Re: [Dwarf-Discuss] variable locations - safe use as lvalues

[Adrian Prantl via Dwarf-Discuss]

> On Jan 20, 2020, at 2:20 PM, Frank Ch. Eigler via Dwarf-Discuss 
> <dwarf-discuss@lists.dwarfstd.org> wrote:
> 
> Hi -
> 
> I have a question about variable location lists, but not their
> encoding, the use they are suitable for.  The basic debugging scenario
> is just reading variable values, for which this is fine, especially
> when high-quality compilers emit exquisitely detailed data for their
> optimized code.
> 
> But what about writes - as though one could edit the program to insert
> an assignment, and resume?  A whole slew of complications come up:
> 
> - trying to modify a variable permanently, but the compiler only
>  emitted -some- of its locations
> 
> - trying to modify a variable (and only one), but the compiler put two
>  variables into the same location at that PC
> 
> - expressions using that value as input might have already started
>  to be computed, so it may be too late to change it at the PC
>  in question
> 
> - ... and undoubtedly other complications exist!
> 
> A debugger cannot currently be told that any particular variable
> location expression is safe to use as an lvalue, something roughly
> "exclusive, exhaustive, -O0-equivalent".  I believe most debuggers
> don't even handle the multiple-locations case for writes at all.  I
> don't know why - assume complications are rare?  or we have kind of
> papered over the problem?
> 
> As a DWARF standard level matter, descriptive rather than prescriptive
> as it is, what might be the minimum extension required to communicate
> lvalue-safety to a debugger?  A DW_OP_lvalue_safe assertive wrapper
> around a real expression?


My reading of the Location Description was that only Single Simple Memory and 
Register Location Descriptions (2.6.1.1.2, 2.6.1.1.3) are safe to use as 
L-values in the debugger. In practice, even that may be difficult, since even 
without optimizations a too-clever register allocator or instruction selector 
may insert a copy of a register or may re-use a previously loaded copy of a 
memory location instead of loading it again for each use. Something like a 
DW_OP_describes_all_locations flag would be needed to communicate what the 
compiler did to the debugger.

>From my experience with LLVM, it might take some work to implement this 
>correctly, since compilers typically only track one value for debug info 
>purposes and don't yet have the infrastructure to keep track of alternative 
>locations.

-- adrian
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