reuse_pool: only do acquire_clock if we reused from a different thread

Author: RalfJung

src/tools/miri/src/alloc_addresses/mod.rs

@@ -171,7 +171,9 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                     memory_kind,
                     ecx.get_active_thread(),
                 ) {
-                    if let Some(data_race) = &ecx.machine.data_race {
+                    if let Some(clock) = clock
+                        && let Some(data_race) = &ecx.machine.data_race
+                    {
                         data_race.acquire_clock(&clock, ecx.get_active_thread());
                     }
                     reuse_addr

src/tools/miri/src/alloc_addresses/reuse_pool.rs

@@ -78,14 +78,15 @@ impl ReusePool {
         subpool.insert(pos, (addr, size, thread, clock));
     }
 
+    /// Returns the address to use and optionally a clock we have to synchronize with.
     pub fn take_addr(
         &mut self,
         rng: &mut impl Rng,
         size: Size,
         align: Align,
         kind: MemoryKind,
         thread: ThreadId,
-    ) -> Option<(u64, VClock)> {
+    ) -> Option<(u64, Option<VClock>)> {
         // Determine whether we'll even attempt a reuse. As above, we don't do reuse for stack addresses.
         if kind == MemoryKind::Stack || !rng.gen_bool(self.address_reuse_rate) {
             return None;
@@ -122,6 +123,7 @@ impl ReusePool {
         let (chosen_addr, chosen_size, chosen_thread, clock) = subpool.remove(idx);
         debug_assert!(chosen_size >= size && chosen_addr % align.bytes() == 0);
         debug_assert!(cross_thread_reuse || chosen_thread == thread);
-        Some((chosen_addr, clock))
+        // No synchronization needed if we reused from the current thread.
+        Some((chosen_addr, if chosen_thread == thread { None } else { Some(clock) }))
     }
 }

src/tools/miri/src/concurrency/vector_clock.rs

@@ -152,7 +152,7 @@ impl VClock {
     }
 
     /// Get a mutable slice to the internal vector with minimum `min_len`
-    /// elements, to preserve invariants this vector must modify
+    /// elements. To preserve invariants, the caller must modify
     /// the `min_len`-1 nth element to a non-zero value
     #[inline]
     fn get_mut_with_min_len(&mut self, min_len: usize) -> &mut [VTimestamp] {