1 files changed, 59 insertions, 1 deletions

diff --git a/compiler/rustc_session/src/code_stats.rs b/compiler/rustc_session/src/code_stats.rs
index 0dfee92f4..cabe1c96b 100644
--- a/compiler/rustc_session/src/code_stats.rs
+++ b/compiler/rustc_session/src/code_stats.rs
@@ -1,5 +1,6 @@
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lock;
+use rustc_span::def_id::DefId;
 use rustc_span::Symbol;
 use rustc_target::abi::{Align, Size};
 use std::cmp;
@@ -65,9 +66,29 @@ pub struct TypeSizeInfo {
     pub variants: Vec<VariantInfo>,
 }
 
+pub struct VTableSizeInfo {
+    pub trait_name: String,
+
+    /// Number of entries in a vtable with the current algorithm
+    /// (i.e. with upcasting).
+    pub entries: usize,
+
+    /// Number of entries in a vtable, as-if we did not have trait upcasting.
+    pub entries_ignoring_upcasting: usize,
+
+    /// Number of entries in a vtable needed solely for upcasting
+    /// (i.e. `entries - entries_ignoring_upcasting`).
+    pub entries_for_upcasting: usize,
+
+    /// Cost of having upcasting in % relative to the number of entries without
+    /// upcasting (i.e. `entries_for_upcasting / entries_ignoring_upcasting * 100%`).
+    pub upcasting_cost_percent: f64,
+}
+
 #[derive(Default)]
 pub struct CodeStats {
     type_sizes: Lock<FxHashSet<TypeSizeInfo>>,
+    vtable_sizes: Lock<FxHashMap<DefId, VTableSizeInfo>>,
 }
 
 impl CodeStats {
@@ -101,6 +122,14 @@ impl CodeStats {
         self.type_sizes.borrow_mut().insert(info);
     }
 
+    pub fn record_vtable_size(&self, trait_did: DefId, trait_name: &str, info: VTableSizeInfo) {
+        let prev = self.vtable_sizes.lock().insert(trait_did, info);
+        assert!(
+            prev.is_none(),
+            "size of vtable for `{trait_name}` ({trait_did:?}) is already recorded"
+        );
+    }
+
     pub fn print_type_sizes(&self) {
         let type_sizes = self.type_sizes.borrow();
         let mut sorted: Vec<_> = type_sizes.iter().collect();
@@ -196,4 +225,33 @@ impl CodeStats {
             }
         }
     }
+
+    pub fn print_vtable_sizes(&self, crate_name: &str) {
+        let mut infos = std::mem::take(&mut *self.vtable_sizes.lock())
+            .into_iter()
+            .map(|(_did, stats)| stats)
+            .collect::<Vec<_>>();
+
+        // Primary sort: cost % in reverse order (from largest to smallest)
+        // Secondary sort: trait_name
+        infos.sort_by(|a, b| {
+            a.upcasting_cost_percent
+                .total_cmp(&b.upcasting_cost_percent)
+                .reverse()
+                .then_with(|| a.trait_name.cmp(&b.trait_name))
+        });
+
+        for VTableSizeInfo {
+            trait_name,
+            entries,
+            entries_ignoring_upcasting,
+            entries_for_upcasting,
+            upcasting_cost_percent,
+        } in infos
+        {
+            println!(
+                r#"print-vtable-sizes {{ "crate_name": "{crate_name}", "trait_name": "{trait_name}", "entries": "{entries}", "entries_ignoring_upcasting": "{entries_ignoring_upcasting}", "entries_for_upcasting": "{entries_for_upcasting}", "upcasting_cost_percent": "{upcasting_cost_percent}" }}"#
+            );
+        }
+    }
 }