined stack frames. That's the main use case for -gmlt.
So I did vaguely remember this correctly. Glad to know.
>
>
> ** To Eric **
> Could you comment on the way we are generating stack frame
> information and in particular how inlining is handled, i.e., does
> an inlined function showed up in the stack frame information
> (seems unlikely, but who knows :)).
>
> -Quentin
>
> On Oct 30, 2013, at 10:24 AM, Philip Reames
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>
>> 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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>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>
>
>
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