From 963e15784db9122ae6c1907f8ccac5b38d7ab82e Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Sat, 6 Jul 2024 20:41:52 -0400
Subject: [PATCH] Remove in-progress allocation decoding states

---
 .../rustc_middle/src/mir/interpret/mod.rs     | 80 +++++--------------
 1 file changed, 20 insertions(+), 60 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
index 4e95e600b5ab9..fccf34986f196 100644
--- a/compiler/rustc_middle/src/mir/interpret/mod.rs
+++ b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -12,9 +12,7 @@ use std::fmt;
 use std::io;
 use std::io::{Read, Write};
 use std::num::NonZero;
-use std::sync::atomic::{AtomicU32, Ordering};
 
-use smallvec::{smallvec, SmallVec};
 use tracing::{debug, trace};
 
 use rustc_ast::LitKind;
@@ -159,14 +157,9 @@ pub fn specialized_encode_alloc_id<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>>(
     }
 }
 
-// Used to avoid infinite recursion when decoding cyclic allocations.
-type DecodingSessionId = NonZero<u32>;
-
 #[derive(Clone)]
 enum State {
     Empty,
-    InProgressNonAlloc(SmallVec<[DecodingSessionId; 1]>),
-    InProgress(SmallVec<[DecodingSessionId; 1]>, AllocId),
     Done(AllocId),
 }
 
@@ -180,13 +173,7 @@ pub struct AllocDecodingState {
 impl AllocDecodingState {
     #[inline]
     pub fn new_decoding_session(&self) -> AllocDecodingSession<'_> {
-        static DECODER_SESSION_ID: AtomicU32 = AtomicU32::new(0);
-        let counter = DECODER_SESSION_ID.fetch_add(1, Ordering::SeqCst);
-
-        // Make sure this is never zero.
-        let session_id = DecodingSessionId::new((counter & 0x7FFFFFFF) + 1).unwrap();
-
-        AllocDecodingSession { state: self, session_id }
+        AllocDecodingSession { state: self }
     }
 
     pub fn new(data_offsets: Vec<u64>) -> Self {
@@ -200,7 +187,6 @@ impl AllocDecodingState {
 #[derive(Copy, Clone)]
 pub struct AllocDecodingSession<'s> {
     state: &'s AllocDecodingState,
-    session_id: DecodingSessionId,
 }
 
 impl<'s> AllocDecodingSession<'s> {
@@ -222,52 +208,28 @@ impl<'s> AllocDecodingSession<'s> {
 
         // Check the decoding state to see if it's already decoded or if we should
         // decode it here.
-        let alloc_id = {
-            let mut entry = self.state.decoding_state[idx].lock();
-
-            match *entry {
-                State::Done(alloc_id) => {
-                    return alloc_id;
-                }
-                ref mut entry @ State::Empty => {
-                    // We are allowed to decode.
-                    match alloc_kind {
-                        AllocDiscriminant::Alloc => {
-                            // If this is an allocation, we need to reserve an
-                            // `AllocId` so we can decode cyclic graphs.
-                            let alloc_id = decoder.interner().reserve_alloc_id();
-                            *entry = State::InProgress(smallvec![self.session_id], alloc_id);
-                            Some(alloc_id)
-                        }
-                        AllocDiscriminant::Fn
-                        | AllocDiscriminant::Static
-                        | AllocDiscriminant::VTable => {
-                            // Fns and statics cannot be cyclic, and their `AllocId`
-                            // is determined later by interning.
-                            *entry = State::InProgressNonAlloc(smallvec![self.session_id]);
-                            None
-                        }
+        let mut entry = self.state.decoding_state[idx].lock();
+        let alloc_id = match *entry {
+            State::Done(alloc_id) => {
+                return alloc_id;
+            }
+            State::Empty => {
+                // We are allowed to decode.
+                match alloc_kind {
+                    AllocDiscriminant::Alloc => {
+                        // If this is an allocation, we need to reserve an
+                        // `AllocId` so we can decode cyclic graphs.
+                        let alloc_id = decoder.interner().reserve_alloc_id();
+                        Some(alloc_id)
                     }
-                }
-                State::InProgressNonAlloc(ref mut sessions) => {
-                    if sessions.contains(&self.session_id) {
-                        bug!("this should be unreachable");
-                    } else {
-                        // Start decoding concurrently.
-                        sessions.push(self.session_id);
+                    AllocDiscriminant::Fn
+                    | AllocDiscriminant::Static
+                    | AllocDiscriminant::VTable => {
+                        // Fns and statics cannot be cyclic, and their `AllocId`
+                        // is determined later by interning.
                         None
                     }
                 }
-                State::InProgress(ref mut sessions, alloc_id) => {
-                    if sessions.contains(&self.session_id) {
-                        // Don't recurse.
-                        return alloc_id;
-                    } else {
-                        // Start decoding concurrently.
-                        sessions.push(self.session_id);
-                        Some(alloc_id)
-                    }
-                }
             }
         };
 
@@ -317,9 +279,7 @@ impl<'s> AllocDecodingSession<'s> {
             }
         });
 
-        self.state.decoding_state[idx].with_lock(|entry| {
-            *entry = State::Done(alloc_id);
-        });
+        *entry = State::Done(alloc_id);
 
         alloc_id
     }