[LLVMdev] RFC: Metadata attachments to function definitions

[Xinliang David Li]

Duncan,  thanks for working on it.

David


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.
>
>         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.  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.  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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