[LLVMdev] RFC: Metadata attachments to function definitions
Reid Kleckner
rnk at google.com
Wed Apr 15 08:59:03 PDT 2015
Conventional wisdom, which is often wrong, says that bytes in Function are
precious. In the past we've bent over backwards to put bits on Function and
have DenseMaps in LLVMContexts and Modules. I think this is probably the
*wrong* approach for debug info, which, when it's being used, is used
everywhere.
Have you considered something like TinyPtrVector, for a cost of one pointer
when it's not being used? It has higher access costs than SmallVector, but
when there's exactly one piece of metadata (the subprogram), it's pretty
good.
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.
>
> 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()`.
>
> 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. (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.)
>
> >
> >> 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?
> >>
>
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