[llvm-dev] RFC: Introduce DW_OP_LLVM_memory to describe variables in memory with dbg.value

[Alex Bradbury via llvm-dev]

On 5 September 2017 at 21:00, Reid Kleckner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Debug info today handles two cases reasonably well:
> 1. At -O0, dbg.declare does a good job describing variables that live at
> some known stack offset
> 2. With optimizations, variables promoted to SSA can be described with
> dbg.value
>
> This leaves behind a large hole in our optimized debug info: variables that
> cannot be promoted, typically because they are address-taken. This is
> https://llvm.org/pr34136, and this RFC is mostly about addressing that.
>
> The status today is that instcombine removes all dbg.declares and
> heuristically inserts dbg.values where it can identify the value of the
> variable in question. This prevents us from having misleading debug info,
> but it throws away information about the variable’s location in memory.

Hi Reid, thanks for writing such a clear summary of the problem that
this RFC addresses. I was wondering if there is any sort of
methodology for quantifying the "quality" of debug information? e.g.
the gap between the current debug information and 'ideal' debug info?

Thanks,

Alex