Handle atomics in GTO (#7160)

GTO removes fields that are never read and also removes sets to those
fields. Update the pass to add a seqcst fence when removing a seqcst set
to preserve its effect on the global order of seqcst operations.

1 files changed, 17 insertions, 7 deletions

diff --git a/src/passes/GlobalTypeOptimization.cpp b/src/passes/GlobalTypeOptimization.cpp
index 0baecdf43..b2ba23b0f 100644
--- a/src/passes/GlobalTypeOptimization.cpp
+++ b/src/passes/GlobalTypeOptimization.cpp
@@ -514,13 +514,23 @@ struct GlobalTypeOptimization : public Pass {
           // operations here: the trap on a null ref happens after the value,
           // which might have side effects.
           Builder builder(*getModule());
-          auto flipped = getResultOfFirst(curr->ref,
-                                          builder.makeDrop(curr->value),
-                                          getFunction(),
-                                          getModule(),
-                                          getPassOptions());
-          replaceCurrent(
-            builder.makeDrop(builder.makeRefAs(RefAsNonNull, flipped)));
+          auto* flipped = getResultOfFirst(curr->ref,
+                                           builder.makeDrop(curr->value),
+                                           getFunction(),
+                                           getModule(),
+                                           getPassOptions());
+          Expression* replacement =
+            builder.makeDrop(builder.makeRefAs(RefAsNonNull, flipped));
+          if (curr->order == MemoryOrder::SeqCst) {
+            // If the removed set is sequentially consistent, we must insert a
+            // seqcst fence to preserve the effect on the global order of seqcst
+            // operations. No fence is necessary for release sets because there
+            // are no reads for them to synchronize with given that we are
+            // removing the field.
+            replacement =
+              builder.makeSequence(replacement, builder.makeAtomicFence());
+          }
+          replaceCurrent(replacement);
         }
       }