[llvm-dev] [RFC] DebugInfo: A different way of specifying variable locations post-isel

[Jeremy Morse via llvm-dev]

Hi Vedant, thanks for the detailed response,

On Tue, Feb 25, 2020 at 7:23 AM Vedant Kumar <vedant_kumar at apple.com> wrote:
> > Finally, being forced to always specify both the machine location and
> > the program location at the same time (in a single DBG_VALUE)
> > introduces un-necessary burdens. In MachineSink, when we sink between
> > blocks an instruction that defines a vreg, we chose to sink DBG_VALUE
> > instructions referring to that vreg too to avoid losing the variable
> > location. This un-necessarily risks re-ordering assignments,
>
> So under the proposed scheme, would the dbg value not be sunk? Ah, so then you’d get the dbg use before def scenario, which you argue has some nice properties below.

Indeed -- I'm aware of previous efforts to suppress dbg-use-before-def
in the past, but I'm not aware of why it's not supported (it does
/feel/ wrong at the very least). Perhaps the reason why was because it
couldn't be described post-regalloc.

(As an aside, this would move us in the direction of making it easier
to change LLVM-IR instructions, but harder to determine what a
variable location is, through the middle stages of the compiler).

> > I'd like to suggest that we can represent variable locations in the
> > codegen backend / MIR with three things:
> > * The instruction that defines the value of the variable,
> > * The operand of that instruction into which the value is written,
> > * The position in the instruction stream where the assignment of this
> > value to the variable occurs
> >
> > That's effectively modifying a machine location from being a {v,}reg,
> > into being a "defining instruction" and operand. This is closer to the
> > LLVM-IR form of a machine location, where the SSA Value and its
> > computation are synonymous. Exactly how this is represented in-memory
> > and in-printed-MIR I haven't thought a lot about; probably by
> > attaching metadata to instructions and having DBG_VALUE use a metadata
> > operand rather than referring to a vreg. Specifying machine locations
> > like this would have the following benefits:
> > * Both DBG_VALUEs and defining instructions are independent and can
> > be moved within the function without loss of information, and without
> > needing to consider so much context,
> > * Likewise, vregs can be rewritten / merged / deleted without the
> > need to update any debug metadata. Only instruction deletion /
> > morphing would need some sort of change,
>
> For deletion, that makes sense, we might consider introducing an MI level salvageDI method to help with that.
>
> For replacement/morphing, wouldn’t MachineInstr::RAUW be able to update the new-style debug uses?

I guess it would, on the assumption that a machineinstr having its
operands changed doesn't actually change the value(s) that it
computes.

> > The three instruction scheduling passes would become significantly
> > easier to deal with: they would only have to replace DBG_VALUE
> > instructions in the correct order, not worry about their operands.
> > Various debug facilities in SimpleRegisterCoalescing, MachineSink, and
> > large amounts of LiveDebugVariables would become redundant, as we
> > wouldn't need to maintain a register location through optimisations.
>
> I view this as a very large potential upside. LiveDebugVariables is incredibly complex and is a huge compile-time bear. Defining away large chunks of its job would be really nice.
>
> Would this mean that an equivalence class in LiveDebugVariables would consist exclusively of UserValues referring to the same variable (as opposed to including UserValues that share a vreg as well)?

It believe so -- although I can't say I have a complete understanding
of LiveDebugVariables' equivalence classes. That aspect of an
equivalence class could be replaced with the identity of the defining
instruction instead, but, I think the interval-splitting that
LiveDebugVariables does wouldn't be necessary any more and so
computing equivalence wouldn't be necessary either.

> > How then do we translate this new kind of machine location into
> > DWARF/CodeView variable locations, which need to know which register
> > to look in? The answer is: LiveDebugValues [3]. We already perform a
> > dataflow analysis in LiveDebugValues of where values "go" after
> > they're defined: we can use that to take "defining instructions" and
> > determine register / stack locations. We would need to track values
> > from the defining instruction up to the DBG_VALUE where that value
> > becomes a variable location, after which it's no different from the
> > LiveDebugValues analysis that we perform today. LiveDebugValues'
> > ability to track values through stack spills and restores would become
> > a critical feature (it isn't today), as we would no longer generate
> > stack locations during register allocation.
>
> Do you expect that handling for the current and new-style DBG_VALUEs could coexist in LiveDebugValues? Could that be done by e.g. introducing a new debug instr MI (DBG_INSTR_REF)?

I believe it should be relatively straightforwards: we would have an
additional kind of location-record (i.e. VarLoc) that, instead of
identifying a _variable_, identified the instruction/operand that
computed a value. Handling a DBG_INSTR_REF as you suggest would mean
looking up such a VarLoc by instruction/operand to find its register,
then creating a register location VarLoc for the variable as we do
today. Propagating the location of an instruction/operand VarLoc would
happen in exactly the same way as variable locations today, it's just
following a value as it's moved around registers.

> > Limitations
> >
> > The largest problem with this idea is that not all variable values are
> > defined by instructions: PHIs are values that are defined by control
> > flow. To deal with this pre-regalloc, we could move LiveDebugVariables
> > to run before phi-elimination.
>
> I don’t really follow this. Are you suggesting stripping out debug values before phi elim, and replacing them after virtregrewrite? How would the re-inserted debug instrs refer to values produced by phis?
>
> > My understanding is that the register
> > allocation phase of LLVM starts there and ends after virtregrewriter,
> > and it'd be legitimate to say "we do special things for these passes".
> > After regalloc however, there would need to be some way of specifying
> > a block and a register, where entry to the block defines a variable
> > value in that register. This isn't pretty; but IMO is the
> > representation closest to the truth.
>
> Oh, you answer this here. So LiveDebugVariables would need to figure out, for each debug-use-of-phi, 1) which block to put it in and 2) which register+variable gets defined. This is different enough from the new-style debug instr to potentially warrant its own instruction (DBG_PHI?).

Yes and no; I think 1) should be un-necessary, or at least not be
difficult in any way. The DBG_PHIs would refer to a block and
register, identifying the PHIs value at a single program location, the
start of the block where it is defined. This is (98% certain) easy to
identify in virtregrewriter with a location query, and avoids
considering live ranges and interval splitting. We wouldn't need to
try and know where the PHI value is later in the block or in other
blocks: the same LiveDebugValues mechanism as DBG_INSTR_REF would be
able to determine the PHI values register location at the position of
a later DBG_PHI instruction.

(Again, I've put little thought into how that would be represented in
memory or MIR).

> > I feel like this would be a better way of representing variable
> > locations in the codegen backend; my fear is that this is a lot of
> > work, and I don't know what appetite there is for change amongst other
> > interested parties. Thus I'd be interested in any kind of feedback as
> > to whether a) this is a good idea, b) whether this category of change
> > is what people want, and c) whether this is seen as being achievable.
>
> Imho this is a good idea and I’d like to see something like this happen. For it to ”really happen” we (Apple) would probably need a way to transition to the new representation incrementally (e.g. to toggle a flag to get back the old representation). I’m not yet sure about what all that would really entail.

That's great to hear! When you say toggling a flag, is that a flag for
a whole compilation, or returning to the old representation at
specific passes? I don't think it'd be too difficult to have two
implementations in the codebase and pick one to use, switching between
the two mid-flight is a lot harder of course.

> > Being able to introduce this change incrementally presents some
> > challenges: while the way of representing variable locations described
> > above is more expressive than the current way, converting between one
> > and the other requires running the LiveDebugValues analysis,
>
> This sounds like you’re planning to change the existing DBG_VALUE instruction. Have you considered keeping it — unmodified — and introducing new debug instructions for the new semantics? The migration story for that seems a lot simpler.

I hadn't thought about keeping it; you're right, it'd be a lot easier
(and a lot more testable) to do it that way. That also lines up with
having a toggle flag to allow incremental transition.

> If we can delete large parts of live debug variables this will be worth it imho. But perhaps part of the plan here should include splitting up live debug values into smaller files to simplify it?

Splitting up LiveDebugValues sounds like a good plan, entry value
development has added a certain amount of complexity to it too.

Thanks for the feedback!

--
Thanks,
Jeremy