diff --git a/compiler/rustc_smir/src/rustc_internal/mod.rs b/compiler/rustc_smir/src/rustc_internal/mod.rs
index 2f4bc5a0499..d0515df9ad5 100644
--- a/compiler/rustc_smir/src/rustc_internal/mod.rs
+++ b/compiler/rustc_smir/src/rustc_internal/mod.rs
@@ -107,12 +107,13 @@ impl<'tcx> Tables<'tcx> {
 
     fn create_alloc_id(&mut self, aid: AllocId) -> stable_mir::AllocId {
         // FIXME: this becomes inefficient when we have too many ids
-        if let Some(i) = self.alloc_ids.iter().position(|a| *a == aid) {
-            return stable_mir::AllocId(i);
-        };
-        let id = self.def_ids.len();
-        self.alloc_ids.push(aid);
-        stable_mir::AllocId(id)
+        if let Some(i) = self.alloc_ids.get(&aid) {
+            return *i;
+        } else {
+            let id = self.def_ids.len();
+            self.alloc_ids.insert(aid, stable_mir::AllocId(id));
+            stable_mir::AllocId(id)
+        }
     }
 
     pub(crate) fn create_span(&mut self, span: Span) -> stable_mir::ty::Span {
@@ -136,7 +137,7 @@ pub fn run(tcx: TyCtxt<'_>, f: impl FnOnce()) {
         Tables {
             tcx,
             def_ids: fx::FxIndexMap::default(),
-            alloc_ids: vec![],
+            alloc_ids: fx::FxIndexMap::default(),
             spans: vec![],
             types: vec![],
         },
diff --git a/compiler/rustc_smir/src/rustc_smir/mod.rs b/compiler/rustc_smir/src/rustc_smir/mod.rs
index 144f1625b04..42688613a61 100644
--- a/compiler/rustc_smir/src/rustc_smir/mod.rs
+++ b/compiler/rustc_smir/src/rustc_smir/mod.rs
@@ -196,7 +196,7 @@ impl<S, R: PartialEq> PartialEq<R> for MaybeStable<S, R> {
 pub struct Tables<'tcx> {
     pub tcx: TyCtxt<'tcx>,
     pub def_ids: FxIndexMap<DefId, stable_mir::DefId>,
-    pub alloc_ids: Vec<AllocId>,
+    pub alloc_ids: FxIndexMap<AllocId, stable_mir::AllocId>,
     pub spans: Vec<rustc_span::Span>,
     pub types: Vec<MaybeStable<stable_mir::ty::TyKind, Ty<'tcx>>>,
 }