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[SR-7613] Missed constant propagation #50155

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swift-ci opened this issue May 5, 2018 · 6 comments
Open

[SR-7613] Missed constant propagation #50155

swift-ci opened this issue May 5, 2018 · 6 comments
Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@swift-ci
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swift-ci commented May 5, 2018

Previous ID SR-7613
Radar None
Original Reporter nuclearace (JIRA User)
Type Bug
Environment

swift 4.1 macOS

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: bacab61987ce077c0e5aca37f8eecfca

Issue Description:

It seems in the following example, Constants.a isn't being identified as a constant, and properly propagated to use sites.

struct Constants {
  static let a = MemoryLayout<Int>.size * 8
  static let b = Int.bitWidth
}

dump(Constants.a)
dump(Constants.b)

Godbolt link: https://godbolt.org/g/d7ti3U
@davezarzycki
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I looked into this briefly and I could be wrong, but it seems like SIL analysis is almost completely oblivious to `Builtin.sizeof()` and the fact that the result is constant.

@belkadan
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belkadan commented May 7, 2018

cc @eeckstein

@belkadan
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belkadan commented May 7, 2018

Note that Builtin.sizeof isn't always a constant, because of resilient structs and enums. But the compiler also knows which types are resilient and which aren't.

@eeckstein
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By design SIL does not know anything about datatype sizes.

@davezarzycki
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Having SIL not knowing anything about datatype sizes (by default) is reasonable and good. That being said, what would go wrong if SIL special cased `Builtin.sizeof` and inlining it when the datatype is known to be fixed sized and fragile?

@eeckstein
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Definitely doable (I just wanted to give an explanation why it is not done currently).

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself
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@belkadan @davezarzycki @eeckstein @swift-ci