[llvm-dev] Stripping Debug Locations on cross BB moves, part 2 (PR31891)

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 10 08:58:10 PST 2017

On Thu, Feb 9, 2017 at 5:16 PM Adrian Prantl <aprantl at apple.com> wrote:

> > On Feb 9, 2017, at 4:09 PM, Robinson, Paul <paul.robinson at sony.com>
> wrote:
> >
> > Sorry to be late to the party, I was out yesterday.
> >
> > I do want to point out that there are different motivations for the two
> cases.  When moving/hoisting a single instruction, the motivation is to
> avoid bad debugger stepping and bad sample-profile data (we get a lot of
> complaints about bad debugger stepping, btw) for which we pay the price of
> not having totally precise instruction-to-source mapping.  I think this is
> the right tradeoff, fwiw.  But, when *combining* or merging instructions
> (with different source locations), the problem is that we *can't* provide a
> correct source location, because there is not a single correct source
> location and we can't do a one-to-many mapping.  As it happens, we're using
> the same mechanism to solve these two different problems, i.e. erase the
> debug location from the moved/combined instruction, but the motivations are
> still different.
> >
> > Why the fuss about motivation?  Because it means the moved-call and
> merged-call cases are different, and might want different solutions.  It
> would be fine to add an "I've been moved" flag to the debug location of a
> moved instruction, with whatever necessary effects that flag might need to
> have; but it would *not* be fine to use this flag for merged/combined
> instructions, because in those cases we do not have a correct "original"
> source location to preserve.  In the latter case, we still need to use
> line-0 for calls, because there's no other correct thing to do.
> >
> > Probably getMergedLocation should optionally return line-0 instead of
> null; whether it's an optional don’t-return-null parameter or a separate
> getMergedLocationForCall API is an implementation detail.
> We need something to this end to support getMergedLocationForCall for
> CallInstructions. I'm not yet sure what the best API design for this. It
> feels dangerous to let the caller decide whether a nullptr is acceptable.

Yeah, I'd just have getMergedLocation check if it's merging a call if
that's the way we want to go. (assuming that's sufficient - I guess it
probably is - a call couldn't move implicitly due to its user moving or
anything, it can't be a ConstantExpr I wouldn't think)

> >  The code-motion cases won't be using getMergedLocation anyway, so if we
> want to make a new API to set a "been-moved" flag, that seems pretty
> orthogonal to the merged-call problem.  Or, of course, we can always skip
> the flag idea, using line-0 for moved calls and null for moved
> anything-else.
> David, do you want to make an argument for why the "been-moved" flag is
> superior over using line-0?

Would allow us to keep the/a source location for other things like asan,
but we aren't already - so I'm fine with line 0 (& presumably no/zero

> > There was also a question about whether the SCE debugger would tolerate
> a call instruction with line-0.  I expect it's fine, but I will check.  I
> might not have an answer right away as I am off again tomorrow.
> Thanks for checking!
> -- adrian
> > --paulr
> >
> >
> > From: David Blaikie [mailto:dblaikie at gmail.com]
> > Sent: Thursday, February 09, 2017 8:28 AM
> > To: Adrian Prantl
> > Cc: llvm-dev; Dehao Chen; Eric Christopher; Reid Kleckner; Robinson, Paul
> > Subject: Re: Stripping Debug Locations on cross BB moves, part 2
> (PR31891)
> >
> >
> >
> > On Wed, Feb 8, 2017 at 10:32 AM Adrian Prantl <aprantl at apple.com> wrote:
> > On Feb 8, 2017, at 10:17 AM, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Wed, Feb 8, 2017 at 9:56 AM Adrian Prantl <aprantl at apple.com> wrote:
> > On Feb 8, 2017, at 9:44 AM, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Wed, Feb 8, 2017 at 9:36 AM Adrian Prantl <aprantl at apple.com> wrote:
> >
> > > On Feb 8, 2017, at 9:25 AM, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > > So Reid came across a case where the current strategy (dropping
> locations when they move across basic blocks) proves to be at odds with
> another precept we have: inlinable calls must have locations, so that
> if/when they are inlined the location can be accurately described (due to
> the nature of our IR representation of inlining, a location must be given
> for the call site so that the DIEs for the inlined subroutine can be
> described)
> > >
> > > Any ideas on how we should approach this?
> > >
> > > We could have a bit in DebugLoc (or other choice of representation)
> for the non-line location. For the line table this would work the same as
> absent DebugLoc - for calls it would cause call_file/line/discriminator (do
> we have a call_discriminator? We probably should, don't know if we do) to
> be omitted which is something the LLVM IR representation currently can't
> express.
> >
> > Even if the function calls are not inlined it could be preferable to
> keep a debug location. It's a very odd debugging experience to be stopped
> at a breakpoint and then do "up" (or just "bt") to look at the parent frame
> and not have a location for the call in the parent frame.
> >
> > Usually what will happen is it will have a location, it'll be the
> flow-on from the previous location (ie: we won't emit any line entry for
> the call, so it naturally ends up at the line of whatever instruction came
> before).
> >
> > Is the stripping in this case motivated by correctness for profiling or
> just to smooth the line table to improve the stepping experience?
> >
> > Mostly it seems the people pushing/contributing patches are motivated by
> profile correctness, as far as I understand (with the line table/debugging
> behavior 'improvement' being a nod to "there's at least an argument to be
> made that this could be justified by debuggability too"). At least that's
> the impression I get. I may've misunderstood.
> >
> > If its about correctness for profiling, we should consider having a
> separate profiling location or a skip-for-profiling bit as David suggests,
> that honored when the CU has the new tune-for-profiling flag set.
> >
> > I think that might be a bit too much, in my mind, for the
> tune-for-profiling flag. Adding things to make the profile more accurate
> seems good - removing things and 'hurting' debuggability because you opted
> for profiling seems like a different and more difficult-to-tolerate
> situation.
> >
> > If it is about a better stepping experience, we just shouldn't do this
> for call sites.
> >
> > Why? A call could produce weird stepping behavior as much as a non-call
> instruction, and could be similarly mis-attributed. CSE could hoist two
> distinct calls to pure functions into a common block and we'd pick one
> location - the stack trace and location would be confusing to the debugger
> user (it could end up indicating that the if branch was taken when it was
> really the else branch, being the canonical example)
> >
> > Calls don't seem fundamentally special here - except due to limitations
> of the IR representation.
> >
> > The CSE example is a good one, though I *think* in this case we will end
> up calling getMergedDebugLocation() which will return a line 0 location.
> >
> > I think getMergedDebugLocation (or whatever it's called) currently
> returns a null location, which is a bit different from line 0.
> >
> > Indeed it does, I misremembered. Looks like we have the same latent
> problem with all users of getMergedDebugLocation(), too.
> >
> > Yep - though I imagine not all of them are liable to move calls.
> >
> >
> > Line zero is tricky to use as it can make the line table larger (file
> size)/worse for the user (similarly jumpy behavior, possibly, depending on
> how the DWARF consumer handles it).
> >
> > LLDB handles line 0 on call sites well (and we could easily fix it if it
> didn't), I can't speak for gdb or the sce debugger. Do we have any data?
> >
> > Not off-hand, no.
> >
> >
> >
> >
> > Which brings me to the next idea: Couldn't we just assign a line 0
> location instead of removing the DebugLoc and everybody would be happy?
> >
> > Will make the line table larger, which might not be ideal (I remember
> Paul tried this, then went to only using zero loc at the start of basic
> blocks to ensure that block ordering didn't affect locations by causing
> un-lined instructions at the start of a block to inherit the location at
> the end of the previous block). But maybe for this new use case/concern the
> cost might be worthwhile? Not sure.
> >
> > If we just limit it to function calls, the size increase might be
> acceptable. We'd have to measure it.
> >
> > Ah, true
> >
> >
> >
> > Or should we treat moving an instruction to a different BB different
> from CSE?
> >
> > CSE's just the most compelling example, even if it's only a single move
> it could still be confusing to the user and the profile (eg: the block
> might be conditional, the destination unconditional - so as a debugger user
> you'd still be mislead into believing the condition is true because your
> code is executing, when that's not the case)
> >
> > - Dave
> >
> >
> > -- adrian
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