[llvm-dev] [DebugInfo] Different representations of optimised-out variables in DWARF

[Jeremy Morse via llvm-dev]

Hi,

This was "[llvm-dev] [DebugInfo] The current status of debug values
using multiple machine locations" but I don't want to de-rail Stephens
thread,

Paul wrote:
> I'm not actually sure what causes variables to be dropped from the DWARF
> entirely, as opposed to them existing but having an unknown location for
> their entire scope; however, outside of our desire to use dwarfdump to
> analyze our debug info it's simply more efficient to omit variables with no
> location, since they inflate the debug info size and I don't believe
> there's any practical value in having them.

David wrote:
> When does this ^ happen? In optimized builds we include all local variables
> in a "variables" attachment to the DISubprogram, so we shouldn't be losing
> variables entirely.
> [...]
> I think it's pretty important that we keep them. It helps a user understand
> that they've not mistyped the name of a variable, etc [...]

This is something that's bothered me for a while, as it messes with
our statistics when changing how variable locations are tracked. Take
this completely contrived C file:

    int foo(int bar) {
      int baz = 12 + bar;
      return baz;
    }

    int qux(int quux) {
      int xyzzy = foo(quux);
      return xyzzy;
    }

Using clang ef0dcb50630 and options "-O3 -g -c", llvm-locstats reports
the object file has five variables in it. If you emit LLVM-IR, and
replace the first operand of all "llvm.dbg.value" intrinsic
invocations with "undef" and compile the IR with llc, then
llvm-locstats still reports five variables. However: if you instead
/delete/ all the invocations of "llvm.dbg.value", four variables are
reported by llvm-locstats. This indicates there's an observable
difference in the way we represent optimised-out variables in DWARF.

The difference between the object files is the way they represent the
inlined copy of "foo", here's the output with undef dbg.values,
followed by the output when I delete all the dbg.value intrinsics:

     DW_TAG_inlined_subroutine
       DW_AT_abstract_origin (0x0000004e "foo")
       DW_AT_low_pc  (0x0000000000000010)
       DW_AT_high_pc (0x0000000000000013)
       DW_AT_call_file       ("/tmp/test.c")
       DW_AT_call_line       (7)
       DW_AT_call_column     (0x0f)

       DW_TAG_formal_parameter
             DW_AT_abstract_origin       (0x0000005a "bar")

      NULL

and:

     DW_TAG_inlined_subroutine
       DW_AT_abstract_origin (0x00000048 "foo")
       DW_AT_low_pc  (0x0000000000000010)
       DW_AT_high_pc (0x0000000000000013)
       DW_AT_call_file       ("/tmp/test.c")
       DW_AT_call_line       (7)
       DW_AT_call_column     (0x0f)

     NULL

When there are dbg.value intrinsics present, then the inlined
subroutine gets an empty DW_TAG_format_parameter that links back to
the abstract origin. If there are no dbg.value intrinsics present, it
does not. As far as I understand it, consumers can still determine
that "bar" exists in the inlined subroutine by looking at the inlined
subroutines abstract origin. This is what the "retained nodes"
collection preserves.

llvm-locstats / llvm-dwarfdump --statistics should probably be taught
to look at the inlined subroutines abstract origin to find all
variables, however, it seems unwise to have internal compiler state
reflected in the output file in the way it is above. The cause of the
empty DW_TAG_formal_parameter being created in
DwarfDebug::collectEntityInfo [0] -- it distinguishes between a
variable that has no location intrinsics, and a variable that has only
empty location intrinsics. Putting a filter in to skip variables with
only empty locations avoids the difference in output, and reduce the
size of .debug_info on a stage2reldeb clang build by about 20Mb, or
~1%.

To ensure this email contains a question: would there be any
objections to adding that filter, and obliging consumers to look in
the inlined subroutines abstract origin to find optimised-out
variables, instead of giving them a list per-inlined-instance?

[0] https://github.com/llvm/llvm-project/blob/70e251497c4e26f8cfd85e745459afff97c909ce/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp#L1779

--
Thanks,
Jeremy