library: Compute RUST_EXCEPTION_CLASS from native-endian bytes

[workingjubilee]

This makes it appear correctly in hexdumps on both LE and BE platforms.

As discussed extensively in https://github.com/rust-lang/rust/pull/130381/files#r1760034980

[rustbot]

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[thomcc]

Nice. Not going to rollup this in case it causes issues and is needed to be identified in a bisection.

@bors r+ rollup=never

[bors]

📌 Commit f21f4cd has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit f21f4cd with merge 150247c...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 150247c to master...

[rust-timer]

Finished benchmarking commit (150247c): 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)	-3.3%	[-3.3%, -3.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-3.3%, -3.3%]	1

Max RSS (memory usage)

Results (primary 2.9%, secondary -2.2%)

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)	2.9%	[2.9%, 2.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	2.9%	[2.9%, 2.9%]	1

Cycles

Results (primary -2.5%, secondary 6.8%)

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)	6.8%	[6.8%, 6.8%]	1
Improvements ✅ (primary)	-2.5%	[-2.9%, -2.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.5%	[-2.9%, -2.1%]	2

Binary size

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

Bootstrap: 771.075s -> 771.117s (0.01%)
Artifact size: 341.37 MiB -> 341.36 MiB (-0.00%)

[nbdd0121]

Note that the Itanium exception ABI says that the convention is that high bit are for vendor and low bits are for language: https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html.

Perhaps put a relnote to say that the langauge is now renamed to MOZ? ;)

[workingjubilee]

Note that the Itanium exception ABI says that the convention is that high bit are for vendor and low bits are for language: https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html.

Perhaps put a relnote to say that the langauge is now renamed to MOZ? ;)

✔️ MOZ from RUST

[RalfJung]

More like ZOM, no? "The low 4 bytes" would have a different order on little-endian than on big-endian.

If all other languages use from_be_bytes, then maybe we should, too?

[RalfJung]

Cc @rust-lang/libs