llvm dev - Dec 2021 - Question regarding correctness of debug information generated by LLC

> Sorry for the late reply, this is my first day back after a long
> holiday weekend. The gist of your reply seems to be that it's a little
> hard to determine what might be going wrong because there is no debug
> information in the IR I shared. There was debug information in my
> original IR, it's true, but I was still able to reproduce the issue I
> was encountering whether or not the debug information was present, so
> it got removed during the process of minimization. The reason I
> considered this normal is because LLC still generates CFI/CFA
> directives in the assembly even when no debug information is present
> in the IR, and my understanding was that this was the information that
> the debugger used in order to unwind the stack.
Ah, okay, I misunderstood.  The CFI/CFA directives are not what I
normally think of as debug information, although of course they are
used to build the unwind tables in the .debug_frame section (or in
the .eh_frame section, if you're not producing debug info).

I suspect you are not targeting an X86-family architecture?  I was
not able to persuade llc for x86_64 Ubuntu to emit the tail calls
that you describe.  Could you provide a complete llc command line
(including triple) that demonstrates the problem for you?

Thanks,
--paulr

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.3 LTS
Release:        20.04
Codename:       focal
$ ~/llvm-project/build/bin/llc --version
LLVM (http://llvm.org/):
  LLVM version 13.0.0
  Optimized build.
  Default target: x86_64-unknown-linux-gnu
  Host CPU: znver1

  Registered Targets:
    x86    - 32-bit X86: Pentium-Pro and above
    x86-64 - 64-bit X86: EM64T and AMD64
$ ~/llvm-project/build/bin/llc -mtriple=x86_64-unknown-linux-gnu -O0 debug.ll

If you inspect the debug.s, you will see that all the `tail call
tailcc` calls which are actually in tail position will be compiled to
the `jmp` instruction, as expected. The other calls are either not
`tailcc` calling convention, not marked as a tail call (because in the
original unminimized source, they were not in tail position), or are
not now in tail position (because they were in the original source,
but the minimized version does not put them in that position).

On Tue, Nov 30, 2021 at 3:51 PM <paul.robinson at sony.com>
wrote:
>
> > Sorry for the late reply, this is my first day back after a long
> > holiday weekend. The gist of your reply seems to be that it's a
little
> > hard to determine what might be going wrong because there is no debug
> > information in the IR I shared. There was debug information in my
> > original IR, it's true, but I was still able to reproduce the
issue I
> > was encountering whether or not the debug information was present, so
> > it got removed during the process of minimization. The reason I
> > considered this normal is because LLC still generates CFI/CFA
> > directives in the assembly even when no debug information is present
> > in the IR, and my understanding was that this was the information that
> > the debugger used in order to unwind the stack.
>
> Ah, okay, I misunderstood.  The CFI/CFA directives are not what I
> normally think of as debug information, although of course they are
> used to build the unwind tables in the .debug_frame section (or in
> the .eh_frame section, if you're not producing debug info).
>
> I suspect you are not targeting an X86-family architecture?  I was
> not able to persuade llc for x86_64 Ubuntu to emit the tail calls
> that you describe.  Could you provide a complete llc command line
> (including triple) that demonstrates the problem for you?
>
> Thanks,
> --paulr
>

-- 
Dwight Guth
Chief Information Officer
Runtime Verification, Inc.

Email: dwight.guth at runtimeverification.com

> $ ~/llvm-project/build/bin/llc -mtriple=x86_64-unknown-linux-gnu -O0
> debug.ll
Thanks!  That is very helpful.

I see a `jmp` from sender12 to apply_rule_6300, from apply_rule_6300
to sender4, and from apply_rule_6299 to sender1.  Does that match
your expectations?  I want to make sure I'm looking at what you're
looking at.
> > The issue I am having is that it would appear that the debug
> > information generated for my program indicates a different value for
> > the $rsp register within the `apply_rule_6870` call frame at each of
> > the two different call sites of `scanStackRoots`. The value seems
> > correct at the first call site. However, at the second call site, the
> > value appears incorrect and gdb actually cannot find the `main` stack
> > frame when it tries to do a backtrace.
Looking at the paths to scanStackRoots, the call sequence and
my take on where you're having the problem look like this:

main
  apply_rule_6870
    sender12
      scanStackRoots // here, gdb can find apply_rule_6870 fine
      (jmp) apply_rule_6300
        (jmp) sender4
          apply_rule_6299
            (jmp) sender1
              apply_rule_6297
                koreAllocAndCollect_p1s_blocks
                  koreCollect
                    scanStackRoots // here, there's a problem

I'm not as fluent in x86 as perhaps I should be, but I do see
one thing that makes me wonder if it's entirely correct.  The
end of apply_rule_6300 looks like this:

    popq %rax
    .cfi_def_cfa_offset 8 # so far so good
    addq $48, %rsp
    jmp sender4

My guess is that the code generator is adjusting the stack
frame size because sender4 has a much shorter argument list
than apply_rule_6300, and it's possible that the .cfi directives
aren't describing that correctly.  Truthfully, I don't know
enough about .cfi directives to say whether they *can* describe
that correctly.

You could work around this by making sender4 not be tail-callable,
perhaps?

This is definitely worth filing a bug; unfortunately, the project
is transitioning from Bugzilla to github issues, and the transition
is not complete, which means there is literally no way to file a
bug at the moment.  When that opens up, though, if you could file it
(calling it "unwind info" rather than "debug info" to avoid
confusion)
that would be very much appreciated!  I'll leave a note to myself to
ping this thread when the transition is done.

Thanks,
--paulr