[llvm-dev] DW_OP_implicit_pointer design/implementation in general

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 14 13:42:48 PST 2019

On Thu, Nov 14, 2019 at 1:33 PM David Blaikie <dblaikie at gmail.com> wrote:

> 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).

Hey, what'd you know, GCC actually produces what I described:

  DW_TAG_array_type [6] *
    DW_AT_type [DW_FORM_ref4]       (cu + 0x0069 => {0x00000069} "int")
    DW_AT_sibling [DW_FORM_ref4]    (cu + 0x0083 => {0x00000083})

    DW_TAG_subrange_type [7]
      DW_AT_type [DW_FORM_ref4]     (cu + 0x0083 => {0x00000083} "long
unsigned int")
      DW_AT_upper_bound [DW_FORM_exprloc]   (DW_OP_fbreg -40, DW_OP_deref)

No artificial variable the way Clang does it:

    DW_TAG_variable [4]
                  DW_AT_location [DW_FORM_exprloc]      (DW_OP_fbreg -24)
                  DW_AT_name [DW_FORM_strp]     ( .debug_str[0x000000a6] =
                  DW_AT_type [DW_FORM_ref4]     (cu + 0x0074 =>
{0x00000074} "long unsigned int")
                  DW_AT_artificial [DW_FORM_flag_present]       (true)

      DW_AT_type [DW_FORM_ref4]     (cu + 0x007b => {0x0000007b} "int[]")

  DW_TAG_array_type [7] *
    DW_AT_type [DW_FORM_ref4]       (cu + 0x006d => {0x0000006d} "int")

  DW_TAG_subrange_type [8]
    DW_AT_type [DW_FORM_ref4]     (cu + 0x008b => {0x0000008b}
    DW_AT_lower_bound [DW_FORM_data1]     (0x00)
    DW_AT_count [DW_FORM_ref4]    (cu + 0x0051 => {0x00000051})

Might be nice to tidy that up some time. GCC's been doing this even in
DWARF-2 mode (where it just uses FORM_black for the upper bound, and as far
back as GCC 6.0 at least.

>> 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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