Remove unnecessary fence in GTO output (#7310)

GlobalTypeOptimization removes struct fields that have no reads, then
optimizes out any sets of those fields. We were previously being overly
conservative and replacing optimized seqcst sets with fences, but in
fact those sets cannot possibly synchronize with anything because there
are no reads. Stop emitting the unnecessary fences.

Author: tlively

src/passes/GlobalTypeOptimization.cpp

@@ -532,15 +532,9 @@ struct GlobalTypeOptimization : public Pass {
           needEHFixups = true;
           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());
-          }
+          // We only remove fields with no reads, so if this set is atomic,
+          // there are no reads it can possibly synchronize with and we do not
+          // need a fence.
           replaceCurrent(replacement);
         }
       }

test/lit/passes/gto-removals.wast

@@ -1593,18 +1593,15 @@
   ;; CHECK-NEXT:    )
   ;; CHECK-NEXT:   )
   ;; CHECK-NEXT:  )
-  ;; CHECK-NEXT:  (block
-  ;; CHECK-NEXT:   (drop
-  ;; CHECK-NEXT:    (ref.as_non_null
-  ;; CHECK-NEXT:     (block (result (ref $A))
-  ;; CHECK-NEXT:      (drop
-  ;; CHECK-NEXT:       (i32.const 1)
-  ;; CHECK-NEXT:      )
-  ;; CHECK-NEXT:      (local.get $0)
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.as_non_null
+  ;; CHECK-NEXT:    (block (result (ref $A))
+  ;; CHECK-NEXT:     (drop
+  ;; CHECK-NEXT:      (i32.const 1)
   ;; CHECK-NEXT:     )
+  ;; CHECK-NEXT:     (local.get $0)
   ;; CHECK-NEXT:    )
   ;; CHECK-NEXT:   )
-  ;; CHECK-NEXT:   (atomic.fence)
   ;; CHECK-NEXT:  )
   ;; CHECK-NEXT: )
   (func $sets (param (ref $A))
@@ -1619,8 +1616,7 @@
       (local.get 0)
       (i32.const 1)
     )
-    ;; This requires a fence to keep the effect on the global order of seqcst
-    ;; operations.
+    ;; Same with a seqcst set.
     (struct.atomic.set $A 0
       (local.get 0)
       (i32.const 1)