[LLVMdev] RFC: Metadata attachments to function definitions

[David Blaikie]

On Fri, Apr 17, 2015 at 7:15 PM, Duncan P. N. Exon Smith
<dexonsmith at apple.com> wrote:
>
>> On 2015 Apr 15, at 10:06, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>
>>
>> On Tue, Apr 14, 2015 at 10:59 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>>
>> > On 2015 Apr 14, at 21:46, David Blaikie <dblaikie at gmail.com> wrote:
>> >
>> > On Tue, Apr 14, 2015 at 9:33 PM, Duncan P. N. Exon Smith
>> > <dexonsmith at apple.com> wrote:
>> >>
>> >> `Function` definitions should support `MDNode` attachments, with a
>> >> similar syntax to instructions:
>> >>
>> >>    define void @foo() nounwind !attach !0 {
>> >>      unreachable
>> >>    }
>> >>    !0 = !{}
>> >>
>> >> Attachments wouldn't be allowed on declarations, just definitions.
>> >>
>> >> There are two open problems this can help with:
>> >>
>> >> 1. For PGO, we need somewhere to attach the function entry count.
>> >>    Attaching to the function definition is a simple solution.
>> >
>> > No comment - can't say I know anything about that.
>> >
>> >>
>> >>        define void @foo() !prof !0 {
>> >>          unreachable
>> >>        }
>> >>        !0 = !{i32 987}
>> >>
>> >> 2. In debug info, we repeatedly build up a map from `Function` to the
>> >>    canonical `MDSubrogram` for it.
>> >
>> > Sounds great - I'd imagine this working somewhat like the way I've
>> > made implicit special members & other non-standard members of class
>> > types work in the debug info metadata, which is to say that the
>> > children reference the parent, but the parent doesn't reference the
>> > children (in this case, that would mean things like comdat folding in
>> > LTO would 'just work' - whichever function we picked would keep its
>> > debug info and attachment to its CU - the other CU would just appear
>> > not to have that function anymore - we might need a special case for
>> > comdat folding where one copy has debug info and another copy doesn't,
>> > make sure we move the metadata over if we're picking one without debug
>> > info).
>> >
>> > Though that will mean that whenever we want to walk all the
>> > subprograms of a CU, we'd have to build it by walking all the
>> > Functions in a module - it might be worth checking if/when/where we
>> > "need" to do that - I suspect it's rare and maybe can be made entirely
>> > unnecessary.
>>
>> I *think* we only do this once, and it's (ironically) to create the
>> "Subprogram -> CompileUnit" map.
>>
>> Right, I thought that might be the case.
>>
>> That highlights a problem that my proposal doesn't solve on its own.
>> While this proposal creates a "Function -> Subprogram" map, there still
>> isn't a "Subprogram -> CompileUnit" map -- that would still (for now)
>> be stored implicitly via `MDCompileUnit::getSubprograms()`.
>>
>> I guess this is (also?) what I was thinking about.
>>
>> Why isn't this map encoded in the `scope:` chain?  Because many of the
>> `scope:` chains currently terminate at `MDFile`s or `null` instead of
>> `MDCompileUnit`s.  Why?  Because LTO type uniquing needs scope chains
>> to be identical.
>>
>> Ah, right.
>>
>> (side note: sometimes need to end in MDFile or we might need an equivalent of DILexicalBlockFile for the CU - to switch files within the same CU (things defined in headers, etc))
>
> Ah, okay.  I thought we could just replace them with pointers to the
> compile unit.  Something like `DIFileScope` with `scope:` and
> `file:` fields would probably work?  (Which means I shouldn't have
> merged the two types of "file" nodes when I introduced the new
> hierarchy.  Boo.)

I'd have to double-check - maybe this is a non-issue & I've
misremembered/misunderstood things here (DILexicalBlockFile is needed
because we don't put file info on the DebugLocs to keep them small -
maybe the MDSubprograms (& MDNamespace, etc) should/could/(already
does?) just have a file attribute directly?)

>
>>
>>   (I have a vague plan for fixing this, too: (1) move
>> ownership of Metadata to the Module (so metadata isn't leaked by
>> `lto_module_dispose()`), (2) add a "StringRef -> MDType" map to the
>> Module (only for types with an ODR-style UUID), (3) delete the concept
>> of `MDString`-based type refs and update lib/Linker to rely on the
>> "StringRef -> MDType" map in the destination Module, (4) make all
>> `scope:` chains terminate at an `MDCompileUnit` and drop "scope"-ness
>> of `MDFile`, and (5) finally drop the `subprograms:` field from
>> `MDCompileUnit`.  But I'm not confident about step 4 yet.)
>>
>> Sounds plausible.
>>
>> (side note: any plans to do the scope-based textual IR that was thrown around during the prototype stages? It'd be another readability (& writability) improvement to not have to manually walk all the scope chains.
>
> Vague plans, but yes.  Doing it the way I want is blocked on
> removing type refs (maybe among other things?).
>
>> Anyway, maybe after most/all the functionality improvements are made we could do a maintainability pass & see whether we could get to a practically writable/readable format... I'm still not sure how likely that is, given the fact that debug info necessarily /describes/ the source the user wrote, so it's always going to be more verbose, but maybe it's achievable)
>
> Yeah, I think this is a great idea.
>
>>
>>
>> >
>> >> Keeping this mapping accurate takes
>> >>    subtle logic in `lib/Linker` (see PR21910/PR22792) and it's
>> >>    expensive to compute and maintain.  Attaching it directly to the
>> >>    `Function` designs away the problem.
>> >>
>> >>        define void @foo() !dbg !0 {
>> >>          unreachable
>> >>        }
>> >>        !0 = !MDSubprogram(name: "foo", function: void ()* @foo)
>> >>
>> >> Thoughts?
>> >>
>> >> Moving onto implementation, I'd provide the same generic API that
>> >> `Instruction` has, and wouldn't bother with the "fast path" API for
>> >> `!dbg`.  Moreover, the generic path wouldn't be slow.  Since there are
>> >> fewer functions than instructions, we can afford to store the
>> >> attachments directly on the `Function` instead of off in the
>> >> `LLVMContext`.
>> >>
>> >> It's not clear to me just how precious memory is in `Function`; IIRC
>> >> it's sitting at 168B right now for x86-64.  IMO, a `SmallVector<..., 1>`
>> >> -- cost of 64B -- seems fine.  I'll start with this if I don't hear any
>> >> objections; we can optimize later if necessary.
>> >>
>> >> Otherwise, I could hack together a custom vector-like object with the
>> >> "small string" optimization.
>> >
>> > Not important, but I'm missing something: what're you picturing that
>> > would be different from/better than SmallVector?
>> >
>>
>> Data storage would be:
>>
>>     struct Data {
>>       struct value_type {
>>         unsigned Tag;
>>         TrackingMDNodeRef MD;
>>       };
>>
>>       unsigned Capacity;
>>       union {
>>         unsigned LargeSize;
>>         unsigned SmallTag;
>>       } Unsigned;
>>
>>       AlignedCharArrayUnion<
>>           value_type *,     // LargeArray
>>           TrackingMDNodeRef // SmallMD
>>         > Pointer;
>>     };
>>     static_assert(sizeof(Data) == sizeof(void *) + sizeof(unsigned) * 2,
>>                   "Wrong size");
>>
>> Two advantages over `SmallVector<value_type, 1>`:
>>
>>   - 32-bits each for size and capacity (instead of 64-bits).
>>   - Domain knowledge of `value_type` allows aggressive unions.
>>
>> Can't provide as much API as `std::vector<>`, but the API for metadata
>> attachments is small and opaque:
>>
>>     bool hasMetadata(unsigned Tag) const;
>>     MDNode *getMetadata(unsigned Tag) const;
>>     void setMetadata(unsigned Tag, const MDNode *MD);
>>     void getAllMetadata(
>>         SmallVectorImpl<std::pair<unsigned, const MDNode *>> &MDs) const;
>>
>> >>  Cost would be 16B per `Function`, with the
>> >> same malloc/heap costs as `SmallVector<..., 1>`.  But I haven't seen a
>> >> profile that suggests this would be worth the complexity...
>> >>
>> >> Any opinions?
>> >>
>