[cfe-dev] RFC: Adding constructor homing to clang's limited debug info mode

[David Blaikie via cfe-dev]

Hey Amy - thanks for the awesome work on this, I'm really excited to/hope
to see this behavior get rolled into debug-info-kind=limited soon.

Mechanically speaking, I'm guessing we might want to change the default for
debug-info-kind (on platforms where the default is already 'limited') to
'constructor' initially - and maybe wait a while/for an LLVM release before
removing 'constructor' and rolling the behavior into 'limited' to remove
the differentiation/extra code to handle that.

As for testing:

The GDB test suite doesn't really have a "pass/fail" kind of mentality, so
far as I know, the best you can do is "does it fail more". But honestly
even when I was running the GDB test suite against Clang years ago, the
first thing I did was add the -fstandalone-debug (/-fno-limit-debug-info)
because it wasn't really designed to cope with some of the optimizations in
there (there are currently 3 optimizations in that bucket: complete type
(if the type is only used for pointers that are never dereferenced, only
emit a declaration of the type even if a definition is available), vtable
homing, and template explicit instantiation decl/def homing - GCC
implements vtable homing but not the other two, and the first one trips up
at least several GDB tests).

Rumeet ( rdhindsa at google.com ) might be able to help you with getting data
from the GDB test suite as we run it internally for LLVM release validation.

> Compared clang.pdb files in a Windows LLVM debug build, and looked at the
list of types that are no longer complete with the constructor homing
change. We looked into some of these types, and they are constructed in
functions that aren't used in clang.exe. In this case, it makes sense to
not emit complete debug information.

Would it be possible to compare the types from the object files rather than
the final linked binary - there's still a risk that some of those missing
types would be in unused libraries that never even needed to be
built/attempted to be linked into the final clang binary, but might reduce
the linker reachability stripping of object files an make the comparison
close enough to put in a spreadsheet/provide more definitive "these are all
the types that were no longer emitted and here's why they weren't/why
that's OK", rather than a sampling? (admittedly, it's always still a bit of
a sampling - testing clang, rather than some/all other binaries/etc)

On Wed, May 6, 2020 at 11:19 PM Amy Huang via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Recently I added a level to clang's debug info
> (-debug-info-kind=constructor) based on Reid's constructor type homing
> idea. Since classes typically live in header files and have no natural
> "home", class type information is emitted in every translation unit where
> it is required to be complete, which results in a lot of duplicate debug
> info. Constructor type homing attempts to reduce the amount of duplicate
> debug info by emitting debug info for classes only where the constructor is
> emitted. It's based on the assumption that if complete debug info for a
> class is needed, then it should be constructed somewhere in the program.
> Currently this applies to classes with nontrivial, user-defined
> constructors. For classes with no constructors or no nontrivial
> constructors, there is no change, so debug info is still emitted
> everywhere. This RFC proposes using constructor homing as part of clang's
> limited debug info level.
>
> Currently clang's limited debug info mode already has some optimizations
> for limiting the amount of class debug info; for example, we emit debug
> info for dynamic classes only when the vtable is emitted, since the program
> will fail to link if the vtable is not provided.
>
> Link to the original patch adding `-debug-info-kind=constructor`:
> https://reviews.llvm.org/D72427
> Patch for the proposed change: https://reviews.llvm.org/D79147
>
> *Some numbers from the LLVM build*
> This is a comparison I did on my machine of total object file size in
> Clang debug builds, with and without constructor homing
> (-debug-info-kind=limited vs. constructor). In general it seems to reduce
> the total object file size by 30-50%.
> on Linux
> before: 9345 MB
> after: 4553 MB
>
> on Windows
> before: 6096 MB
> after: 3979 MB
>
> We also enabled this in Chromium a few months ago, and saw a similar
> change in object / split dwarf file size.
>
> *Testing*
> There isn't a very comprehensive way that I know of to test debug info. I
> tested a few things:
> - Ran the LLDB test suite with this change as part of
> -debug-info-kind=limited. This caught an edge case with constexpr
> constructors. After fixing this, the LLDB test suite now passes with this
> mode (minus one test case that happens to fail, and is updated in the
> proposed patch)
> -Compared clang.pdb files in a Windows LLVM debug build, and looked at the
> list of types that are no longer complete with the constructor homing
> change. We looked into some of these types, and they are constructed in
> functions that aren't used in clang.exe. In this case, it makes sense to
> not emit complete debug information.
> -Enabled this in Chromium and we haven't yet received any bug reports
> about debug info.
>
> We talked about running the GDB test suite with Clang in this mode, but
> the GDB test suite doesn't currently pass with Clang. Triaging GDB test
> suite failures is probably more than a month of work, so we don't plan to
> pursue it.
>
> Any feedback is welcome! Especially looking for opinions on whether this
> should be the default for limited debug info, or if more testing is
> preferred, what that might look like.
>
> -Amy
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