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

src/compiler/simplified-lowering.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/simplified-lowering.h"
 
 #include <limits>
 
 #include "src/address-map.h"
 #include "src/base/bits.h"
@@ skipping 1853 matching lines @@
       }
 
       case IrOpcode::kAllocate: {
         ProcessInput(node, 0, UseInfo::TruncatingWord32());
         ProcessRemainingInputs(node, 1);
         SetOutput(node, MachineRepresentation::kTagged);
         return;
       }
       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: {
         FieldAccess access = FieldAccessOf(node->op());
         WriteBarrierKind write_barrier_kind = WriteBarrierKindFor(
             access.base_is_tagged, access.machine_type.representation(),
             access.offset, access.type, node->InputAt(1));
         ProcessInput(node, 0, UseInfoForBasePointer(access));
         ProcessInput(node, 1, TruncatingUseInfoFromRepresentation(
                                   access.machine_type.representation()));
@@ skipping 1376 matching lines @@
         isolate(), graph()->zone(), callable.descriptor(), 0, flags,
         Operator::kNoProperties);
     to_number_operator_.set(common()->Call(desc));
   }
   return to_number_operator_.get();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8