Ahmed Bougacha 61069bd5a3 [Support] Add SipHash-based 16-bit ptrauth ABI-stable hash. (#93902) ...

This finally wraps the now-lightly-modified SipHash C reference
implementation, for the main interface we need (16-bit ptrauth
discriminators).

The exact algorithm is the little-endian interpretation of the
non-doubled (i.e. 64-bit) result of applying a SipHash-2-4 using the
constant seed `b5d4c9eb79104a796fec8b1b428781d4` (big-endian), with the
result reduced by modulo to the range of non-zero discriminators (i.e.
`(rawHash % 65535) + 1`).

By "stable" we mean that the result of this hash algorithm will the same
across different compiler versions and target platforms.

The 16-bit hashes are used extensively for the AArch64 ptrauth ABI,
because AArch64 can efficiently load a 16-bit immediate into the high
bits of a register without disturbing the remainder of the value, which
serves as a nice blend operation.

16 bits is also sufficiently compact to not inflate a loader relocation.
We disallow zero to guarantee a different discriminator from the places
in the ABI that use a constant zero.

Co-authored-by: John McCall <rjmccall@apple.com>

2024-06-14 17:57:12 -07:00

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