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

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.

1 files changed, 7 insertions, 9 deletions

diff --git a/compiler/rustc_query_system/src/dep_graph/serialized.rs b/compiler/rustc_query_system/src/dep_graph/serialized.rs
index 59c1333fb5a..edddfda6242 100644
--- a/compiler/rustc_query_system/src/dep_graph/serialized.rs
+++ b/compiler/rustc_query_system/src/dep_graph/serialized.rs
@@ -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);