[llvm-dev] Conditional references between globals in LLVM IR

[Reid Kleckner via llvm-dev]

Can this be expressed with comdat groups? I believe GlobalDCE should
already handle those in the way that you want: if any member is referenced,
the whole group is retained, if no members are referenced, the whole group
is dropped.

There is a slight wrinkle that MachO doesn't support comdats, but that is
the IR feature that we use to express this idea.

On Mon, Aug 16, 2021 at 5:27 PM Kuba Mracek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi llvm-dev!
>
> I'm trying to add dead-stripability of types into the Swift compiler, and
> for that I need some new affordance in the LLVM IR to be able to express
> the condition that says "this global can be removed if either of these
> other globals are removable" -- for a concrete example see below. As pcc
> has pointed out to me, we already have a !associated metadata, and what I'm
> looking for is basically a generalization of this concept.
>
>   @a = internal global i32 1
>   @b = internal global i32 2
>   @assoc = internal global [2 x i32*] { i32* @a, i32* @b } !1
>
>   ... other code here possibly using or not using @a and @b ...
>
>   ; Somehow I want to express that the references from @assoc to @a and @b
> are not to be taken into account
>   ; when doing global liveness analysis, and presence of @assoc in
> @llvm.used and @llvm.compiler.used should
>   ; be ignored as well. Instead the references from @assoc to @a and @b
> are conceptually supposed to be "weak"
>   ; references that will not keep targets alive if there are no "strong"
> (regular) references. And if either @a or @b
>   ; (or both) do get removed, then @assoc should also be removed.
>   !1 = !{ ... }
>
>   ; We need to mention @assoc in @llvm.used or @llvm.compiler.used,
> otherwise @assoc is trivially removable.
>   @llvm.used = appending global [...] { @assoc }
>
> This example above basically shows the situation of Swift's Protocol
> Conformance Records, which don't have any incoming static references and
> instead they are scanned and processed by the runtime (by looking those
> records up via APIs from the dynamic linker), so they need the @llvm.used
> marker to survive into the final binary. They reference two other globals,
> a type record and a protocol record, which means that today they actually
> keep both the type and the protocol alive transitively, and therefore
> prohibit dead-stripping of those. So practically, the @llvm.used marker is
> necessary but also too conservative at the same time, and I want to relax
> removal rules on these globals -- specifically these Protocol Conformance
> Records and some other records in similar situations.
>
> What I'm looking for is 1) a design for how to represent this in LLVM IR,
> 2) and an implementation in GlobalDCE that would actually follow these
> rules and remove globals whenever allowed. For the latter, me and Manman
> have prepared a patch, see <https://reviews.llvm.org/D104496>, which does
> the job by adding a new specially named metadata -- !llvm.used.conditional
> -- that has a specific structure that can express these "conditionally used
> globals". But mainly I'd like to figure out the former -- what should this
> look like in the IR. Should we try to extend the existing !associated
> metadata? If yes, can we teach GlobalDCE to optimize based on !associated
> markers? Should we have a list like !llvm.used.conditional instead? Should
> it subsume !associated metadata and replace it?
>
> Thanks ahead for any feedback!
>
> Kuba
>
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