[llvm-dev] RFC: metadata attachments for global variables

Mehdi Amini via llvm-dev
Fri, 06 May 2016 23:38:00 -0000

> On May 6, 2016, at 4:06 PM, Adrian Prantl <aprantl at apple.com>
wrote:
>
>>
>> On May 6, 2016, at 4:01 PM, Mehdi Amini <mehdi.amini at
apple.com> wrote:
>>
>>
>>> On May 6, 2016, at 3:53 PM, Adrian Prantl <aprantl at
apple.com> wrote:
>>>
>>>>
>>>> On May 6, 2016, at 3:48 PM, Adrian Prantl via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>>
>>>>> On May 6, 2016, at 3:40 PM, Mehdi Amini <mehdi.amini at
apple.com> wrote:
>>>>>
>>>>>
>>>>>> On May 6, 2016, at 3:17 PM, Adrian Prantl <aprantl
at apple.com> wrote:
>>>>>>
>>>>>>
>>>>>>> On May 6, 2016, at 1:17 PM, Peter Collingbourne via
llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>>>> 1) Lets us reverse the DIGlobalVariable ->
GlobalVariable edge, which should hopefully clear the way for removing the
llvm.dbg.cu named metadata node.
>>>>>>
>>>>>> Not to spoil all the fun, but I’m not sure if this will
bring us much closer to removing the llvm.dbg.cu node. The reason the
llvm.dbg.cu node exits is so we can find all DICompileUnits, because the
DICompileUnit holds debug info that is not referenced by any IR. This includes
things like DIImportedEntity (think C++ “using”), enums, and macros.
>>>>>
>>>>> I understand the fact that DICompileUnit holds debug
information that are useful and not referenced directly from the IR.
>>>>>
>>>>> What I wonder is, after all edges are pointing from the IR
toward the metadata graph, why do you need to keep a compile unit (and the
associated retained informations) if nothing in the IR (transitively) points to
this compile unit? (i.e. it is unreachable from the IR)
>>>>
>>>> Couple of examples:
>>>> - because the compile unit could contain definitions for an
enum that is useful during expression evaluation
>>
>> Do we need to preserve an enum type that is not used anywhere in the
program? (i.e. we dropped all functions that were using it)
>
> Yes, to support code like:
>
> enum {
> MY_CONSTANT = 42;
> };
>
> int i = MY_CONSTANT;


There is still a piece I'm missing: in this example the enum is used to
initialize "i", which will have an edge to the CU to be kept alive,
and this CU should retain the enum.

>
>>
>>>> - because it contains constants that are useful (XNU for
example has this version.c file:
http://opensource.apple.com/source/xnu/xnu-1456.1.26/config/version.c).
>>
>> Is it a feature that we need to keep the debug info for a constant
global even though we dead-strip the global?
>> I'd have assumed that the constant being present in the binary, you
wouldn't duplicate the initializer of the constant in the dwarf itself and
that accessing the constant was enough.
>
> Similar example as above:
>
> const int my_magic_number = ...; // gets optimized away, but is still
useful

"still useful" is hand-wavy. That is not clear to me (other than a
programmer saying "I want a pony").


>>>> - When clang compiles debug info for a module it compiles an
otherwise empty CU with debug info for all types in the module.
>>>
>>> - (something we recently discussed on cfe-commits for better
supporting dtrace/ctfconvert) because the frontend implemented a -gfull option
that added all types to the CU’s list retained types.
>>
>> Ok, you can have the named metadata to retain specific CU, but that
could be the exception and not the rule.
>
> Given the enum example, I’m not entirely convinced that there will be many
CUs that are not the exception, but generally, yes, that would work.

I meant the exception is the specific use case of -gfull. Not the enum case.

--
Mehdi