clawhdf5: parse local files through the slice, and cache the contiguous view

Since M2 the facade handed the metadata parsers its FileData view, so a
local file ran the parsers monomorphised for FileData, whose Storage impl
worked out contiguous() (patched and overlay checks, two range conversions)
on every structure read. A metadata walk of h5stat_newgrat.h5 (35,001
groups: open, entries and attrs of each) was about 4.5% slower than at
8f59b2e.

- FileData works out its contiguous slice once at open (a borrow of its own
  heap/mapped buffer, kept as a pointer; see the SAFETY notes).
- with_bytes! hands the in-memory slice to the format parsers when the file
  has one (header parsing, attributes, group listings and lookups, path
  resolution, shared messages, VL decoding), so local files run the [u8]
  parsers as before; storage-backed files still get FileData.

Provisional A/B on tank (load 5-11), best of 30, 5 alternating rounds:
walk 26.07-26.37 ms at 8f59b2e, 27.24-28.03 ms before this commit,
26.52-26.98 ms after. Caching alone did not move it (27.02-27.47 ms); the
dispatch did. File::open read_f32 on 32M f32 (contiguous, chunked, gzip)
stays within noise of 8f59b2e.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5/src/reader.rs

@@ -64,6 +64,24 @@ impl Backing {
     }
 }
 
+/// Evaluate `$body` with `$d` bound to the bytes the `clawhdf5_format`
+/// parsers read: the in-memory slice when the file has one (a `Vec`, an
+/// mmap), so local files run the parsers monomorphised for `[u8]` — the
+/// slice code, as before the range-read migration — and otherwise the
+/// [`FileData`] itself, whose reads go to the storage.
+macro_rules! with_bytes {
+    ($data:expr, |$d:ident| $body:expr) => {{
+        let data: &FileData = $data;
+        match data.contiguous() {
+            Some($d) => $body,
+            None => {
+                let $d = data;
+                $body
+            }
+        }
+    }};
+}
+
 /// The file's bytes, viewed from the superblock on and up to the end of
 /// file the superblock records. A file may start with a user block (the
 /// superblock at 512, 1024, …); every HDF5 address is relative to the
@@ -93,8 +111,28 @@ struct FileData {
     /// then); every object lookup here fails with this error, and no
     /// metadata is read from the file's own, possibly stale, bytes.
     image_error: Option<FormatError>,
+    /// [`Self::contiguous`], worked out once at open: every structure a
+    /// parser reads asks for it, and the patched/overlay checks and range
+    /// conversions behind it cost a local metadata walk a few percent.
+    contiguous: Option<WholeView>,
 }
 
+/// A borrow of the HDF5 data held by a [`FileData`]'s own `backing` or
+/// `patched` buffer (see [`FileData::contiguous`]).
+#[derive(Clone, Copy)]
+struct WholeView {
+    ptr: *const u8,
+    len: usize,
+}
+
+// SAFETY: a `WholeView` is only a borrow of bytes owned (through `backing`
+// or `patched`) by the `FileData` holding it, which is `Send + Sync`: the
+// bytes are never written after open, so sharing the pointer across
+// threads is sharing a `&[u8]`.
+unsafe impl Send for WholeView {}
+// SAFETY: as above.
+unsafe impl Sync for WholeView {}
+
 impl FileData {
     /// Locate the superblock and parse it. A truncated file is refused, and
     /// bytes past the recorded end of file are not read, as in libhdf5.
@@ -127,17 +165,17 @@ impl FileData {
             ImageView::Patched(p) => (Some(p), None),
             ImageView::Unloadable(e) => (None, Some(e)),
         };
-        Ok((
+        let mut data = Self {
-            Self {
             backing,
             base: base as u64,
             end: end as u64,
             patched,
             overlay: Vec::new(),
             image_error,
-            },
+            contiguous: None,
-            superblock,
+        };
-        ))
+        data.contiguous = data.find_contiguous();
+        Ok((data, superblock))
     }
 
     /// [`Self::new`] for a [`Storage`] backend: the same checks, through
@@ -152,7 +190,11 @@ impl FileData {
             patched: None,
             overlay: Vec::new(),
             image_error: None,
+            contiguous: None,
         };
+        // Worked out again below, once the end of file and any cache image
+        // are known.
+        data.contiguous = data.find_contiguous();
         let superblock = Superblock::parse_in(&data, 0)?;
         data.end = base + superblock.data_end(base, file_len)?;
         match superblock_ext::cache_image_state_in(&data, &superblock)? {
@@ -167,13 +209,37 @@ impl FileData {
                     .collect();
             }
         }
+        data.contiguous = data.find_contiguous();
         Ok((data, superblock))
     }
 
     /// The HDF5 data as one slice, when the file is in memory (a `Vec`, an
     /// mmap, or a storage that holds it all and has no cache image to lay
     /// over it).
+    #[inline]
     fn contiguous(&self) -> Option<&[u8]> {
+        // SAFETY: `find_contiguous` borrowed these bytes from `backing` or
+        // `patched`, which this `FileData` owns and never changes after
+        // open. They live on the heap or in a mapping (a `Vec`'s buffer, an
+        // mmap, a private copy, or a buffer inside the `Arc`'d storage), so
+        // they stay put when the `FileData` moves, and they live as long as
+        // `self`.
+        self.contiguous
+            .map(|v| unsafe { core::slice::from_raw_parts(v.ptr, v.len) })
+    }
+
+    /// [`Self::contiguous`], worked out from `backing` and `patched`.
+    fn find_contiguous(&self) -> Option<WholeView> {
+        let bytes = self.compute_contiguous()?;
+        Some(WholeView {
+            ptr: bytes.as_ptr(),
+            len: bytes.len(),
+        })
+    }
+
+    /// The HDF5 data as one slice, from `backing` and `patched` (see
+    /// [`Self::contiguous`]).
+    fn compute_contiguous(&self) -> Option<&[u8]> {
         if let Some(p) = &self.patched {
             return p.get(usize::try_from(self.base).ok()?..usize::try_from(self.end).ok()?);
         }
@@ -224,6 +290,7 @@ impl FileData {
 }
 
 impl Storage for FileData {
+    #[inline]
     fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
         if let Some(all) = self.contiguous() {
             return all.read_at(offset, len);
@@ -261,6 +328,7 @@ impl Storage for FileData {
             .collect())
     }
 
+    #[inline]
     fn as_contiguous(&self) -> Option<&[u8]> {
         self.contiguous()
     }
@@ -400,8 +468,11 @@ impl File {
     ///
     /// The path uses `/` separators (e.g., `"group1/values"`).
     pub fn dataset(&self, path: &str) -> Result<Dataset<'_>, Error> {
-        let data = self.data.meta()?;
+        let addr = with_bytes!(self.data.meta()?, |d| group_v2::resolve_path_any_in(
-        let addr = group_v2::resolve_path_any_in(data, &self.superblock, path)?;
+            d,
+            &self.superblock,
+            path
+        ))?;
         let hdr = self.parse_header(addr)?;
         if !has_message(&hdr, MessageType::DataLayout) {
             return Err(Error::NotADataset(path.to_string()));
@@ -447,8 +518,11 @@ impl File {
     /// The path uses `/` separators (e.g., `"sensors"`).
     /// Use `"/"` or `""` for the root group.
     pub fn group(&self, path: &str) -> Result<Group<'_>, Error> {
-        let data = self.data.meta()?;
+        let addr = with_bytes!(self.data.meta()?, |d| group_v2::resolve_path_any_in(
-        let addr = group_v2::resolve_path_any_in(data, &self.superblock, path)?;
+            d,
+            &self.superblock,
+            path
+        ))?;
         Ok(Group {
             file: self,
             address: addr,
@@ -560,13 +634,13 @@ impl File {
     /// [`AttrValue::Raw`] attribute. Variable-length strings are resolved in
     /// this file's global heap; see [`Dataset::read_string`] for the values.
     pub fn decode_strings(&self, datatype: &Datatype, raw: &[u8]) -> Result<Vec<String>, Error> {
-        crate::vlen::decode_strings(
+        with_bytes!(&self.data, |d| crate::vlen::decode_strings(
-            &self.data,
+            d,
             datatype,
             raw,
             self.offset_size(),
             self.length_size(),
-        )
+        ))
     }
 
     /// Like [`decode_strings`](Self::decode_strings) for variable-length
@@ -577,13 +651,13 @@ impl File {
         datatype: &Datatype,
         raw: &[u8],
     ) -> Result<Vec<Vec<u8>>, Error> {
-        crate::vlen::decode_string_bytes(
+        with_bytes!(self.data.meta()?, |d| crate::vlen::decode_string_bytes(
-            self.data.meta()?,
+            d,
             datatype,
             raw,
             self.offset_size(),
             self.length_size(),
-        )
+        ))
     }
 
     /// Decode the variable-length sequences in `raw`, a buffer of elements
@@ -595,22 +669,22 @@ impl File {
         datatype: &Datatype,
         raw: &[u8],
     ) -> Result<Vec<Vec<T>>, Error> {
-        crate::vlen::decode_vlen(
+        with_bytes!(self.data.meta()?, |d| crate::vlen::decode_vlen(
-            self.data.meta()?,
+            d,
             datatype,
             raw,
             self.offset_size(),
             self.length_size(),
-        )
+        ))
     }
 
     fn parse_header(&self, address: u64) -> Result<ObjectHeader, FormatError> {
-        ObjectHeader::parse_in(
+        with_bytes!(self.data.meta()?, |d| ObjectHeader::parse_in(
-            self.data.meta()?,
+            d,
             address,
             self.superblock.offset_size,
             self.superblock.length_size,
-        )
+        ))
     }
 
     fn offset_size(&self) -> u8 {
@@ -688,8 +762,12 @@ impl<'f> Group<'f> {
         &self,
     ) -> Result<(HashMap<String, AttrValue>, Vec<FormatError>), Error> {
         let hdr = self.file.parse_header(self.address)?;
-        let data = &self.file.data;
+        with_bytes!(&self.file.data, |d| read_attrs(
-        read_attrs(data, &hdr, self.file.offset_size(), self.file.length_size())
+            d,
+            &hdr,
+            self.file.offset_size(),
+            self.file.length_size()
+        ))
     }
 
     /// Get a dataset within this group by name.
@@ -719,14 +797,13 @@ impl<'f> Group<'f> {
     /// stored densely.
     pub fn attr(&self, name: &str) -> Result<Option<AttrValue>, Error> {
         let hdr = self.file.parse_header(self.address)?;
-        let data = &self.file.data;
+        with_bytes!(&self.file.data, |d| read_attr(
-        read_attr(
+            d,
-            data,
             &hdr,
             name,
             self.file.offset_size(),
             self.file.length_size(),
-        )
+        ))
     }
 
     /// The object header address of the child called `name`: the entry of
@@ -734,8 +811,12 @@ impl<'f> Group<'f> {
     /// name index rather than by listing the group (see
     /// [`group_v2::resolve_child`]).
     fn child_address(&self, name: &str) -> Result<u64, Error> {
-        let data = self.file.data.meta()?;
+        with_bytes!(self.file.data.meta()?, |d| group_v2::resolve_child_in(
-        group_v2::resolve_child_in(data, &self.file.superblock, self.address, name)
+            d,
+            &self.file.superblock,
+            self.address,
+            name
+        ))
         .map_err(Error::Format)
     }
 
@@ -757,8 +838,9 @@ impl<'f> Group<'f> {
     /// [`group_v2::resolve_group_children`]); dangling, external and
     /// user-defined links are left out.
     fn children(&self) -> Result<Vec<GroupEntry>, Error> {
-        let data = self.file.data.meta()?;
+        with_bytes!(self.file.data.meta()?, |d| {
-        group_v2::resolve_group_children_in(data, &self.file.superblock, self.address)
+            group_v2::resolve_group_children_in(d, &self.file.superblock, self.address)
+        })
         .map_err(Error::Format)
     }
 }
@@ -1332,13 +1414,12 @@ impl<'f> Dataset<'f> {
     pub fn attrs_with_errors(
         &self,
     ) -> Result<(HashMap<String, AttrValue>, Vec<FormatError>), Error> {
-        let data = &self.file.data;
+        with_bytes!(&self.file.data, |d| read_attrs(
-        read_attrs(
+            d,
-            data,
             &self.header,
             self.file.offset_size(),
             self.file.length_size(),
-        )
+        ))
     }
 
     /// The attribute called `name`, or `None` if it has none by that name
@@ -1346,14 +1427,13 @@ impl<'f> Dataset<'f> {
     /// that name, found without reading the other attributes when they are
     /// stored densely.
     pub fn attr(&self, name: &str) -> Result<Option<AttrValue>, Error> {
-        let data = &self.file.data;
+        with_bytes!(&self.file.data, |d| read_attr(
-        read_attr(
+            d,
-            data,
             &self.header,
             name,
             self.file.offset_size(),
             self.file.length_size(),
-        )
+        ))
     }
 
     /// Verify this dataset's content against its stored provenance hash
@@ -1393,12 +1473,14 @@ impl<'f> Dataset<'f> {
             .iter()
             .find(|m| m.msg_type == msg_type)
             .map(|msg| {
+                with_bytes!(&self.file.data, |d| {
                     clawhdf5_format::shared_message::message_data_in(
-                    &self.file.data,
+                        d,
                         msg,
                         self.file.offset_size(),
                         self.file.length_size(),
                     )
+                })
                 .map_err(Error::Format)
             })
             .transpose()