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seqs.nim: added a remark about C++ optimizers

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This commit is contained in:
Andreas Rumpf 2019-03-13 10:05:05 +01:00
commit 436ec57f83
1 changed files with 8 additions and 0 deletions
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8
lib/core/seqs.nim
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@ -36,6 +36,14 @@ when false:
proc `=trace`[T](s: NimSeqV2[T]) = proc `=trace`[T](s: NimSeqV2[T]) =
for i in 0 ..< s.len: `=trace`(s.data[i]) for i in 0 ..< s.len: `=trace`(s.data[i])
#[
Keep in mind that C optimizers are bad at detecting the connection
between ``s.p != nil`` and ``s.len != 0`` and that these are intermingled
with user-level code that accesses ``s.len`` only, never ``s.p`` directly.
This means the check for whether ``s.p`` needs to be freed should
be ``s.len == 0`` even though that feels slightly more awkward.
]#
proc `=destroy`[T](s: var seq[T]) = proc `=destroy`[T](s: var seq[T]) =
var x = cast[ptr NimSeqV2[T]](addr s) var x = cast[ptr NimSeqV2[T]](addr s)
var p = x.p var p = x.p