[turbofan] Handle word32 truncation in word32->tagged representation change.

Similarly to the word32->float64 case, we interpret word32 as uint32 if
the value is word32 truncated. This is fine because the users declared
they only care about mod 2^32 of the value (that's what word32
truncation means).

This CL also removes the ad-hoc handling of this situation
(https://codereview.chromium.org/2311903002).

BUG=chromium:644048

Review-Url: https://codereview.chromium.org/2312003005
Cr-Commit-Position: refs/heads/master@{#39224}

src/compiler/representation-change.cc

@@ -149,7 +149,8 @@ Node* RepresentationChanger::GetRepresentationFor(
       return GetTaggedPointerRepresentationFor(node, output_rep, output_type);
     case MachineRepresentation::kTagged:
       DCHECK(use_info.type_check() == TypeCheckKind::kNone);
-      return GetTaggedRepresentationFor(node, output_rep, output_type);
+      return GetTaggedRepresentationFor(node, output_rep, output_type,
+                                        use_info.truncation());
     case MachineRepresentation::kFloat32:
       DCHECK(use_info.type_check() == TypeCheckKind::kNone);
       return GetFloat32RepresentationFor(node, output_rep, output_type,
@@ -262,7 +263,8 @@ Node* RepresentationChanger::GetTaggedPointerRepresentationFor(
 }
 
 Node* RepresentationChanger::GetTaggedRepresentationFor(
-    Node* node, MachineRepresentation output_rep, Type* output_type) {
+    Node* node, MachineRepresentation output_rep, Type* output_type,
+    Truncation truncation) {
   // Eagerly fold representation changes for constants.
   switch (node->opcode()) {
     case IrOpcode::kNumberConstant:
@@ -312,7 +314,10 @@ Node* RepresentationChanger::GetTaggedRepresentationFor(
       op = simplified()->ChangeInt31ToTaggedSigned();
     } else if (output_type->Is(Type::Signed32())) {
       op = simplified()->ChangeInt32ToTagged();
-    } else if (output_type->Is(Type::Unsigned32())) {
+    } else if (output_type->Is(Type::Unsigned32()) ||
+               truncation.IsUsedAsWord32()) {
+      // Either the output is uint32 or the uses only care about the
+      // low 32 bits (so we can pick uint32 safely).
       op = simplified()->ChangeUint32ToTagged();
     } else {
       return TypeError(node, output_rep, output_type,

src/compiler/representation-change.h

@@ -249,7 +249,7 @@ class RepresentationChanger final {
                                           MachineRepresentation output_rep,
                                           Type* output_type);
   Node* GetTaggedRepresentationFor(Node* node, MachineRepresentation output_rep,
-                                   Type* output_type);
+                                   Type* output_type, Truncation truncation);
   Node* GetFloat32RepresentationFor(Node* node,
                                     MachineRepresentation output_rep,
                                     Type* output_type, Truncation truncation);

src/compiler/simplified-lowering.cc

@@ -929,9 +929,6 @@ class RepresentationSelector {
     }
     // Convert inputs to the output representation of this phi, pass the
     // truncation truncation along.
-    if (output == MachineRepresentation::kTagged) {
-      truncation = Truncation::Any();
-    }
     UseInfo input_use(output, truncation);
     ProcessInput(node, 1, input_use);
     ProcessInput(node, 2, input_use);
@@ -956,9 +953,6 @@ class RepresentationSelector {
 
     // Convert inputs to the output representation of this phi, pass the
     // truncation along.
-    if (output == MachineRepresentation::kTagged) {
-      truncation = Truncation::Any();
-    }
     UseInfo input_use(output, truncation);
     for (int i = 0; i < node->InputCount(); i++) {
       ProcessInput(node, i, i < values ? input_use : UseInfo::None());
@@ -2446,10 +2440,6 @@ class RepresentationSelector {
         // the sigma's type.
         MachineRepresentation output =
             GetOutputInfoForPhi(node, TypeOf(node->InputAt(0)), truncation);
-
-        if (output == MachineRepresentation::kTagged) {
-          truncation = Truncation::Any();
-        }
         VisitUnop(node, UseInfo(output, truncation), output);
         if (lower()) DeferReplacement(node, node->InputAt(0));
         return;