<div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Nov 14, 2019 at 1:53 PM Robinson, Paul <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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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:<u></u><u></u></p>
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<p class="MsoNormal">void inline_me(foo *ptr) {<u></u><u></u></p>
<p class="MsoNormal"> does something with ptr->x or *ptr;<u></u><u></u></p>
<p class="MsoNormal">}<u></u><u></u></p>
<p class="MsoNormal">void caller() {<u></u><u></u></p>
<p class="MsoNormal"> foo actual_obj;<u></u><u></u></p>
<p class="MsoNormal"> inline_me(&actual_obj);<u></u><u></u></p>
<p class="MsoNormal">}<u></u><u></u></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.<u></u><u></u></p>
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<p class="MsoNormal">I think it is not widely applicable outside of that kind of scenario.</p></div></div></blockquote><div><br></div><div>Any ideas why it wouldn't be more general to handle cases where the variable isn't named? Such as:<br><br>foo source();<br>void f(foo);<br>inline void sink(foo* p) {<br> f(*p);<br>}<br>int main() {<br> sink(&source());</div><div>}</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-US"><div class="gmail-m_-2404182418550447038WordSection1"><p class="MsoNormal"><u></u><u></u></p>
<p class="MsoNormal">--paulr<u></u><u></u></p>
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<p class="MsoNormal"><b>From:</b> David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> <br>
<b>Sent:</b> Thursday, November 14, 2019 4:34 PM<br>
<b>To:</b> Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank">aprantl@apple.com</a>><br>
<b>Cc:</b> <a href="mailto:AlokKumar.Sharma@amd.com" target="_blank">AlokKumar.Sharma@amd.com</a>; Robinson, Paul <<a href="mailto:paul.robinson@sony.com" target="_blank">paul.robinson@sony.com</a>>; Jonas Devlieghere <<a href="mailto:jdevlieghere@apple.com" target="_blank">jdevlieghere@apple.com</a>>; llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<b>Subject:</b> Re: DW_OP_implicit_pointer design/implementation in general<u></u><u></u></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" target="_blank">aprantl@apple.com</a>> wrote:<u></u><u></u></p>
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> 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?
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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.<u></u><u></u></p>
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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>
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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.<u></u><u></u></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)<u></u><u></u></p>
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-- adrian<br>
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> <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<u></u><u></u></p>
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