[llvm-dev] Debug Locations for Optimized Code

[Philip Reames via llvm-dev]

FYI, if we do end up deciding that asan needs a stronger guarantee than 
debug info can provide, we have another mechanism in tree which is 
available for this purpose. Operand bundles can be associated with a 
callsite today to provide a guaranteed side table of value locations at 
runtime.  We use the "deopt" bundle type for exactly this purpose and 
it's explicitly part of the design to be stronger than debug info and 
accept the performance impact that implies while trying to minimize it 
as much as possible.  We might have to extend the notion of operand 
bundles to other instruction types, but the fundamental mechanism is 
already in the IR.

Philip

On 12/07/2016 09:01 AM, Robinson, Paul via llvm-dev wrote:
>
> I don't see how ASan and debuggers are different. It feels like both 
> need reasonably accurate source location attribution for any 
> instruction. ASan just happens to care more about loads and stores 
> than interactive stepping debuggers.
>
> Actually they are pretty different in their requirements.
>
> ASan cares about *accurate* source location info for *specific* 
> instructions, the ones that do something ASan cares about. The source 
> attributions for any other instruction is irrelevant to ASan.  The 
> source attributions for these instructions *must* survive optimization.
>
> Debuggers care about *useful* source location info for *sets* of 
> instructions, i.e. the instructions related to some particular source 
> statement.  If that set is only 90% complete/accurate, instead of 
> 100%, generally that doesn't adversely affect the user experience.  If 
> you step past statement A, and happen to execute one or two 
> instructions from the next statement B before you actually stop, 
> generally that is not important to the user.  Debuggers are able to 
> tolerate a moderate amount of slop in the source attributions, because 
> absolute accuracy is not critical to correct operation of the 
> debugger.  This is why optimizations can get away with dropping 
> attributions that are difficult to represent accurately.
>
> ASan should be able to encode source info for just the instructions it 
> cares about, e.g. pass an index or other encoded representation to the 
> RT calls.  Being actual parameters, they will survive any correct 
> optimization, unlike today's situation where multiple calls might be 
> merged by an optimization, damaging the correctness of ASan reports.  
> (We've see this exact thing happen.)  ASan does not need a line table 
> mapping all instructions back to their source; it needs a parameter at 
> each call (more or less). It does need a file table, that's the main 
> bit of redundancy with debug info that I see happening.
>
> --paulr
>
> *From:*Reid Kleckner [mailto:rnk at google.com]
> *Sent:* Wednesday, December 07, 2016 8:23 AM
> *To:* Robinson, Paul
> *Cc:* Hal Finkel; David Blaikie; llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Debug Locations for Optimized Code
>
> On Wed, Dec 7, 2016 at 7:39 AM, Robinson, Paul via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
> When we are looking at a situation where an instruction is merely *moved*
> from one place to another, retaining the source location and having a
> less naïve statement-marking tactic could help the debugging experience
> without perturbing other consumers (although one still wonders whether
> profiles will get messed up in cases where e.g. a loop invariant gets
> hoisted out of a cold loop into a hot predecessor).
>
> When we are looking at a situation where two instructions are *merged* or
> *combined* into one, and the original two instructions had different
> source locations, that's a separate problem.  In that case there is no
> single correct source location for the new instruction, and typically
> erasing the source location will give a better debugging experience (also
> a less misleading profile).
>
> My personal opinion is that having sanitizers *rely* on debug info for
> accurate source attribution is just asking for trouble.  It happens to
> work at –O0 but cannot be considered reliable in the face of optimization.
> IMO this is a fundamental design flaw; debug info is best-effort and full
> of ambiguities, as shown above. Sanitizers need a more reliable
> source-of-truth, i.e. they should encode source info into their own
> instrumentation.
>
> I don't see how ASan and debuggers are different. It feels like both 
> need reasonably accurate source location attribution for any 
> instruction. ASan just happens to care more about loads and stores 
> than interactive stepping debuggers.
>
> It really doesn't make sense for ASan to invent another mechanism to 
> track source location information. Any mechanism we build would be so 
> redundant with debug info that, as an implementation detail, we would 
> find a way to make them use the same storage when possible. With that 
> in mind, maybe we should really find a way to mark source locations as 
> "hoisted" or "sunk" so that we can suppress them from our line tables 
> or do something else clever.
>
>
>
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