[turbofan] Lower Smi field loads to Word32 field loads.

If a Smi field load is truncated to Word32, we can just load the upper
32 bits on 64-bit architectures (with 32-bit Smis), which avoids the
nasty shift right that we'd have to perform otherwise to untag it.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2140553002
Cr-Commit-Position: refs/heads/master@{#37630}

src/compiler/simplified-lowering.cc

@@ -1871,9 +1871,31 @@ class RepresentationSelector {
       }
       case IrOpcode::kLoadField: {
         FieldAccess access = FieldAccessOf(node->op());
-        ProcessInput(node, 0, UseInfoForBasePointer(access));
-        ProcessRemainingInputs(node, 1);
-        SetOutput(node, access.machine_type.representation());
+        MachineRepresentation representation =
+            access.machine_type.representation();
+        // If we are loading from a Smi field and truncate the result to Word32,
+        // we can instead just load the high word on 64-bit architectures, which
+        // is exactly the Word32 we are looking for, and therefore avoid a nasty
+        // right shift afterwards.
+        // TODO(bmeurer): Introduce an appropriate tagged-signed machine rep.
+        if (truncation.TruncatesToWord32() &&
+            representation == MachineRepresentation::kTagged &&
+            access.type->Is(Type::TaggedSigned()) && SmiValuesAre32Bits()) {
+          VisitUnop(node, UseInfoForBasePointer(access),
+                    MachineRepresentation::kWord32);
+          if (lower()) {
+            // Morph this Smi load field into an int32 load field.
+            access.machine_type = MachineType::Int32();
+            access.type = type_cache_.kInt32;
+#if V8_TARGET_LITTLE_ENDIAN
+            access.offset += kPointerSize / 2;
+#endif
+            NodeProperties::ChangeOp(node,
+                                     jsgraph_->simplified()->LoadField(access));
+          }
+        } else {
+          VisitUnop(node, UseInfoForBasePointer(access), representation);
+        }
         return;
       }
       case IrOpcode::kStoreField: {