[llvm-dev] DW_OP_implicit_pointer design/implementation in general

[David Blaikie via llvm-dev]

On Thu, Nov 14, 2019 at 1:53 PM Robinson, Paul <paul.robinson at sony.com>
wrote:

> 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:
>
>
>
> void inline_me(foo *ptr) {
>
>  does something with ptr->x or *ptr;
>
> }
>
> void caller() {
>
>   foo actual_obj;
>
>   inline_me(&actual_obj);
>
> }
>
>
>
> 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.
>
>
>
> I think it is not widely applicable outside of that kind of scenario.
>

Any ideas why it wouldn't be more general to handle cases where the
variable isn't named? Such as:

foo source();
void f(foo);
inline void sink(foo* p) {
  f(*p);
}
int main() {
  sink(&source());
}


> --paulr
>
>
>
> *From:* David Blaikie <dblaikie at gmail.com>
> *Sent:* Thursday, November 14, 2019 4:34 PM
> *To:* Adrian Prantl <aprantl at apple.com>
> *Cc:* AlokKumar.Sharma at amd.com; Robinson, Paul <paul.robinson at sony.com>;
> Jonas Devlieghere <jdevlieghere at apple.com>; llvm-dev <
> llvm-dev at lists.llvm.org>
> *Subject:* Re: DW_OP_implicit_pointer design/implementation in general
>
>
>
>
>
>
>
> On Thu, Nov 14, 2019 at 1:27 PM Adrian Prantl <aprantl at apple.com> wrote:
>
>
>
> > 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.
>
>
> 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).
>
>
> 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.
>
>
>
> 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)
>
>
>
>
>
> -- 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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