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

[Brian Gesiak via llvm-dev]

Apologies for the slow response here Jeremy. Your reply has been incredibly
helpful so far, I just need to try adding 'llvm.dbg.addr' myself to confirm
that works. Thank you! - Brian Gesiak

On Thu, Feb 6, 2020 at 11:04 AM Jeremy Morse <jeremy.morse.llvm at gmail.com>
wrote:

> Hi Brian,
>
> Thanks for working on coroutines, the debugging experience, and in
> particular thanks for the comprehensive write-up!,
>
> On Thu, Feb 6, 2020 at 1:19 PM Brian Gesiak via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Specifically, I’m trying to improve lldb’s behavior when showing
> > variables in the current stack frame, when that frame corresponds to a
> > coroutine function.
>
> [...]
>
> Everything in the IR appears correct to my eyes, although I know next
> to nothing about coroutines and might have missed something. The
> simplest explanation of why the variable location goes missing can be
> seen in the disassembly:
>
> > ```
> >     0x401885 <+373>: movq   -0x8(%rbp), %rax
> >     0x401889 <+377>: movl   $0x0, 0x40(%rax)
> >     0x401890 <+384>: X movl   0x28(%rax), %edx
> >     0x401893 <+387>: X addl   $0x1, %edx
> >     0x401896 <+390>: X movl   %edx, 0x28(%rax)
> >     0x401899 <+393>: X movl   0x40(%rax), %edx
> >     0x40189c <+396>: addl   $0x1, %edx
> >     0x40189f <+399>: movl   %edx, 0x40(%rax)
> > ->  0x4018a2 <+402>: movl   0x28(%rax), %esi
> > ```
>
> Where I've marked with 'X' before the mnemonic the instructions that
> the variable location list covers. The location of "i" is correctly
> given as edx from its load to its store, and ends when edx is
> overwritten with the value of "j". In all the rest of the code, the
> variables value is in memory, and the DWARF data doesn't record this.
>
> Ideally debug info would track variables when they're stored to memory
> -- however we don't automatically know whether any subsequent store to
> memory will overwrite that variable, and so we don't track locations
> into memory. PR40628 [0] is an example of what can go wrong, where we
> described a variable as being in memory, but didn't know when that
> location was overwritten.
>
> If whatever's producing the coroutine IR has guarantees about where
> and when variables are loaded/stored from/to memory, it should be
> possible to put more information into the IR, so that the rest of LLVM
> doesn't have to guess. For example, this portion of IR:
>
>   %15 = load i32, i32* %i.reload.addr62, align 4, !dbg !670
>   call void @llvm.dbg.value(metadata i32 %15, metadata !659, metadata
> !DIExpression()), !dbg !661
>   %inc19 = add nsw i32 %15, 1, !dbg !670
>   call void @llvm.dbg.value(metadata i32 %inc19, metadata !659,
> metadata !DIExpression()), !dbg !661
>   store i32 %inc19, i32* %i.reload.addr62, align 4, !dbg !670
>
> Could have a call to llvm.dbg.addr(metadata i32 *%i.reload.addr66,
> ...) inserted after the store, indicating that the variable is located
> in memory. This should work (TM) so long as that memory is never
> overwritten with something that isn't the current value of "i" on
> every path after the call to llvm.dbg.addr; and on every path after
> the call to llvm.dbg.addr, when the variable is loaded form memory,
> there's a call to llvm.dbg.value to indicate that the variable is
> located somewhere other than memory now.
>
> Providing that extra information should improve the location coverage
> for your example, certainly when unoptimised. However, I believe (80%)
> this method isn't safe against optimisation, because (for example)
> dead stores can be deleted by LLVM passes without deleting the call to
> llvm.dbg.addr, pointing the variable location at a stale value in
> memory. Unfortunately I'm not aware of a facility or technique that
> protects against this right now. (CC Reid who I think ran into this
> last?).
>
> Note that there's some support for tracking variables through stack
> spills in post-isel debug data passes, however those loads and stores
> operate in well defined ways, and general loads and stores might not.
>
> [0] https://bugs.llvm.org/show_bug.cgi?id=40628
>
> --
> Thanks,
> Jeremy
>
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