[llvm-dev] [DebugInfo] Different representations of optimised-out variables in DWARF
Jeremy Morse via llvm-dev
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?
--
Thanks,
Jeremy
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via llvm-dev
> -----Original Message-----
> From: Jeremy Morse <jeremy.m...@gmail.com>
> Sent: Wednesday, January 27, 2021 10:26 AM
> To: David Blaikie <dbla...@gmail.com>; Robinson, Paul
> <paul.r...@sony.com>; Tozer, Stephen <stephe...@sony.com>; llvm-
> dev <llvm...@lists.llvm.org>
> Subject: [DebugInfo] Different representations of optimised-out variables
> in DWARF
>
> 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:
personal names from outgoing email (or maybe it's just Outlook365's fault).
> > 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 [...]
a scope just because they get optimized away.
Rereading the description of concrete instance trees (DWARF 5,
section 3.3.8.2) it explicitly permits omitting a useless entry
(has only abstract_origin as an attribute, and no children) (p.85
item 1). I think it is legal to omit these, and I would go so far
as to say it's specifically legal to omit formal_parameter DIEs
with no attributes (other than abstract_origin).
--paulr
>
> [0] https://urldefense.com/v3/__https://github.com/llvm/llvm-
> project/blob/70e251497c4e26f8cfd85e745459afff97c909ce/llvm/lib/CodeGen/Asm
> Printer/DwarfDebug.cpp*L1779__;Iw!!JmoZiZGBv3RvKRSx!tY6GfPI-
> g0N1h42dFceJMSVgUrJkaLldqFUlVzWL2QrrVEbrcltoibOrpKdWjMiz8g$
Djordje Todorovic via llvm-dev
>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?
Sent: Wednesday, January 27, 2021 4:26 PM
To: David Blaikie <dbla...@gmail.com>; Paul Robinson <paul.r...@sony.com>; Tozer, Stephen <Stephe...@sony.com>; llvm-dev <llvm...@lists.llvm.org>
Subject: [llvm-dev] [DebugInfo] Different representations of optimised-out variables in DWARF
David Blaikie via llvm-dev
We should never produce DWARF like this:
DW_TAG_formal_parameter
DW_AT_abstract_origin (0x0000005a "bar")
And locstats/dwarfdump statistics should not produce different results if it reads DWARF like that compared to DWARF missing this inlined instance entirely.
Djordje Todorovic via llvm-dev
DW_AT_abstract_origin (0x0000005a "bar")
Sent: Wednesday, January 27, 2021 9:40 PM
To: Jeremy Morse <jeremy.m...@gmail.com>
Cc: llvm-dev <llvm...@lists.llvm.org>
Subject: Re: [llvm-dev] [DebugInfo] Different representations of optimised-out variables in DWARF
Jeremy Morse via llvm-dev
David wrote:
> We should never produce DWARF like this:
A patch to achieve that: https://reviews.llvm.org/D95617
On Thu, Jan 28, 2021 at 10:19 AM Djordje Todorovic
<Djordje....@syrmia.com> wrote:
> IMPO, this works intuitively, but if you think that we should handle 2) case in some other way (e.g. just to ignore such parameter), please let me know.
As far as I understand it, we shouldn't be counting the number of
variables "processed", and instead be counting:
* The total number of variables for each scope, by using the inlined
subprograms abstract origins to find in-scope variables that have no
location,
* The number of concrete variables that have a DW_AT_location
The first number should remain stable regardless of how well we do at
tracking variable locations.
David Blaikie via llvm-dev
Hi,
David wrote:
> We should never produce DWARF like this:
A patch to achieve that: https://reviews.llvm.org/D95617
On Thu, Jan 28, 2021 at 10:19 AM Djordje Todorovic
<Djordje....@syrmia.com> wrote:
> IMPO, this works intuitively, but if you think that we should handle 2) case in some other way (e.g. just to ignore such parameter), please let me know.
As far as I understand it, we shouldn't be counting the number of
variables "processed", and instead be counting:
* The total number of variables for each scope, by using the inlined
subprograms abstract origins to find in-scope variables that have no
location,
* The number of concrete variables that have a DW_AT_location
The first number should remain stable regardless of how well we do at
tracking variable locations.
Yep, +1 to that.
Said another way, the presence or absence of an inline variable/formal_parameter with only an abstract_origin should not change the answer. And more specifically, the coverage shouldn't go /down/ when we add a single byte of location information to a variable that was previously uncovered. That variable having no coverage should already be recorded in the stats (which it isn't currently, as I understand it). To do this, as Jeremy said - we should go searching for variables in the abstract DIE tree, even if they don't appear in the inlined tree.
(I'm /vaguely/ remembering some particular issue with matching up inlined scope entities to their abstract origin... maybe something about inlined scopes that needed to be emitted/had entities in them even when they were zero length or something... but I can't quite recall right now. Oh well)
- Dave