Speed up SVH computation by using Fingerprint::combine().

[michaelwoerister]

Existing fingerprint values can be merged into one fingerprint via the rather efficient Fingerprint::combine() method. We should make use of that during SVH computation instead of re-hashing all those fingerprints.

See here:

rust/src/librustc/hir/map/collector.rs

Lines 123 to 163 in 2e33c89

	pub(super) fn finalize_and_compute_crate_hash(self,
	crate_disambiguator: CrateDisambiguator,
	cstore: &CrateStore,
	commandline_args_hash: u64)
	-> (Vec<MapEntry<'hir>>, Svh) {
	let mut node_hashes: Vec<_> = self
	.hir_body_nodes
	.iter()
	.map(|&(def_path_hash, dep_node_index)| {
	(def_path_hash, self.dep_graph.fingerprint_of(dep_node_index))
	})
	.collect();
	node_hashes.sort_unstable_by(|&(ref d1, _), &(ref d2, _)| d1.cmp(d2));
	let mut upstream_crates: Vec<_> = cstore.crates_untracked().iter().map(|&cnum| {
	let name = cstore.crate_name_untracked(cnum).as_str();
	let disambiguator = cstore.crate_disambiguator_untracked(cnum)
	.to_fingerprint();
	let hash = cstore.crate_hash_untracked(cnum);
	(name, disambiguator, hash)
	}).collect();
	upstream_crates.sort_unstable_by(|&(name1, dis1, _), &(name2, dis2, _)| {
	(name1, dis1).cmp(&(name2, dis2))
	});
	let (_, crate_dep_node_index) = self
	.dep_graph
	.with_task(DepNode::new_no_params(DepKind::Krate),
	&self.hcx,
	((node_hashes, upstream_crates),
	(commandline_args_hash,
	crate_disambiguator.to_fingerprint())),
	identity_fn);
	let svh = Svh::new(self.dep_graph
	.fingerprint_of(crate_dep_node_index)
	.to_smaller_hash());
	(self.map, svh)
	}

[srinivasreddy]

I am taking it.

[michaelwoerister]

Thank you, @srinivasreddy! Let me know if you need any further directions.

[Pulkit07]

@srinivasreddy are you still working on it or can I take this one?

[michaelwoerister]

Feel free to give it a shot if you are still interested, @Pulkit07!

[hrvolapeter]

Hello @michaelwoerister I would like to take this one, but first I will need a bit more explanation since I'm new to this project.

You are suggestings using Fingerprint::combine() method I suppose you meant instead of this

.map(|&(def_path_hash, dep_node_index)| { 
             (def_path_hash, self.dep_graph.fingerprint_of(dep_node_index)) 
         }) 

create just one fingerprint.

There is a commit that is doing what I described, but I'm not sure if it's fine https://github.com/retep007/rust/commit/0c3bf9d82c1dd8c7304ac241bc610efc85ab87fc , particularly if merging vector of hashes to one can't affect something else

[hrvolapeter]

@michaelwoerister did you have time to check my commit?

[michaelwoerister]

Oh, sorry for the delay, @retep007! I remember taking a look and then completely forgot replying to you.

The code combining the fingerprints looks good. However, we have to keep the sorting step in order to make the hash more stable. Otherwise exchanging to functions in the source code would lead to a different hash, even though such a change makes no semantic difference.