[dart2js] Do some load elimination in simplifier

Doing load elimination of final fields earlier generates more
opportunities for GVN.  This helps when the same value is 'boxed' in a
wrapper object and the 'unboxed' multiple times.

Change-Id: Id3a2b392a7563bae7a4cd1f8b2763c5ac1b377b5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432502
Commit-Queue: Stephen Adams <[email protected]>
Reviewed-by: Mayank Patke <[email protected]>
Reviewed-by: Nate Biggs <[email protected]>

Author: rakudrama

pkg/compiler/lib/src/ssa/optimize.dart

@@ -1821,9 +1821,47 @@ class SsaInstructionSimplifier extends HBaseVisitor<HInstruction>
       }
     }
 
+    // HFieldGet of a final field from an allocator can be replaced with the
+    // field's value.
+    //
+    // Load Elimination is more powerful as it can handle non-final fields, but
+    // doing this earlier in the simplifier generates more opportunities for the
+    // first GVN pass.
+    if (receiver is HCreate && !node.isAssignable) {
+      int index = _indexOfFieldInAllocatorInputs(
+        receiver.element,
+        node.element,
+      );
+      if (index >= 0) return receiver.inputs[index];
+    }
+
     return node;
   }
 
+  int _indexOfFieldInAllocatorInputs(ClassEntity cls, FieldEntity field) {
+    final classCache = _indexOfFieldInAllocatorCache ??= {};
+    final fieldCache = classCache[cls] ??= {};
+    final index = fieldCache[field];
+    if (index != null) return index;
+
+    int argumentIndex = 0;
+    _closedWorld.elementEnvironment.forEachInstanceField(cls, (
+      _,
+      FieldEntity member,
+    ) {
+      FieldAnalysisData fieldData = _closedWorld.fieldAnalysis.getFieldData(
+        member,
+      );
+      int index = (fieldData.isElided || fieldData.isInitializedInAllocator)
+          ? -1
+          : argumentIndex++;
+      fieldCache[member] = index;
+    });
+    return fieldCache[field]!;
+  }
+
+  Map<ClassEntity, Map<FieldEntity, int>>? _indexOfFieldInAllocatorCache;
+
   @override
   HInstruction visitGetLength(HGetLength node) {
     HInstruction receiver = node.receiver;