Remove leaf bias from BTreeMap's BoxedNode and NodeRef #67980
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r? @cramertj (rust_highfive has picked a reviewer for you, use r? to override) |
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The job Click to expand the log.I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact |
hanna-kruppe
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Jan 8, 2020
| /// `Box<InternalNode<K, V>>`. However, it contains no information as to which of the two types | ||
| /// of nodes is actually behind the box, and, partially due to this lack of information, has no | ||
| /// destructor. | ||
| /// `Box<InternalNode<K, V>>` or `Box<NodeHeader<(), ())` (i.e. EMPTY_ROOT_NODE). |
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The shared root is neither boxed nor uniquely owned by one BoxedNode, so although it's good to note that this pointer can refer to the shared root, wording it like this is misleading IMO. I'd suggest the following outline instead:
- This is usually an owned pointer to a LeafNode/InternalNode, but it can also be a pointer (no ownership) to EMPTY_ROOT_NODE
- No destructor in part because we don't know which of these variants it is
- All of these types have a
NodeHeader<K, V>prefix (evenEMPTY_ROOT: NodeHeader<(), ()>), so that's the pointee type we store
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I'm confused by and can't find a description of "prefix" in a pointer context. Does that mean the data it points at always has at least the size and layout of NodeHeader<K, V>? Not that I know any better word for it. Does "prefix" also imply that pointer alignment is at least as high as that of NodeHeader<K, V>?
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To be precise, I mean that all of InternalNode<K, V>, LeafNode<K, V> and NodeHeader<(), ()>:
- are at least as large as
NodeHeader<K, V> - have alignment at least as large as
NodeHeader<K, V> - store the same kinds of data, at the same offsets, as a
NodeHeader<K, V>in the firstsize_of::<NodeHeader<K, V>>
This is the reason why as_header() is unconditionally safe. Alignment in particular is probably worth calling out separately, as it's not really implied by "prefix" (e.g., you might say i32 is a prefix of #[repr(C)] struct { a: i32, b: i64 } but alignment differs).
| } | ||
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| impl<K, V> BoxedNode<K, V> { | ||
| fn from_leaf(node: Box<LeafNode<K, V>>) -> Self { | ||
| BoxedNode { ptr: Box::into_unique(node) } | ||
| unsafe { | ||
| BoxedNode { ptr: Unique::new_unchecked(Box::into_raw(node) as *mut NodeHeader<K, V>) } |
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I see that this is consistent with from_internal but I'm tempted to suggest rewriting both into Box::into_unique(node).cast() to get rid of the gratuitous unsafe.
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Done, and alloc tests and Miri are happy
| @@ -510,7 +513,7 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> { | |||
| /// This also implies you can invoke this member on the shared root, but the resulting pointer | |||
| /// might not be properly aligned and definitely would not allow accessing keys and values. | |||
| fn as_leaf_mut(&mut self) -> *mut LeafNode<K, V> { | |||
| self.node.as_ptr() | |||
| unsafe { &mut *(self.node.as_ptr() as *mut LeafNode<K, V>) } | |||
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Creating a reference here risks UB and is also completely pointless. Just use self.node.as_ptr() as *mut LeafNode<K, V>.
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Done, and alloc tests and Miri are happy
| @@ -204,7 +207,7 @@ impl<K, V> Root<K, V> { | |||
| Root { | |||
| node: unsafe { | |||
| BoxedNode::from_ptr(NonNull::new_unchecked( | |||
| &EMPTY_ROOT_NODE as *const _ as *const LeafNode<K, V> as *mut _, | |||
| &EMPTY_ROOT_NODE as *const _ as *const NodeHeader<K, V> as *mut _, | |||
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Nit: can now be simplified to NonNull::from(&EMPTY_ROOT_NODE).
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The compiler and I think you're wrong there: something must be done to transmute the NodeHeader<(), ()> to NodeHeader<K, V>. Why we also have to tag it as mutable, beats me.
Earlier, I got off on a tangent to adjust the pointer in BoxedNode to this actual type of EMPTY_ROOT_NODE, which implies removing K and V altogether, or pretending to understand PhantomData, but I managed to squeeze that monster back into the box before it ate me.
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I forgot about the type parameters, mea culpa. NonNull::from(&EMPTY_ROOT_NODE).cast() should work, though.
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Meanwhile, I managed to reproduce and hopefully fix the failing pr build. Awaiting a long build. |
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I know nothing about the gdb pretty printing, so I would prefer if the changes to it were kept to the bare minimum required by the other changes in this PR, or alternatively, if someone who knows anything about the pretty printers reviewed those parts. |
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I'll first submit a PR fixing and testing pretty printing as is now, and they can judge whether the near-invalid memory access is cool or cripple. |
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Ping from triage: |
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This PR is now blocked by #68098 and won't automatically merge after it (or will need changes). As far as I know, I've addressed all change requests. |
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Nothing to see here, folks. I just played around in the GitHub interface and rebased on the #68098 commit. |
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Yes, AFAIK there's nothing to be done here before #68098 is merged and once that's done this PR can be rebased and most likely merged (NB: the first commit here has a different hash than the commit in that PR, so some manual |
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Well spotted, I think that before rebasing this one on that one, I rebased that one on master and forgot to push. |
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Closing for now (and it looks to be a very long time) because being based on another PR's commit misguides GitHub. |
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…, r=Mark-Simulacrum More documentation and simplification of BTreeMap's internals Salvage the documentation and simplification from rust-lang#67980, without changing the type locked down by debuginfo. r? @rkruppe
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…, r=Mark-Simulacrum More documentation and simplification of BTreeMap's internals Salvage the documentation and simplification from rust-lang#67980, without changing the type locked down by debuginfo. r? @rkruppe
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Improve readability - there is no genuine problem here, after #56648.
BoxedNodeandNodeRefcontain a pointer to a node that is typed asLeafNode. In case of EMPTY_ROOT_NODE, that pointer may not be properly aligned and does not point to anything matchingLeafNode<K, V>. OnlyNodeRef's documentation mentions this.It seemed more logical to me to give these pointers as type the least common denominator,
NodeHeader<K, V>(which only exists since #56648). That simplifies the simplest accessas_headerand levels the playing field for leaf and internal nodes.One could argue that most nodes are in fact leaves, and most access to nodes is to their leaf-portion, keys and values. Then I'd still like to fix the documentation of
BoxedNode.