Prefer -1 for None

[scottmcm]

Currently we pick "weird" numbers like 1114112 for None::<char>. While that's not wrong, it's kinda unnatural -- a human wouldn't make that choice.

This PR instead picks -1 for thinge like None::<char> -- like clang's WEOF -- and None::<bool> and such.

Any enums with more than one niched value (so not Result nor Option) remain as they were before. Also we continue to use 0 when that's possible -- -1 is only preferred when zero isn't possible.

---

Inspired when someone in discord posted an example like this https://rust.godbolt.org/z/W94s9qdYW and I thought it was odd that we're currently picking -9223372036854775808 to be the value to store to mark an Option<Vec<_>> as None. (Especially since that needs an 8-byte immediate on x64, and writing -1 is only a 4-byte immediate.)

[scottmcm]

Note that this arm just kinda gave up before; it never even considered whether putting it next to start might be better.

View changes since the review

[scottmcm]

@bors try @rust-timer queue

[scottmcm]

It helps to remember to git add all the changes 🤦

[scottmcm]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: ed76e05 (ed76e056cbb36453c64da4dd8ffccac4e093f819, parent: 2f201bccb3a7fb5a85b0fcfcc0a020a946d6d58a)

[rust-timer]

Finished benchmarking commit (ed76e05): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	0.5%	[0.0%, 1.5%]	9
Improvements ✅ (primary)	-0.3%	[-2.9%, -0.2%]	86
Improvements ✅ (secondary)	-0.3%	[-1.8%, -0.0%]	62
All ❌✅ (primary)	-0.3%	[-2.9%, 0.3%]	87

Max RSS (memory usage)

Results (secondary 1.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	6.5%	[6.5%, 6.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.4%	[-4.4%, -4.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.9%, secondary -4.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-2.9%	[-3.0%, -2.9%]	2
Improvements ✅ (secondary)	-6.2%	[-7.6%, -3.4%]	3
All ❌✅ (primary)	-2.9%	[-3.0%, -2.9%]	2

Binary size

Results (primary -0.2%, secondary -0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.0%, 0.6%]	2
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.9%	[-2.1%, -0.3%]	3
All ❌✅ (primary)	-0.2%	[-0.6%, -0.1%]	5

Bootstrap: 491.618s -> 493.312s (0.34%)
Artifact size: 394.25 MiB -> 396.31 MiB (0.52%)

[scottmcm]

Wow, those results are way better than I expected 😅

Seems pretty clearly a net win to me.
@rustbot label: +perf-regression-triaged

[rustbot]

r? @chenyukang

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 72 candidates
• Random selection from 18 candidates

[chenyukang]

seems a good perf improvement, lgtm, but need other's review.
@rustbot reroll