[turbofan] Improve representation selection for type guard.

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 712 matching lines @@
     MachineRepresentation representation = GetInfo(node)->representation();
     return IsAnyTagged(representation);
   }
 
   bool OneInputCannotBe(Node* node, Type* type) {
     DCHECK_EQ(2, node->op()->ValueInputCount());
     return !GetUpperBound(node->InputAt(0))->Maybe(type) ||
            !GetUpperBound(node->InputAt(1))->Maybe(type);
   }
 
-  void ConvertInput(Node* node, int index, UseInfo use) {
+  // Converts input {index} of {node} according to given UseInfo {use},
+  // assuming the type of the input is {input_type}. If {input_type} is null,
+  // it takes the input from the input node {TypeOf(node->InputAt(index))}.
+  void ConvertInput(Node* node, int index, UseInfo use,
+                    Type* input_type = nullptr) {
     Node* input = node->InputAt(index);
     // In the change phase, insert a change before the use if necessary.
     if (use.representation() == MachineRepresentation::kNone)
       return;  // No input requirement on the use.
     DCHECK_NOT_NULL(input);
     NodeInfo* input_info = GetInfo(input);
     MachineRepresentation input_rep = input_info->representation();
     if (input_rep != use.representation() ||
         use.type_check() != TypeCheckKind::kNone) {
       // Output representation doesn't match usage.
       TRACE("  change: #%d:%s(@%d #%d:%s) ", node->id(), node->op()->mnemonic(),
             index, input->id(), input->op()->mnemonic());
       TRACE(" from ");
       PrintOutputInfo(input_info);
       TRACE(" to ");
       PrintUseInfo(use);
       TRACE("\n");
+      if (input_type == nullptr) {
+        input_type = TypeOf(input);
+      }
       Node* n = changer_->GetRepresentationFor(
-          input, input_info->representation(), TypeOf(input), node, use);
+          input, input_info->representation(), input_type, node, use);
       node->ReplaceInput(index, n);
     }
   }
 
   void ProcessInput(Node* node, int index, UseInfo use) {
     switch (phase_) {
       case PROPAGATE:
         EnqueueInput(node, index, use);
         break;
       case RETYPE:
@@ skipping 2047 matching lines @@
                   MachineRepresentation::kFloat64);
         if (lower()) NodeProperties::ChangeOp(node, Float64Op(node));
         return;
       case IrOpcode::kFrameState:
         return VisitFrameState(node);
       case IrOpcode::kStateValues:
         return VisitStateValues(node);
       case IrOpcode::kObjectState:
         return VisitObjectState(node);
       case IrOpcode::kTypeGuard: {
-        // We just get rid of the sigma here. In principle, it should be
-        // possible to refine the truncation and representation based on
-        // the sigma's type.
+        // We just get rid of the sigma here, choosing the best representation
+        // for the sigma's type.
+        Type* type = TypeOf(node);
         MachineRepresentation representation =
-            GetOutputInfoForPhi(node, TypeOf(node->InputAt(0)), truncation);
+            GetOutputInfoForPhi(node, type, truncation);
 
-        // For now, we just handle specially the impossible case.
-        MachineRepresentation output = TypeOf(node)->IsInhabited()
-                                           ? representation
-                                           : MachineRepresentation::kNone;
-
-        VisitUnop(node, UseInfo(representation, truncation), output);
+        // Here we pretend that the input has the sigma's type for the
+        // conversion.
+        UseInfo use(representation, truncation);
+        if (propagate()) {
+          EnqueueInput(node, 0, use);
+        } else if (lower()) {
+          ConvertInput(node, 0, use, type);
+        }
+        ProcessRemainingInputs(node, 1);
+        SetOutput(node, representation);
         if (lower()) DeferReplacement(node, node->InputAt(0));
         return;
       }
 
       case IrOpcode::kFinishRegion:
         VisitInputs(node);
         // Assume the output is tagged pointer.
         return SetOutput(node, MachineRepresentation::kTaggedPointer);
 
       case IrOpcode::kReturn:
@@ skipping 878 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