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.byref always optimizes sink call into a bitwise memcopy if =sink is disabled #24327

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.byref always optimizes sink call into a bitwise memcopy if =sink is disabled #24327

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ringabout
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@ringabout ringabout commented Oct 18, 2024 •
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closes #75
ref nim-lang/threading#79

Or only when =sink is disabled for byRef

Araq
Araq reviewed Oct 18, 2024
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compiler/injectdestructors.nim Outdated
@@ -277,7 +277,8 @@ proc deepAliases(dest, ri: PNode): bool =
proc genSink(c: var Con; s: var Scope; dest, ri: PNode; flags: set[MoveOrCopyFlag] = {}): PNode =
if (c.inLoopCond == 0 and (isUnpackedTuple(dest) or IsDecl in flags or
(isAnalysableFieldAccess(dest, c.owner) and isFirstWrite(dest, c)))) or
isNoInit(dest) or IsReturn in flags:
isNoInit(dest) or IsReturn in flags or
tfByRef in dest.typ.flags:
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I think the logic should be "it's .byref and also has its =sink disabled".

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make sense

@ringabout ringabout changed the title .byref always optimizes sink call into a bitwise memcopy .byref always optimizes sink call into a bitwise memcopy if =sink is disabled Oct 19, 2024
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Araq commented Oct 19, 2024

This is still not correct as it can introduce leaks for code like var x = create(); x = create(). The idea here is that .byref types do not have any bitwise copies. We need to rethink this. It probably means the whole idea of =sink() .error is just wrong. =sink is fine and doesn't copy and shallow copies are prevented by .byref.

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darkestpigeon commented Oct 21, 2024 •
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I usually use =copy() .error and =sink() .error for types that are used to communicate between multiple threads (perhaps some foreign) and the object needs to be pinned to a particular address. This both signals intent and prevents dumb errors (although moving an object by accident is rare). Will .byref cover this case correctly?

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Araq commented Oct 22, 2024

I think so, yes, but making =sink an .error just because you want to tie the object to a stack location is wrong regardless.

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Problem with the create* API of synchronization primitives
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@ringabout @Araq @darkestpigeon