1583953 - mozilla::Span iterators not optimized out in operator==

Status: RESOLVED FIXED

(Core :: MFBT, enhancement)

Description

[Adrian Wielgosik [:adrian17]]

When profiling the microbenchmark from bug 1420440 comment 0 on nightly and stable, I noticed the following stack close to the top of leaf functions list:

3.3% js::SharedScriptDataHasher::match(js::SharedScriptData *,RefPtr<js::SharedScriptData> const &)
3.2%    bool std::equal<mozilla::span_details::span_iterator<mozilla::Span<const unsigned char,18446744073709551615>,0>,...
3.0%        bool std::_Equal_unchecked<mozilla::span_details::span_iterator<mozilla::Span<const unsigned char,18446744073709551615>,0>,...

It seems like Clang* isn't able to optimize out the release assertions in Span iterators and produces a heavy loop.
Changing Span's operator== implementation from std::equal(l.begin(), l.end(), r.begin() to std::equal(l.data(), l.data() + l.size(), r.data() to avoid the iterators does let the compiler generate a memcmp/bcmp call instead.

Same probably applies for operator< and std::lexicographical_compare.

I'm assuming the release assertions can't be dropped, so I propose at least updating the std::equal, std::lexicographical_compare calls.

*For the record, using GCC (or disabling iterator assertions) seems to generate better code, but still not memcmp.

Comment 1

[Adrian Wielgosik [:adrian17]]

Attachment: Bug 1583953 - avoid assert-heavy iterators in Span comparisons. r?hsivonen

This lets us avoid pointlessly checking release assertions in a safe comparison loop.
In particular, for Span<char>, this lets the compiler use memcmp, speeding up comparisons by over an order of magnitude.

Comment 2

[Adrian Wielgosik [:adrian17]]

Small try run (as currently there is only a single user of these operators, in RuntimeScriptDataHasher):
https://treeherder.mozilla.org/#/jobs?repo=try&revision=9d0b1dd08c5a5c5e9561825913680f5527bec042

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/10de08776fe8
avoid assert-heavy iterators in Span comparisons. r=jwalden

Comment 4

[Bogdan Tara[:bogdan_tara | bogdant]]

https://hg.mozilla.org/mozilla-central/rev/10de08776fe8