panic_unwind: Use global_asm! for IMGREL relocations

[Darksonn]

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Rust consteval cannot construct 32-bit values that contain the offset between two symbols, but this is required for panic_unwind on some architectures because unwinding on Windows SEH uses image-base-relative pointers for the panic information (which makes them fit in 32-bit rather than 64-bit).

Currently, this is worked around by initializing these globals on panic with atomic stores. This works, but is undefined behavior as the panic runtime reads the values with non-atomic loads, which leads to a data race.

Instead, utilize global_asm! to create the globals with @IMGREL relocations so that the linker constructs the relative pointers for us at link or load time.

For context: #t-compiler > relative pointers in windows SEH panic_unwind @ 💬

AI assistance was involved with writing the assembly code.

[rustbot]

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[Fulgen301]

(Nitpick: I'd clarify that they're offsets from the image base and not __ImageBase, even if that's the exact same address.)

View changes since the review

[LawnGnome]

Re-rolling, as my asm is absolutely not good enough for this.

@rustbot reroll

[Mark-Simulacrum]

Definitely not familiar with the Windows aspects of this... but happy to approve given Bjorn has taken a look with a response to the comment (either telling me I'm wrong or changing things :)

View changes since this review

[Mark-Simulacrum]

@bors try jobs=msvc

[rust-bors]

💔 Test for ee5da48 failed: CI. Failed jobs:

• try - aarch64-msvc-1 (web logs, enhanced plaintext logs)
• try - aarch64-msvc-2 (web logs, enhanced plaintext logs)

[Darksonn]

@bors try jobs=msvc

[rust-bors]

💔 Test for aa22e3c failed: CI. Failed jobs:

• try - aarch64-msvc-1 (web logs, enhanced plaintext logs)
• try - aarch64-msvc-2 (web logs, enhanced plaintext logs)
• try - dist-aarch64-msvc (web logs, enhanced plaintext logs)

[Darksonn]

@bors try jobs=msvc

[rust-bors]

☀️ Try build successful (CI)
Build commit: 976338d (976338db6bad68e99abbdc7370002f113822c140)
Base parent: 7088e4b (7088e4b63a9516ebfbfe2ab2d999cf01a528ac14)

[Mark-Simulacrum]

r=me with rebase (and maybe commits squashed a bit)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Darksonn]

@bors try jobs=msvc

[rust-bors]

☀️ Try build successful (CI)
Build commit: 47b8902 (47b8902dd55a82ada8730a99f6366c46b1ad8835)
Base parent: cc05892 (cc05892c8346313865afd91ca12ee1fde6d3603c)

[Darksonn]

@bors r=Mark-Simulacrum

[rust-bors]

📌 Commit 4339216 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[rust-timer]

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (ae960b0): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 4.1%, 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)	4.1%	[4.1%, 4.1%]	1
Regressions ❌ (secondary)	1.9%	[1.0%, 3.7%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.9%	[-2.9%, -2.9%]	1
All ❌✅ (primary)	4.1%	[4.1%, 4.1%]	1

Cycles

Results (primary -0.5%, secondary 2.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	4.2%	[3.1%, 5.3%]	2
Improvements ✅ (primary)	-2.1%	[-2.2%, -2.1%]	2
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-0.5%	[-2.2%, 2.8%]	3

Binary size

Results (secondary -0.0%)

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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 475.812s -> 474.909s (-0.19%)
Artifact size: 403.37 MiB -> 402.66 MiB (-0.18%)