Armaan Sandhu
31bf94f87d
gcc does not inline sipround() at -O2: its size estimate lands just over
the default max-inline-insns-auto budget, so nm on libsystemd-shared.so
shows a local sipround symbol and each of the ten call sites pays a
call/ret plus a reload and store of the state through the pointer, around
six rounds of ~14 ALU ops. clang inlines it on its own, and gcc does too
at -O3 or with --param max-inline-insns-auto=30, so this is not a missed
optimization to report upstream but the inliner's size heuristic doing
what it is designed to do. The documented way to overrule it for one
function is always_inline.
The SipHash reference implementation this file is derived from inlines
the round manually, by writing it as a SIPROUND macro.
AMD Ryzen AI 7 350, gcc 16.2.1, -O2 -fstack-protector-strong
-fno-strict-aliasing, median of five runs:
input before after speedup
8 B 61.3 ns 9.3 ns 6.63x
16 B 54.6 ns 10.7 ns 5.11x
24 B 50.0 ns 12.6 ns 3.98x
40 B 49.4 ns 16.4 ns 3.01x
64 B 53.2 ns 22.1 ns 2.41x
128 B 67.8 ns 36.9 ns 1.84x
512 B 157.7 ns 128.1 ns 1.23x
4096 B 992.5 ns 964.5 ns 1.03x
The win is concentrated on short inputs because the bulk loop in
siphash24_compress() was the one place gcc did inline the round; the
prologue, the tail and all six rounds of siphash24_finalize() were not.
Short inputs are also what we overwhelmingly hash: hashmap and set keys,
unit names, paths, journal field names and data objects, bus match
strings. clang is unaffected.
Resulting hash values are unchanged; verified identical over every input
length from 0 to 128 bytes, and test-siphash24, test-hash-funcs,
test-hashmap and test-set pass.
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