incr.comp.: Use a struct-of-arrays instead of an array-of-structs for SerializedDepGraph

[michaelwoerister]

This should improve memory consumption and cache locality a bit because we are wasting space on padding bytes at the moment. It should also enable us to bulk-persist parts or all of the graph to disk at once instead of dealing with each value separately.

[michaelwoerister]

Some more information: The thing that could be refactored here is the nodes field in SerializedDepGraph:

rust/src/librustc/dep_graph/serialized.rs

Line 23 in 24e679c

pub nodes: IndexVec<SerializedDepNodeIndex, (DepNode, Fingerprint)>,

At the moment it is an array of (DepNode, Fingerprint) but it could be turned into two arrays, one for DepNode, one for Fingerprint.

Then the following step in DepGraph::serialize() would not be necessary:

rust/src/librustc/dep_graph/graph.rs

Lines 479 to 482 in 24e679c

	let nodes: IndexVec<_, (DepNode, Fingerprint)> =
	current_dep_graph.nodes.iter_enumerated().map(|(idx, &dep_node)| {
	(dep_node, fingerprints[idx])
	}).collect();

One could even try to not copy the arrays at all in DepGraph::serialize(). Instead one could use std::mem::swap() to exchange the data in DepGraph for empty arrays and then just move it into the returned SerializedDepGraph. This would leave the dep-graph in an invalid state though. Not sure if we access it again after serialization (some code might still read from it).

[wesleywiser]

I can take this. Thanks for writing up that additional detail @michaelwoerister!

[michaelwoerister]

Cool! Thanks, @wesleywiser!