SipHasher128: use specific struct layout

[rust.git] / compiler / rustc_data_structures / src / sip128.rs

diff --git a/compiler/rustc_data_structures/src/sip128.rs b/compiler/rustc_data_structures/src/sip128.rs
index 8b91407acffa9bf5822aac25bcbd0ddc70464f9f..5bbc53945edc69ec60cc7ce9cbd296be528bc3e1 100644 (file)
--- a/compiler/rustc_data_structures/src/sip128.rs
+++ b/compiler/rustc_data_structures/src/sip128.rs
@@ -15,7 +15,13 @@
 const BUFFER_SPILL_INDEX: usize = BUFFER_SIZE_ELEMS_SPILL - 1;
 
 #[derive(Debug, Clone)]
+#[repr(C)]
 pub struct SipHasher128 {
+    // The access pattern during hashing consists of accesses to `nbuf` and
+    // `buf` until the buffer is full, followed by accesses to `state` and
+    // `processed`, and then repetition of that pattern until hashing is done.
+    // This is the basis for the ordering of fields below. However, in practice
+    // the cache miss-rate for data access is extremely low regardless of order.
     nbuf: usize,                                      // how many bytes in buf are valid
     buf: [MaybeUninit<u64>; BUFFER_SIZE_ELEMS_SPILL], // unprocessed bytes le
     state: State,                                     // hash State