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<p class="MsoNormal">I’ve been reminded of PR37682, where a function with a reference parameter might spend all its time computing the “referenced” value in a temp, and only move the final value back to the referenced object at the end. This is clearly a situation
that could benefit from DW_OP_implicit_pointer, and there is really no other-object DIE for it to refer to. Given the current spec, the compiler would need to produce a DW_TAG_dwarf_procedure for the parameter DIE to refer to. Appendix D (Figure D.61) has
an example of this construction, although it’s a more contrived source example.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Does it <b>have</b> to be spec’d this way? I think the spec as given is general enough to support DW_OP_implicit_pointer to an aggregate, with different locations for each member. You could probably come up with a way to specify simpler
cases more simply, although you’d need a new DW_OP to do that—there’s no explicit FORM describing the operand of a DW_OP, so we can’t just mess with how the operands are interpreted.<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> Friday, November 15, 2019 12:54 PM<br>
<b>To:</b> Robinson, Paul <paul.robinson@sony.com><br>
<b>Cc:</b> Adrian Prantl <aprantl@apple.com>; AlokKumar.Sharma@amd.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"><o:p> </o:p></p>
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<p class="MsoNormal">On Fri, Nov 15, 2019 at 8:07 AM Robinson, Paul <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">| Any ideas why it wouldn't be more general to handle cases where the variable isn't named?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Couldn’t there be a DIE (flagged as artificial) to describe the return-value temp?<o:p></o:p></p>
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<p class="MsoNormal"><br>
There could be - though there are very few (the array bound example Adrian gave is the only one I know of - and even that seems unnecessary/GCC uses a different (& I think better/clearer/simpler) representation) cases of artificial variables being generated
in Clang/LLVM - it lacks precedent so far as I can tell.<br>
<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> You’d need such a DIE if you wanted the debugger to be able to look at the return value from source() anyway,<o:p></o:p></p>
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<p class="MsoNormal"><br>
Not so far as I know - with GDB (& I assume LLDB) when you call a function and return from it (eg: "finish" or "step" that steps across the end of a function) the debugger prints out the return value (using the DW_AT_type of the DW_TAG_subprogram that was executing
& its knowledge of the ABI to know where/how that value would be stored during the return) & you can actually then query it and do other things using the artificial variable name GDB provides<br>
<br>
(my example was slightly bogus - you can't take the address of a temporary in C++ like that, but you can take a reference to it, so updating & fleshing out the test:<br>
<br>
<span style="font-family:"Courier New"">__attribute__((optnone)) int source() {<br>
return 3;<br>
}<br>
__attribute__((optnone)) void f(int) {<br>
}<br>
inline void sink(const int& p) {<br>
f(p);<br>
}<br>
int main() {<br>
sink(source());<br>
}</span><br>
<br>
& then playing that through GDB:<br>
<span style="font-family:"Courier New""><br>
(gdb) start<br>
Temporary breakpoint 1 at 0x401131: file var.cpp, line 10.<br>
Starting program: /usr/local/google/home/blaikie/dev/scratch/a.out <br>
<br>
Temporary breakpoint 1, main () at var.cpp:10<br>
10 sink(source());<br>
(gdb) s<br>
source () at var.cpp:2<br>
2 return 3;<br>
(gdb) fin<br>
Run till exit from #0 source () at var.cpp:2<br>
main () at var.cpp:10<br>
10 sink(source());<br>
Value returned is $1 = 3<br>
(gdb) s<br>
sink (p=<optimized out>) at var.cpp:7<br>
7 f(p);</span><br>
<br>
It'd be nice if the value of 'p' could be printed there, but it seems without introducing artificial variables, the implicit_pointer doesn't provide a way to do that & that seems to me like an unnecessary limitation & complication in the DWARF and in LLVM's
intermediate representation compared to having 'p's DW_AT_location describe the value being pointed to directly without the need for another variable?<br>
<br>
- Dave<br>
<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">in the context of main() and in the absence of inlining. And given that DIE, implicit_pointer within sink() can refer to it.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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 5:32 PM<br>
<b>To:</b> Robinson, Paul <<a href="mailto:paul.robinson@sony.com" target="_blank">paul.robinson@sony.com</a>><br>
<b>Cc:</b> Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank">aprantl@apple.com</a>>;
<a href="mailto:AlokKumar.Sharma@amd.com" target="_blank">AlokKumar.Sharma@amd.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<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Thu, Nov 14, 2019 at 1:53 PM Robinson, Paul <<a href="mailto:paul.robinson@sony.com" target="_blank">paul.robinson@sony.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">void inline_me(foo *ptr) {<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> does something with ptr->x or *ptr;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">}<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">void caller() {<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> foo actual_obj;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> inline_me(&actual_obj);<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">}<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I think it is not widely applicable outside of that kind of scenario.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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());<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">}<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">--paulr<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Thu, Nov 14, 2019 at 1:27 PM Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank">aprantl@apple.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <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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