1577263 - Use memcmp for nsID::Equals

Status: RESOLVED FIXED

(Core :: XPCOM, task)

Description

[Kris Maglione [:kmag]]

It's been a few years since we last tried this, and compilers have come a long way. Clang compiles memcmp(*this, &aOther, sizeof *this) to a single SIMD operation, and since we now use Clang everywhere, we shouldn't see any variability across platforms.

Comment 1

[Kris Maglione [:kmag]]

Attachment: Bug 1577263: Use memcmp for nsID::Equals. r=froydnj

Compilers have gotten a lot better since the last time we tried this, and the
generated SIMD code for the inlined memcmp is more efficient than our manual
comparison operations.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/3e2987e01215
Use memcmp for nsID::Equals. r=froydnj

Comment 3

[Gerald Squelart (he/him) (not at Mozilla since 2022-09-15)]

FYI, comparing member-by-member gave the exact same code as the SIMD'd memcmp on x86-64 clang, and the same code as the old Equals on ARM:
https://godbolt.org/z/QTmG90

That's what you wanted to achieve, right? (But without the #ifdefs, and no C casts.)
I'll let you decide if it's worth changing again. 😉

Comment 4

[Cristina Coroiu [:ccoroiu]]

https://hg.mozilla.org/mozilla-central/rev/3e2987e01215