[SR-13903] Make ApplyInst a MultipleValueInstruction

[dan-zheng]

Previous ID	SR-13903
Radar	rdar://problem/71913175
Original Reporter	@dan-zheng
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, SILOptimizer, StarterBug
Assignee	sachinvas16 (JIRA)
Priority	Medium

md5: 41f0f57e0ecea221993e51084b3f1975

Issue Description:

Forum question with context: https://forums.swift.org/t/make-applyinst-a-multiplevalueinstruction/42294

---

SIL supports instructions that return multiple values (i.e. multiple result {{SILValue}}s).

However, apply instructions don't return multiple values. Instead, they produce a single value, which may have a tuple type. This leads to much avoidable destructuring and restructuring of tuples, which complicates SIL transformations like autodiff:

// example.swift
@_silgen_name("foo")
func foo() -> (Int, Int) {
  (0, 0)
}

@_silgen_name("bar")
func bar() -> (Int, Int) {
  return foo()
}

$ swiftc -emit-silgen example.swift
sil hidden [ossa] @bar : $@convention(thin) () -> (Int, Int) {
bb0:
  %0 = function_ref @foo : $@convention(thin) () -> (Int, Int)
  %1 = apply %0() : $@convention(thin) () -> (Int, Int)
  (%2, %3) = destructure_tuple %1 : $(Int, Int) // this is not good
  %4 = tuple (%2 : $Int, %3 : $Int)
  return %4 : $(Int, Int)
}

---

Instead, we can change ApplyInst to inherit MultipleResultInstruction.

Steps:

1. Change definition: make ApplyInst inherit MultipleValueInstruction.

Fix obvious compilation errors in the compiler codebase. Fix as many tests as possible.

2. Update and fix ApplyInst users: SIL generation + parsing + verification + printing, analyses and transformations, IRGen.

[typesanitizer]

@swift-ci create

[swift-ci]

Comment by Sachin Vas (JIRA)

theindigamer (JIRA User) Anyone working on this? Should I pick it up?

[typesanitizer]

I don't think anyone is, feel free to assign it to yourself (see Assignee field in top right of web UI) and work on it. 🙂

[typesanitizer]

I think this change will need to be broken up into at least 2 PRs. See https://forums.swift.org/t/make-applyinst-a-multiplevalueinstruction/42294/3 for context.

[swift-ci]

Comment by Saidhon Orifov (JIRA)

@dan-zheng theindigamer (JIRA User) based on my understanding from the forum, this has 2 parts to it, which will need to be broken down into 2 PR's:

1. Change ApplyInst inheritance from SingleValueInstruction -> MultipleValueInstruction

   1. Basically after this change, I am seeing a couple of errors, and most likely tests failing, so will need to fix that, which makes sense.

2. Update SIL generation + parsing + verification + printing, analyses and transformations, IRGen (which I didn't really quite follow tbh, is there more context on this or any guidance?)

[swift-ci]

Comment by Sachin Vas (JIRA)

saidhon (JIRA User) Are you working on this issue?

[dan-zheng]

sachinvas16 (JIRA User): I'll go ahead and assign you to this issue, if you'd like to get started!

As Varun said, you have the ability to assign yourself. Please do so in the future.

Please also change the issue status to "In Progress" after you do get started. This communicates to everyone that active progress is being made by you and that we can expect to see some PR within a few days or weeks. Certainly do not hesitate to ask questions (even small ones) if you get stuck, there are many experts here to help.

Hope this makes sense, thanks!

[swift-ci]

Comment by Sachin Vas (JIRA)

Thank you @dan-zheng, will get started ASAP.

[dan-zheng]

Thanks for the quick reply! I'm excited to see your progress 🙂

[swift-ci]

Comment by Saidhon Orifov (JIRA)

sachinvas16 (JIRA User) Yeah, but I yield back, feel free to take over. Sorry, I got started on it but never assigned it to myself or saw your comments.