[llvm-dev] DW_OP_implicit_pointer design/implementation in general

Sat Nov 16 00:19:29 PST 2019

Hi Dave,

Let me explain my point of view (apology for long mail)

The bigger goal is to enable debugger to be able to display value of as many variables as possible (in theory it can be ALL, practically it may be all important variables).

This can be achieved in multiple ways.

  1.  Keeping the variable in memory
  2.  If optimized out, it can be represented as DWARF expressions (DW_OP_stack_value, DW_OP_lit0, DW_OP_piece).
  3.  If variable is optimized out but its value can be seen in other variable (statement ‘ptr=&obj’ implies (‘*ptr=obj’) that value contained by ‘ptr’ can be seen in ‘obj’) . This is done using DW_OP_implicit_pointer. DW_OP_implicit pointer just tells that value can be implicitly checked in OTHER variable which is present. Its responsibility ends here, the value will be displayed ONLY if the OTHER variable has value (may be using DWARF expressions as in way-2 above).

Coming back to your test case. Thanks for sharing the test case.

If you notice for the same test case value is propagated to function ‘f’.
---------------
__attribute__((optnone)) int source() {
  return 3;
}
__attribute__((optnone)) void f(int i) {
}
inline void sink(const int& p) {
  f(p);
}
int main() {
  sink(source());
}
------------
With gdb
(gdb) bt
#0  f (i=3) at test1.cc:5
#1  0x00000000004004b0 in sink (p=<optimized out>) at test1.cc:7
#2  main () at test1.cc:10

In this case though we are not able to check the variable in ‘sink’ but we can check it in ‘f’. As compiler decides it as non-important in sink while propagates it to ‘f’

Lets change a little to test case.
----------------------
__attribute__((optnone)) int source() {
  return 3;
}
__attribute__((optnone)) void f(const int& i) {
}
inline void sink(const int& p) {
  f(p);
}
int main() {
  sink(source());
}
-----------------------
#0  f (i=@0x7fffffffe3e4: 3) at test2.cc:5
#1  0x00000000004004b8 in sink (p=@0x7fffffffe3e4: 3) at test2.cc:7
#2  main () at test2.cc:10
-----------------------
In this case we can get the value of ‘p’ in ‘sink’ as well, and that happens because compiler considers it important and decides to keep the temporary (store to it).

IMO the variable value not seen is corner case (it doesn’t apply to pointers, it applies to references only when it refers to a temporary and compiler does optimize that temporary). But even if we decide to get this case displayed, it comes under bigger goal of displaying a variable value. We can solve this problem in many ways.

  *   Way-1 above) Keeping the temporary (store to it). It is performance penalty compiler decides if variable is that important.
  *   Way-3 above) DW_OP_implicit_pointer is only implies/redirects a variable to something that is available in Dwarf, so ideally this case doesn’t fall under this. But yes to extend it we can generate artificial DIE for temporary variable as suggested by Paul
  *   Way-2 above) We can use any existing/new Dwarf expression to keep the value in variable itself.

IMO since now DW_OP_implicit_pointer is part of standard. It will be good to comply. Moreover popular tool GNU gdb will work with it which would be added advantage. We can start with existing DW_OP_implicit_pointer, incrementally we can increase the scope of bigger goal of displaying all the important variables (with/without help of implicit pointers). Thanks.

Regards,
Alok

From: David Blaikie <dblaikie at gmail.com>
Sent: Friday, November 15, 2019 11:24 PM
To: Robinson, Paul <paul.robinson at sony.com>
Cc: Adrian Prantl <aprantl at apple.com>; Sharma, Alok Kumar <AlokKumar.Sharma at amd.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

[CAUTION: External Email]

On Fri, Nov 15, 2019 at 8:07 AM Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>> wrote:
| Any ideas why it wouldn't be more general to handle cases where the variable isn't named?

Couldn’t there be a DIE (flagged as artificial) to describe the return-value temp?

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.

  You’d need such a DIE if you wanted the debugger to be able to look at the return value from source() anyway,

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

(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:

__attribute__((optnone)) int source() {
  return 3;
}
__attribute__((optnone)) void f(int) {
}
inline void sink(const int& p) {
  f(p);
}
int main() {
  sink(source());
}

& then playing that through GDB:

(gdb) start
Temporary breakpoint 1 at 0x401131: file var.cpp, line 10.
Starting program: /usr/local/google/home/blaikie/dev/scratch/a.out

Temporary breakpoint 1, main () at var.cpp:10
10        sink(source());
(gdb) s
source () at var.cpp:2
2         return 3;
(gdb) fin
Run till exit from #0  source () at var.cpp:2
main () at var.cpp:10
10        sink(source());
Value returned is $1 = 3
(gdb) s
sink (p=<optimized out>) at var.cpp:7
7         f(p);

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?

- Dave

in the context of main() and in the absence of inlining.  And given that DIE, implicit_pointer within sink() can refer to it.

From: David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>
Sent: Thursday, November 14, 2019 5:32 PM
To: Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>>
Cc: Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>>; AlokKumar.Sharma at amd.com<mailto:AlokKumar.Sharma at amd.com>; Jonas Devlieghere <jdevlieghere at apple.com<mailto:jdevlieghere at apple.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: DW_OP_implicit_pointer design/implementation in general

On Thu, Nov 14, 2019 at 1:53 PM Robinson, Paul <paul.robinson at sony.com<mailto: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<mailto:dblaikie at gmail.com>>
Sent: Thursday, November 14, 2019 4:34 PM
To: Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>>
Cc: AlokKumar.Sharma at amd.com<mailto:AlokKumar.Sharma at amd.com>; Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>>; Jonas Devlieghere <jdevlieghere at apple.com<mailto:jdevlieghere at apple.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto: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<mailto:aprantl at apple.com>> wrote:

> On Nov 14, 2019, at 1:21 PM, David Blaikie <dblaikie at gmail.com<mailto: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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