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Stable hashing: add comments and tests concerning platform-independence

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SipHasher128 implements short_write in an endian-independent way, yet
its write_xxx Hasher trait methods undo this endian-independence by byte
swapping the integer inputs on big-endian hardware. StableHasher then
adds endian-independence back by also byte-swapping on big-endian
hardware prior to invoking SipHasher128.

This double swap may have the appearance of being a no-op, but is in
fact by design. In particular, we really do want SipHasher128 to be
platform-dependent, in order to be consistent with the libstd SipHasher.
Try to clarify this intent. Also, add and update a couple of unit tests.
This commit is contained in:
Tyson Nottingham 2020-09-28 17:34:27 -07:00
parent fc2daaae61
commit d061fee177
4 changed files with 142 additions and 18 deletions
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56
compiler/rustc_data_structures/src/sip128/tests.rs
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@ -1,7 +1,6 @@
use super::*;
use std::hash::{Hash, Hasher};
use std::{mem, slice};
// Hash just the bytes of the slice, without length prefix
struct Bytes<'a>(&'a [u8]);
@ -399,20 +398,55 @@ fn test_hash_no_concat_alias() {
}
#[test]
fn test_write_short_works() {
let test_usize = 0xd0c0b0a0usize;
fn test_short_write_works() {
let test_u8 = 0xFF_u8;
let test_u16 = 0x1122_u16;
let test_u32 = 0x22334455_u32;
let test_u64 = 0x33445566_778899AA_u64;
let test_u128 = 0x11223344_55667788_99AABBCC_DDEEFF77_u128;
let test_usize = 0xD0C0B0A0_usize;
let test_i8 = -1_i8;
let test_i16 = -2_i16;
let test_i32 = -3_i32;
let test_i64 = -4_i64;
let test_i128 = -5_i128;
let test_isize = -6_isize;
let mut h1 = SipHasher128::new_with_keys(0, 0);
h1.write_usize(test_usize);
h1.write(b"bytes");
h1.write(b"string");
h1.write_u8(0xFFu8);
h1.write_u8(0x01u8);
h1.write_u8(test_u8);
h1.write_u16(test_u16);
h1.write_u32(test_u32);
h1.write_u64(test_u64);
h1.write_u128(test_u128);
h1.write_usize(test_usize);
h1.write_i8(test_i8);
h1.write_i16(test_i16);
h1.write_i32(test_i32);
h1.write_i64(test_i64);
h1.write_i128(test_i128);
h1.write_isize(test_isize);
let mut h2 = SipHasher128::new_with_keys(0, 0);
h2.write(unsafe {
slice::from_raw_parts(&test_usize as *const _ as *const u8, mem::size_of::<usize>())
});
h2.write(b"bytes");
h2.write(b"string");
h2.write(&[0xFFu8, 0x01u8]);
assert_eq!(h1.finish128(), h2.finish128());
h2.write(&test_u8.to_ne_bytes());
h2.write(&test_u16.to_ne_bytes());
h2.write(&test_u32.to_ne_bytes());
h2.write(&test_u64.to_ne_bytes());
h2.write(&test_u128.to_ne_bytes());
h2.write(&test_usize.to_ne_bytes());
h2.write(&test_i8.to_ne_bytes());
h2.write(&test_i16.to_ne_bytes());
h2.write(&test_i32.to_ne_bytes());
h2.write(&test_i64.to_ne_bytes());
h2.write(&test_i128.to_ne_bytes());
h2.write(&test_isize.to_ne_bytes());
let h1_hash = h1.finish128();
let h2_hash = h2.finish128();
assert_eq!(h1_hash, h2_hash);
}