[llvm-dev] DW_OP_implicit_pointer design/implementation in general

Adrian Prantl via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 14 13:26:54 PST 2019

> On Nov 14, 2019, at 1:21 PM, David Blaikie <dblaikie at gmail.com> wrote:
> Hey folks,
> Would you all mind having a bit of a design discussion around the feature both at the DWARF level and the LLVM implementation? It seems like what's currently being proposed/reviewed (based on the DWARF feature as spec'd) is a pretty big change & I'm not sure I understand the motivation, exactly.
> The core point of my confusion: Why does describing the thing a pointer points to require describing a named variable that it points to? What if it doesn't point to a named variable? 

Without having looked at the motivational text when the feature was proposed to DWARF, my assumption was that this is similar to how bounds for variable-length arrays are implemented, where a (potentially) artificial variable is created by the compiler in order to have something to refer to. In retrospect I find the entire specification of DW_OP_implicit_pointer to be strangely specific/limited (why one hard-coded offset instead of an arbitrary expression?), but that ship has sailed for DWARF 5 and I'm to blame for not voicing that concern earlier.

-- adrian

> Seems like there should be a way to describe that situation - and that doing so would be a more general solution than one limited to only describing pointers that point to named variables. And would be a simpler implementation in LLVM - without having to deconstruct variables during optimizations, etc, to track one variable's value being concretely related to another variable's value.
> - David

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