Use a specialized varint + bitpacking scheme for DepGraph encoding

2 files changed, 17 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/dep_graph/dep_node.rs b/compiler/rustc_middle/src/dep_graph/dep_node.rs
index afcf08395bb..78a0f82db13 100644
--- a/compiler/rustc_middle/src/dep_graph/dep_node.rs
+++ b/compiler/rustc_middle/src/dep_graph/dep_node.rs
@@ -97,7 +97,7 @@ macro_rules! define_dep_nodes {
             // discriminants of the variants have been assigned consecutively from 0
             // so that just the one comparison suffices to check that the u16 can be
             // transmuted to a DepKind.
-            const VARIANTS: u16 = {
+            pub const VARIANTS: u16 = {
                 let deps: &[DepKind] = &[$(DepKind::$variant,)*];
                 let mut i = 0;
                 while i < deps.len() {
diff --git a/compiler/rustc_middle/src/dep_graph/mod.rs b/compiler/rustc_middle/src/dep_graph/mod.rs
index f79ce08b8ae..87436f9eeed 100644
--- a/compiler/rustc_middle/src/dep_graph/mod.rs
+++ b/compiler/rustc_middle/src/dep_graph/mod.rs
@@ -26,6 +26,7 @@ pub type DepKindStruct<'tcx> = rustc_query_system::dep_graph::DepKindStruct<TyCt
 impl rustc_query_system::dep_graph::DepKind for DepKind {
     const NULL: Self = DepKind::Null;
     const RED: Self = DepKind::Red;
+    const MAX: u16 = DepKind::VARIANTS - 1;
 
     fn debug_node(node: &DepNode, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{:?}(", node.kind)?;
@@ -68,6 +69,21 @@ impl rustc_query_system::dep_graph::DepKind for DepKind {
             op(icx.task_deps)
         })
     }
+
+    #[track_caller]
+    #[inline]
+    fn from_u16(u: u16) -> Self {
+        if u > Self::MAX {
+            panic!("Invalid DepKind {u}");
+        }
+        // SAFETY: See comment on DepKind::VARIANTS
+        unsafe { std::mem::transmute(u) }
+    }
+
+    #[inline]
+    fn to_u16(self) -> u16 {
+        self as u16
+    }
 }
 
 impl<'tcx> DepContext for TyCtxt<'tcx> {