Sort mono items by symbol name

Author: Kobzol

compiler/rustc_middle/src/mir/mono.rs

@@ -566,10 +566,16 @@ impl<'tcx> CodegenUnit<'tcx> {
 
         let mut items: Vec<_> = self.items().iter().map(|(&i, &data)| (i, data)).collect();
         if !tcx.sess.opts.unstable_opts.codegen_source_order {
-            // It's already deterministic, so we can just use it.
-            return items;
+            // The input item list is already produced in a deterministic matter, so we can just
+            // use it.
+            // However, it seems that actually moving related things (probably mainly different
+            // monomorphizations of the same item) close to one another is actually beneficial for
+            // perf.
+            // Sorting by symbol name should not incur any non-determinism.
+            items.sort_by_cached_key(|&(i, _)| i.symbol_name(tcx));
+        } else {
+            items.sort_by_cached_key(|&(i, _)| item_sort_key(tcx, i));
         }
-        items.sort_by_cached_key(|&(i, _)| item_sort_key(tcx, i));
         items
     }