From d210c7fca4163e7708aed61a3df17d32cd68adaf Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Thu, 29 Aug 2024 16:53:07 +0200
Subject: [PATCH] interpret/visitor: make memory order iteration slightly more
 efficient

---
 src/helpers.rs | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/src/helpers.rs b/src/helpers.rs
index 0483745621..a546ad20fe 100644
--- a/src/helpers.rs
+++ b/src/helpers.rs
@@ -630,14 +630,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 self.ecx
             }
 
-            fn aggregate_field_order(memory_index: &IndexVec<FieldIdx, u32>, idx: usize) -> usize {
-                // We need to do an *inverse* lookup: find the field that has position `idx` in memory order.
-                for (src_field, &mem_pos) in memory_index.iter_enumerated() {
-                    if mem_pos as usize == idx {
-                        return src_field.as_usize();
-                    }
-                }
-                panic!("invalid `memory_index`, could not find {}-th field in memory order", idx);
+            fn aggregate_field_iter(
+                memory_index: &IndexVec<FieldIdx, u32>,
+            ) -> impl Iterator<Item = FieldIdx> + 'static {
+                let inverse_memory_index = memory_index.invert_bijective_mapping();
+                inverse_memory_index.into_iter()
             }
 
             // Hook to detect `UnsafeCell`.