[llvm-dev] Why is lldb telling me "variable not available"?

[Jeremy Morse via llvm-dev]

Hi Brian,

On Tue, Feb 25, 2020 at 7:43 PM Brian Gesiak <modocache at gmail.com> wrote:
> In other words, the value of %i is stored on the frame object, on the
> heap, at an offset of 7 into the frame. I'm beginning to think a
> fundamental fix for this issue would be to stop replacing
> llvm.dbg.declare with llvm.dbg.value, and instead replace the
> llvm.dbg.declare with llvm.dbg.addr that points the debugger to the %i
> variable's new permanent location as an offset into the coroutine
> frame object. Does this approach make sense to people on this mailing
> list, who probably know more about how these intrinsics work than I
> do?

This matches a few similar use cases that I'm aware of -- certain
kinds of struct that are passed-by-value according to the language,
but passed-by-reference according to ABI, are treated in that way. In
general, the downside is that the debugger can only observe variable
values when they get written to memory, not when they're computed, as
dbg.values and dbg.declares aren't supposed to be mixed. Observing
variable values slightly later might be an improvement over the
current situation.

Although, I don't think this will work immediately, see below,

> I tried multiple approaches to manually inserting an llvm.dbg.addr
> after the store instruction, as per your suggestion, Jeremy. I used
> llc to compile the IR into an object file that I then linked, and
> inspected the DWARF generated for the file. Unfortunately, inserting
> dbg.addr that operated on the reloaded values didn't lead to any
> change in the DWARF that was produced --  specifically, this didn't
> make a difference:
>
> call void @llvm.dbg.addr(metadata i32* %i.reload.addr62, metadata
> !873, metadata !DIExpression()), !dbg !884

Ouch, I tried this myself, and ran into the same difficulty. I'd
missed that all your functions are marked "optnone" / -O0, which means
a different instruction-selection pass (FastISel) runs, and it turns
out FastISel isn't aware of dbg.addrs existence. Even better, FastISel
doesn't manage to lower any debug intrinsic (including dbg.declare)
that refers to a GEP, because it doesn't have a register location (the
GEP gets folded into a memory addressing mode).

I've hacked together some support in [0], that allows dbg.addr's of
GEPs to be handled. A single dbg.addr at the start of the function
(and no dbg.values) should get you the same behaviour as a
dbg.declare. I suspect the reason why this problem hasn't shown up in
the past is because the coroutine code being generated hits a gap
between "optimised" and "not optimised": I believe all variables in
code that isn't optimised get their own storage (and so will always
have a stack or register location). Wheras in the coroutine code
you're generating the variable address doesn't get storage.

If [0] is useful for you I can get that landed; it'd be good to hear
whether this resolves the dbg.addr intrinsics not having an affect on
the output.

[0] https://github.com/jmorse/llvm-project/commit/40927e6c2b71ec914d937287a0c2ca6c52c01f6b

--
Thanks,
Jeremy