Auto merge of rust-lang#3135 - RalfJung:nonatomic-clock, r=RalfJung

avoid AtomicU64 when a Cell is enough

src/tools/miri/src/clock.rs

@@ -1,4 +1,4 @@
-use std::sync::atomic::{AtomicU64, Ordering};
+use std::cell::Cell;
 use std::time::{Duration, Instant as StdInstant};
 
 /// When using a virtual clock, this defines how many nanoseconds we pretend are passing for each
@@ -59,7 +59,7 @@ enum ClockKind {
     },
     Virtual {
         /// The "current virtual time".
-        nanoseconds: AtomicU64,
+        nanoseconds: Cell<u64>,
     },
 }
 
@@ -82,7 +82,7 @@ impl Clock {
                 // Time will pass without us doing anything.
             }
             ClockKind::Virtual { nanoseconds } => {
-                nanoseconds.fetch_add(NANOSECONDS_PER_BASIC_BLOCK, Ordering::SeqCst);
+                nanoseconds.update(|x| x + NANOSECONDS_PER_BASIC_BLOCK);
             }
         }
     }
@@ -93,7 +93,8 @@ impl Clock {
             ClockKind::Host { .. } => std::thread::sleep(duration),
             ClockKind::Virtual { nanoseconds } => {
                 // Just pretend that we have slept for some time.
-                nanoseconds.fetch_add(duration.as_nanos().try_into().unwrap(), Ordering::SeqCst);
+                let nanos: u64 = duration.as_nanos().try_into().unwrap();
+                nanoseconds.update(|x| x + nanos);
             }
         }
     }
@@ -110,9 +111,7 @@ impl Clock {
         match &self.kind {
             ClockKind::Host { .. } => Instant { kind: InstantKind::Host(StdInstant::now()) },
             ClockKind::Virtual { nanoseconds } =>
-                Instant {
-                    kind: InstantKind::Virtual { nanoseconds: nanoseconds.load(Ordering::SeqCst) },
-                },
+                Instant { kind: InstantKind::Virtual { nanoseconds: nanoseconds.get() } },
         }
     }
 }

src/tools/miri/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(rustc_private)]
+#![feature(cell_update)]
 #![feature(float_gamma)]
 #![feature(map_try_insert)]
 #![feature(never_type)]