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Auto merge of #110634 - saethlin:pointy-decoder, r=cjgillot

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Rewrite MemDecoder around pointers not a slice

This is basically https://github.com/rust-lang/rust/pull/109910 but I'm being a lot more aggressive. The pointer-based structure means that it makes a lot more sense to absorb more complexity into `MemDecoder`, most of the diff is just complexity moving from one place to another.

The primary argument for this structure is that we only incur a single bounds check when doing multi-byte reads from a `MemDecoder`. With the slice-based implementation we need to do those with `data[position..position + len]` , which needs to account for `position + len` wrapping. It would be possible to dodge the first bounds check if we stored a slice that starts at `position`, but that would require updating the pointer and length on every read.

This PR also embeds the failure path in a separate function, which means that this PR should subsume all the perf wins observed in https://github.com/rust-lang/rust/pull/109867.
This commit is contained in:
bors 2023-04-26 02:36:42 +00:00
commit adaac6b166
11 changed files with 174 additions and 130 deletions
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16
compiler/rustc_query_system/src/dep_graph/serialized.rs
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@ -94,21 +94,19 @@ impl<'a, K: DepKind + Decodable<MemDecoder<'a>>> Decodable<MemDecoder<'a>>
{
#[instrument(level = "debug", skip(d))]
fn decode(d: &mut MemDecoder<'a>) -> SerializedDepGraph<K> {
let start_position = d.position();
// The last 16 bytes are the node count and edge count.
debug!("position: {:?}", d.position());
d.set_position(d.data.len() - 2 * IntEncodedWithFixedSize::ENCODED_SIZE);
let (node_count, edge_count) =
d.with_position(d.len() - 2 * IntEncodedWithFixedSize::ENCODED_SIZE, |d| {
debug!("position: {:?}", d.position());
let node_count = IntEncodedWithFixedSize::decode(d).0 as usize;
let edge_count = IntEncodedWithFixedSize::decode(d).0 as usize;
(node_count, edge_count)
});
debug!("position: {:?}", d.position());
let node_count = IntEncodedWithFixedSize::decode(d).0 as usize;
let edge_count = IntEncodedWithFixedSize::decode(d).0 as usize;
debug!(?node_count, ?edge_count);
debug!("position: {:?}", d.position());
d.set_position(start_position);
debug!("position: {:?}", d.position());
let mut nodes = IndexVec::with_capacity(node_count);
let mut fingerprints = IndexVec::with_capacity(node_count);
let mut edge_list_indices = IndexVec::with_capacity(node_count);