[llvm-dev] Debug Locations for Optimized Code

Wed Dec 7 13:03:34 PST 2016

I suspect that you misunderstand where ASan instrumentation is added. Unlike UBSan, which is added by Clang during initial IR generation, ASan instrumentation is added late (at the EP_OptimizerLast extension point).

You are correct, I did not know that.

I don't see any better way to get the location information at that point than using the existing debug info.

Let us distinguish between information carried around in metadata, and information emitted to the object file.
Seems like it would be completely feasible to flag a DebugLoc instance as "do not emit" and yet retain it in the metadata, rather than erasing it (overwriting it with DebugLoc()).  Then the ASan instrumentation can extract the file/line info from it, while we still decline to generate it in the DWARF line table.  ASan gets the info it needs, the debugger doesn't get the information that will distress the user.

Another possibility is to flag the DebugLoc for a moved/combined instruction as "not a statement" and cause this flag to suppress the DWARF "is_stmt" flag.  That idea would need a little more baking but feels like it could have potential.  Some debuggers might have to learn to pay attention to the flag, but that's the debugger's problem.
--paulr

From: Hal Finkel [mailto:hfinkel at anl.gov]
Sent: Wednesday, December 07, 2016 10:19 AM
To: Robinson, Paul
Cc: David Blaikie; llvm-dev at lists.llvm.org; Reid Kleckner
Subject: Re: [llvm-dev] Debug Locations for Optimized Code

________________________________
From: "Paul Robinson" <paul.robinson at sony.com<mailto:paul.robinson at sony.com>>
To: "Reid Kleckner" <rnk at google.com<mailto:rnk at google.com>>
Cc: "Hal Finkel" <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>, "David Blaikie" <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>, llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Sent: Wednesday, December 7, 2016 11:01:57 AM
Subject: RE: [llvm-dev] Debug Locations for Optimized Code
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.
I suspect that you misunderstand where ASan instrumentation is added. Unlike UBSan, which is added by Clang during initial IR generation, ASan instrumentation is added late (at the EP_OptimizerLast extension point). I don't see any better way to get the location information at that point than using the existing debug info.

 -Hal
--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<mailto: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.

--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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