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<p class="MsoNormal">My reading of the DWARF issue is that it was fairly specifically designed to handle the case of a function taking parameters by pointer/reference, which is then inlined, and the caller is passing local objects rather than other pointers/references.
So:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">void inline_me(foo *ptr) {<o:p></o:p></p>
<p class="MsoNormal"> does something with ptr->x or *ptr;<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal">void caller() {<o:p></o:p></p>
<p class="MsoNormal"> foo actual_obj;<o:p></o:p></p>
<p class="MsoNormal"> inline_me(&actual_obj);<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal">After inlining, maintaining a pointer to actual_obj might be sub-optimal, but after a “step in” to inline_me, the user wants to look at an expression spelled *ptr even though the actual_obj might not have a memory address (because fields
are SROA’d into registers, or whatever). This is where DW_OP_implicit_pointer saves the day; *ptr and ptr->x are still evaluatable expressions, which expressions are secretly indirecting through the DIE for actual_obj.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I think it is not widely applicable outside of that kind of scenario.<o:p></o:p></p>
<p class="MsoNormal">--paulr<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> David Blaikie <dblaikie@gmail.com> <br>
<b>Sent:</b> Thursday, November 14, 2019 4:34 PM<br>
<b>To:</b> Adrian Prantl <aprantl@apple.com><br>
<b>Cc:</b> AlokKumar.Sharma@amd.com; Robinson, Paul <paul.robinson@sony.com>; Jonas Devlieghere <jdevlieghere@apple.com>; llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: DW_OP_implicit_pointer design/implementation in general<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Nov 14, 2019 at 1:27 PM Adrian Prantl <<a href="mailto:aprantl@apple.com">aprantl@apple.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><br>
<br>
> On Nov 14, 2019, at 1:21 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:<br>
> <br>
> Hey folks,<br>
> <br>
> 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.<br>
> <br>
> 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?
<br>
<br>
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.<o:p></o:p></p>
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<p class="MsoNormal"><br>
I /sort/ of see that case as a bit different, because the array type needs to refer back into the function potentially (to use frame-relative, etc). I could think of other ways to do that in hindsight (like putting the array type definition inside the function
to begin with & having the count describe the location directly, for instance).<br>
<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">Sure, but we don't have to implement it if we don't find it to be super useful/worthwhile, right? (if something else would be particularly more general/useful we could instead implement that as an extension, though of course there's cost
to that in terms of consumer support, etc)<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<br>
-- adrian<br>
<br>
> <br>
> 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.<br>
> <br>
> - David<o:p></o:p></p>
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