Optimize RcInnerPtr::inc_strong()/inc_weak() instruction count

[mjbshaw]

Inspired by this internals thread: https://internals.rust-lang.org/t/rc-optimization-on-64-bit-targets/16362

The generated assembly is a bit smaller and is a more efficient usage of the CPU's instruction cache. unlikely doesn't impact any of the small artificial tests I've done, but I've included it in case it might help more complex scenarios when this is inlined.

[rust-highfive]

r? @yaahc

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[yaahc]

Looks great, thank you for the PR!

@bors r+

[bors]

📌 Commit 8d14c03 has been approved by yaahc

[Mark-Simulacrum]

@bors rollup=never

[bors]

⌛ Testing commit 8d14c03 with merge 78bca4dd0d02b7d3ec42f55ff0511b4a7894811d...

[Dylan-DPC]

@bors retry (letting rollup go first since queue is long)

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

⌛ Testing commit 8d14c03 with merge 2761af47284e3ef7ac9a027a40e98da242fb60ac...

[Mark-Simulacrum]

@bors retry -- gha-self-hosted restart

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

⌛ Testing commit 8d14c03 with merge bb1a03c...

[bors]

☀️ Test successful - checks-actions
Approved by: yaahc
Pushing bb1a03c to master...

[rust-timer]

Finished benchmarking commit (bb1a03c): comparison url.

Summary:

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: mixed results

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	0	4	0	1	0
mean2	N/A	1.9%	N/A	-0.3%	N/A
max	N/A	3.9%	N/A	-0.3%	N/A

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

Footnotes

1. number of relevant changes ↩

2. the arithmetic mean of the percent change ↩

[pnkfelix]

• The bulk of this regression is attached to secondary benchmark regression-31157 opt (incr-patched: println, incr-full, full), by up to 3.86%
• the self-profile data seems to indicate that the bulk of the cost here is spent in LLVM.
• the cachegrind output seconds that: https://gist.github.com/4fa7403ee2e812e7712c5046e9eb4d72
• This PR is motivated by an improvement to the object code itself generated by the compiler. I guess that improvement did not result in a net win for the compiler itself.
• regression-31157 (rust#31157) is encoding a case where we were seeing a 20x slowdown. So we should not be worrying about 2% or 4% performance losses there.

@rustbot label: +perf-regression-triaged