Improve SC comments

Author: cbeuw

src/concurrency/weak_memory.rs

@@ -320,31 +320,35 @@ impl<'mir, 'tcx: 'mir> StoreBuffer {
                 keep_searching = if store_elem.timestamp <= clocks.clock[store_elem.store_index] {
                     // CoWR: if a store happens-before the current load,
                     // then we can't read-from anything earlier in modification order.
+                    // C++20 §6.9.2.2 [intro.races] paragraph 18
                     log::info!("Stopping due to coherent write-read");
                     false
                 } else if store_elem.loads.borrow().iter().any(|(&load_index, &load_timestamp)| {
                     load_timestamp <= clocks.clock[load_index]
                 }) {
                     // CoRR: if there was a load from this store which happened-before the current load,
                     // then we cannot read-from anything earlier in modification order.
+                    // C++20 §6.9.2.2 [intro.races] paragraph 16
                     log::info!("Stopping due to coherent read-read");
                     false
                 } else if store_elem.timestamp <= clocks.fence_seqcst[store_elem.store_index] {
-                    // The current load, which may be sequenced-after an SC fence, can only read-from
-                    // the last store sequenced-before an SC fence in another thread (or any stores
-                    // later than that SC fence)
+                    // The current load, which may be sequenced-after an SC fence, cannot read-before
+                    // the last store sequenced-before an SC fence in another thread.
+                    // C++17 §32.4 [atomics.order] paragraph 6
                     log::info!("Stopping due to coherent load sequenced after sc fence");
                     false
                 } else if store_elem.timestamp <= clocks.write_seqcst[store_elem.store_index]
                     && store_elem.is_seqcst
                 {
-                    // The current non-SC load can only read-from the latest SC store (or any stores later than that
-                    // SC store)
+                    // The current non-SC load, which may be sequenced-after an SC fence,
+                    // cannot read-before the last SC store executed before the fence.
+                    // C++17 §32.4 [atomics.order] paragraph 4
                     log::info!("Stopping due to needing to load from the last SC store");
                     false
                 } else if is_seqcst && store_elem.timestamp <= clocks.read_seqcst[store_elem.store_index] {
-                    // The current SC load can only read-from the last store sequenced-before
-                    // the last SC fence (or any stores later than the SC fence)
+                    // The current SC load cannot read-before the last store sequenced-before
+                    // the last SC fence.
+                    // C++17 §32.4 [atomics.order] paragraph 5
                     log::info!("Stopping due to sc load needing to load from the last SC store before an SC fence");
                     false
                 } else {true};