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runtime: mix a bit more in arm64 hash function

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We really need 3 mix steps between the data being hashed and the output.
One mix can only spread a 1 bit change to 32 bits. The second mix
can spread to all 128 bits, but the spread is not complete. A third mix
spreads out ~evenly to all 128 bits.

The amd64 version has 3 mix steps.

Fixes #59643

Change-Id: I54ad8686ca42bcffb6d0ec3779d27af682cc96e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/486616
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Keith Randall 2023-04-19 16:41:37 -07:00 committed by Keith Randall
parent 8588a3fa08
commit 22a840aa9b
2 changed files with 6 additions and 2 deletions
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1
src/runtime/asm_arm64.s
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@ -611,6 +611,7 @@ done:
AESE V0.B16, V2.B16
AESMC V2.B16, V2.B16
AESE V0.B16, V2.B16
AESMC V2.B16, V2.B16
VMOV V2.D[0], R0
RET

7
src/runtime/hash_test.go
View File

@ -775,8 +775,11 @@ func TestCollisions(t *testing.T) {
a[j] = byte(n >> 8)
m[uint16(BytesHash(a[:], 0))] = struct{}{}
}
if len(m) <= 1<<15 {
t.Errorf("too many collisions i=%d j=%d outputs=%d out of 65536\n", i, j, len(m))
// N balls in N bins, for N=65536
avg := 41427
stdDev := 123
if len(m) < avg-40*stdDev || len(m) > avg+40*stdDev {
t.Errorf("bad number of collisions i=%d j=%d outputs=%d out of 65536\n", i, j, len(m))
}
}
}