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]>
