Emit the enum discriminant separately for the Encodable macro

[Zoxc]

This changes the Encodable proc macro to first emit the discriminant with a separate match statement, then emit the fields. LLVM can optimize down the first match to just a read of the enum discriminant. This avoids generating a emit_usize call per variant and enums with no fields now optimize down to just a single emit_usize call.

The no variant case is special cased to avoid warnings about unreachable code.

Benchmark	Before	After
	Time	Time	%
🟣 clap:check:unchanged	0.4676s	0.4640s	-0.78%
🟣 hyper:check:unchanged	0.1348s	0.1338s	-0.75%
🟣 regex:check:unchanged	0.3370s	0.3352s	-0.54%
🟣 syn:check:unchanged	0.6326s	0.6281s	-0.71%
🟣 syntex_syntax:check:unchanged	1.8561s	1.8589s	0.15%
Total	3.4282s	3.4200s	-0.24%
Summary	1.0000s	0.9947s	-0.53%

[rustbot]

r? @TaKO8Ki

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[cjgillot]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 7de205e with merge 57130c81008993e359d706fbf762508fdcbaaf69...

[bors]

☀️ Try build successful - checks-actions
Build commit: 57130c81008993e359d706fbf762508fdcbaaf69 (57130c81008993e359d706fbf762508fdcbaaf69)

[rust-timer]

Finished benchmarking commit (57130c81008993e359d706fbf762508fdcbaaf69): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-1.0%, -0.3%]	89
Improvements ✅ (secondary)	-0.6%	[-0.9%, -0.3%]	23
All ❌✅ (primary)	-0.6%	[-1.0%, -0.3%]	89

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.1%	[3.1%, 3.1%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

[cjgillot]

@bors r+

[bors]

📌 Commit 7de205e has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7de205e with merge 58136ff...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 58136ff to master...

[rust-timer]

Finished benchmarking commit (58136ff): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-1.0%, -0.3%]	91
Improvements ✅ (secondary)	-0.6%	[-0.9%, -0.4%]	22
All ❌✅ (primary)	-0.6%	[-1.0%, -0.3%]	91

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.0%	[-3.4%, -2.7%]	2
Improvements ✅ (secondary)	-2.9%	[-4.5%, -1.3%]	2
All ❌✅ (primary)	-3.0%	[-3.4%, -2.7%]	2

Cycles

This benchmark run did not return any relevant results for this metric.