Original commit message:
[strings] improve array index hash distribution
Previously, the hashes stored in a Name's raw_hash_field for decimal
numeric strings (potential array indices) consist of the literal
integer value along with the length of the string. This means
consecutive numeric strings can have consecutive hash values, which
can lead to O(n^2) probing for insertion in the worst case when e.g.
a non-numeric string happen to land in the these buckets.
This patch adds a build-time flag v8_enable_seeded_array_index_hash that
scrambles the 24-bit array-index value stored in a Name's raw_hash_field
to improve the distribution.
x ^= x >> kShift; x = (x * m1) & kMask; // round 1
x ^= x >> kShift; x = (x * m2) & kMask; // round 2
x ^= x >> kShift; // finalize
To decode, apply the same steps with the modular inverses of m1 and m2
in reverse order.
x ^= x >> kShift; x = (x * m2_inv) & kMask; // round 1
x ^= x >> kShift; x = (x * m1_inv) & kMask; // round 2
x ^= x >> kShift; // finalize
where kShift = kArrayIndexValueBits / 2, kMask = kArrayIndexValueMask,
m1, m2 (both odd) are the lower bits of the rapidhash secrets, m1_inv,
m2_inv (modular inverses) are precomputed modular inverse of m1 and m2.
The pre-computed values are appended to the hash_seed ByteArray in
ReadOnlyRoots and accessed in generated code to reduce overhead.
In call sites that don't already have access to the seeds, we read them
from the current isolate group/isolate's read only roots.
To consolidate the code that encode/decode these hashes, this patch
adds MakeArrayIndexHash/DecodeArrayIndexFromHashField in C++ and CSA
that perform seeding/unseeding if enabled, and updates places where
encoding/decoding of array index is needed to use them.
Bug: 477515021
Change-Id: I350afe511951a54c4378396538152cc56565fd55
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7564330
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/main@{#105596}
Refs: 1361b2a49d
Co-authored-by: Joyee Cheung <joyeec9h3@gmail.com>
Backport-PR-URL: https://github.com/nodejs-private/node-private/pull/833
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
PR-URL: https://github.com/nodejs-private/node-private/pull/809
CVE-ID: CVE-2026-21717
e3f4d6a42e