bootstrap: Memoize the LLVM rebuild hash to avoid very slow x check

[Zalathar]

Recently I've encountered a massive regression in the performance of re-running x check after making no changes.

It used to take around 2 seconds, and now it takes 20-30 seconds. That's quite a hassle when r-a runs it every time I save.

After some poking around, what I've found is that each individual call to generate_smart_stamp_hash takes only a modestly long time (around 0.5 sec), but it gets called dozens of times during x check, and that seems to be what's adding up to 20-30 seconds.

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https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/Massive.20regression.20in.20no-op.20.60x.20check.60

cc @onur-ozkan

[rustbot]

r? @albertlarsan68

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[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

[Zalathar]

This is an alternative to #118351.

It's very much intended as a quick-fix to solve the immediate issue of no-op check builds taking way too long (~30 sec instead of ~2 sec).

I'm open to fixing this in better ways, but I think it's worth at least working around this as soon as possible, so that people's dev environments are functional again.

(I assume this is one of those issues that stems from git being way faster on Linux than on other platforms.)

[onur-ozkan]

@bors r+

[bors]

📌 Commit 4f1cf0b has been approved by onur-ozkan

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4f1cf0b with merge b29a1e0...

[bors]

☀️ Test successful - checks-actions
Approved by: onur-ozkan
Pushing b29a1e0 to master...

[rust-timer]

Finished benchmarking commit (b29a1e0): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

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)	2.0%	[2.0%, 2.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 674.949s -> 675.168s (0.03%)
Artifact size: 313.35 MiB -> 313.39 MiB (0.01%)