New upstream version 1.52.1+dfsg1

[rustc.git] / vendor / packed_simd / perf-guide / src / bound_checks.md

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+# Bounds checking
+
+Reading and writing packed vectors to/from slices is checked by default.
+Independently of the configuration options used, the safe functions:
+
+* `Simd<[T; N]>::from_slice_aligned(& s[..])`
+* `Simd<[T; N]>::write_to_slice_aligned(&mut s[..])`
+
+always check that:
+
+* the slice is big enough to hold the vector
+* the slice is suitably aligned to perform an aligned load/store for a `Simd<[T;
+  N]>` (this alignment is often much larger than that of `T`).
+
+There are `_unaligned` versions that use unaligned load and stores, as well as
+`unsafe` `_unchecked` that do not perform any checks iff `debug-assertions =
+false` / `debug = false`. That is, the `_unchecked` methods do still assert size
+and alignment in debug builds and could also do so in release builds depending
+on the configuration options.
+
+These assertions do often significantly impact performance and you should be
+aware of them.