Specialize HashStable for [u8] slices

[Mark-Simulacrum]

Particularly for ctfe-stress-4, the hashing of byte slices as part of the
MIR Allocation is quite hot. Previously, we were falling back on byte-by-byte
copying of the slice into the SipHash buffer (64 bytes long) before hashing a 64
byte chunk, and then doing that again and again; now we use the dedicated byte-slice write.

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 3cd5c95 with merge 32fa510127d3d61a41f76035ff5573c49ea98243...

[bors]

☀️ Try build successful - checks-actions
Build commit: 32fa510127d3d61a41f76035ff5573c49ea98243 (32fa510127d3d61a41f76035ff5573c49ea98243)

[rust-timer]

Queued 32fa510127d3d61a41f76035ff5573c49ea98243 with parent aa5740c, future comparison URL.

[rust-timer]

Finished benchmarking commit (32fa510127d3d61a41f76035ff5573c49ea98243): comparison url.

Summary: This change led to very large relevant improvements 🎉 in compiler performance.

• Very large improvement in instruction counts (up to -8.0% on incr-full builds of ctfe-stress-4)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

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 led to changes in compiler perf.

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

[Mark-Simulacrum]

r? rust-lang/compiler

[oli-obk]

@bors r+

[bors]

📌 Commit 3cd5c95 has been approved by oli-obk

[bors]

⌛ Testing commit 3cd5c95 with merge bdcb528...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing bdcb528 to master...

[rust-timer]

Finished benchmarking commit (bdcb528): comparison url.

Summary: This change led to very large relevant improvements 🎉 in compiler performance.

• Very large improvement in instruction counts (up to -8.0% on incr-full builds of ctfe-stress-4)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression