[do not merge] Remove PackedFingerprint

[tgnottingham]

PackedFingerprint was added by #78646 to reduce the size and alignment of DepNodes. This lowered memory consumption at the cost of increased cycles. Since then, #79589 and #80957 have decreased the number of DepNodes that exist during a compilation session, so PackedFingerprint may not be as beneficial anymore. Let's do a perf run and find out.

r? @ghost

[tgnottingham]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

[bors]

⌛ Trying commit 2b32c8b2e6f1939aa45d6654a3f34f2885b954cf with merge 0a3bc90310fef9bd3563b773ac96ad1720c363de...

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/compiler/rustc_middle/src/dep_graph/dep_node.rs at line 407:
     /// has been removed.
     fn extract_def_id(&self, tcx: TyCtxt<'tcx>) -> Option<DefId> {
         if self.kind.can_reconstruct_query_key() {
-            tcx.queries
-                .on_disk_cache
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/compiler/rustc_middle/src/dep_graph/dep_node.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
-                .as_ref()?
-                .def_path_hash_to_def_id(tcx, DefPathHash(self.hash))
+            tcx.queries.on_disk_cache.as_ref()?.def_path_hash_to_def_id(tcx, DefPathHash(self.hash))
             None
         }
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --stage 2 src/tools/tidy
Build completed unsuccessfully in 0:00:18

[tgnottingham]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

[bors]

⌛ Trying commit d4e42af with merge cf92be18e6cc5980739fdd6c7bfc48659900f777...

[bors]

☀️ Try build successful - checks-actions
Build commit: cf92be18e6cc5980739fdd6c7bfc48659900f777 (cf92be18e6cc5980739fdd6c7bfc48659900f777)

[rust-timer]

Queued cf92be18e6cc5980739fdd6c7bfc48659900f777 with parent a4cbb44, future comparison URL.

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

[rust-timer]

Finished benchmarking try commit (cf92be18e6cc5980739fdd6c7bfc48659900f777): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

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

[tgnottingham]

The instruction counts improve very slightly after reverting (deeply-nested-async is worse, but is probably bogus based on how that benchmark has behaved in the past). The cycles difference is in the noise/variance range. And the max-rss difference isn't large enough to show the through the noise.

But profiling locally under Massif, which produces less noisy results, shows that using packed fingerprints results in memory reductions of about 50MB or 4% for the style-servo-check incr-patched benchmark, for example.

Assuming that result is representative and because the other stats don't hugely favor one approach, I'm inclined to leave things as they are. But I don't have strong feelings one way or the other.