[lldb-dev] DW_OP_deref handling

[Davide Italiano via lldb-dev]

Follow up. This turned out to be a bug in the swift specific support,
so I don't think the DWARF parser code is wrong here.

Thanks!

--
Davide
On Thu, Oct 11, 2018 at 10:52 AM Davide Italiano <dccitaliano at gmail.com> wrote:
>
> Hi Greg,
> I have this issue that I suspect is a bug in lldb’s handling of DW_OP_deref.
> I wasn’t able to craft an easy C++ testcase, so I’ll start from my
> motivating (swift) example.
>
> Given the following code:
>
> func use<T>(_ t : T) {}
> func single<T>(_ t : T) {
>   let x = t
>   use(x)
> }
> let s = "hello"
> single(s)
>
> The IR emitted for the local binding `x` in `single<T>` is an alloca
> containing the address of `x`. Hence, the DWARF, correctly contains a
> DW_OP_deref expression, i.e.:
>
> 0x000000da:                 TAG_variable [9]
>                              AT_location( fbreg -16, deref )
>                              AT_name( "x" )
>                              AT_decl_file(
> "/Users/dcci/work/swift/build/Ninja-RelWithDebInfoAssert+stdlib-RelWithDebInfo/swift-macosx-x86_64/bin/pat.swift"
> )
>                              AT_decl_line( 4 )
>                              AT_type( {0x00000204} ( $sxD ) )
>
> When I debug this with lldb, I expect lldb to print the value that’s
> pointed by %rbp - 16,
> i.e.
> (lldb) register read $rbp
>      rbp = 0x00007ffeefbff860
>
> (lldb) mem read 0x00007ffeefbff850
> 0x7ffeefbff850: 10 f8 bf ef fe 7f 00 00 b8 32 4d 00 01 00 00 00
> .????...?2M.....
> 0x7ffeefbff860: c0 f8 bf ef fe 7f 00 00 64 09 00 00 01 00 00 00
> ?????...d.......
>
> So, the value that I expected to be printed is 0x7ffeefbff810.
>
> What I instead see in the debugger is:
> (T) x = <read memory from 0xe500000000000000 failed (0 of 16 bytes read)>
>
> The value `0xe500000000000000` happens to be the value that’s pointed
> by the address 0x7ffeefbff810, that is:
>
> (lldb) mem read 0x7ffeefbff810
> 0x7ffeefbff810: 00 00 00 00 00 00 00 e5 68 65 6c 6c 6f 00 00 00
> .......?hello...
> 0x7ffeefbff820: 05 00 00 00 00 00 00 00 40 32 4d 00 01 00 00 00
> ........ at 2M.....
>
> Is this an expected behavior? I did expect DW_OP_deref to just do a
> single dereference and not two. It looks like when we call
> `UpdateValue()` in lldb we do have the correct value in `m_value`, but
> GetPointerValue() ends up printing the value that’s *pointed by*
> what’s in `m_value`, i.e.
>
>     m_integer = {
>       U = {
>         VAL = 0x00007ffeefbff810
>         pVal = 0x00007ffeefbff810
>       }
>
> as this is a LoadAddress. Greg, as you wrote this code (and touched it
> last), what do you expect to be the correct semantic here?
>
> Removing a dereference from DW_OP_deref in the DWARFASTParser works
> around the issue, but I’m fairly sure it’s the incorrect fix. Are we
> maybe setting the value type incorrectly?
>
>
> Best,
>
>
> —
>
> Davide