[compiler] Better packing of fields for RepresentationSelector::NodeInfo

Move a field and pack enums in Truncation to save 2 word for
each NodeInfo.

BUG=v8:9684

Change-Id: Ib470019b13a1cb8586c1bc585ff1aff6a88267ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2892664
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74597}

src/compiler/representation-change.h

@@ -16,7 +16,11 @@ namespace compiler {
 // Foward declarations.
 class TypeCache;
 
-enum IdentifyZeros { kIdentifyZeros, kDistinguishZeros };
+enum IdentifyZeros : uint8_t { kIdentifyZeros, kDistinguishZeros };
+constexpr int kIdentifyZerosBits = 1;
+static_assert(static_cast<int>(IdentifyZeros::kDistinguishZeros) <
+                  1 << kIdentifyZerosBits,
+              "kIdentifyZeros must fit into kIdentifyZerosBits");
 
 class Truncation final {
  public:
@@ -94,6 +98,10 @@ class Truncation final {
     kOddballAndBigIntToNumber,
     kAny
   };
+  static constexpr int kTruncationKindBits = 4;
+  static_assert(static_cast<int>(TruncationKind::kAny) <
+                    1 << kTruncationKindBits,
+                "TransactionKind must fit into kTruncationKindBits");
 
   explicit Truncation(TruncationKind kind, IdentifyZeros identify_zeros)
       : kind_(kind), identify_zeros_(identify_zeros) {
@@ -103,8 +111,8 @@ class Truncation final {
   }
   TruncationKind kind() const { return kind_; }
 
-  TruncationKind kind_;
-  IdentifyZeros identify_zeros_;
+  TruncationKind kind_ : kTruncationKindBits;
+  IdentifyZeros identify_zeros_ : kIdentifyZerosBits;
 
   static TruncationKind Generalize(TruncationKind rep1, TruncationKind rep2);
   static IdentifyZeros GeneralizeIdentifyZeros(IdentifyZeros i1,

src/compiler/simplified-lowering.cc

@@ -294,15 +294,17 @@ class RepresentationSelector {
     Type restriction_type() const { return restriction_type_; }
 
    private:
+    // Fields are ordered to avoid mixing byte and word size fields to minimize
+    // padding.
     enum State : uint8_t { kUnvisited, kPushed, kVisited, kQueued };
     State state_ = kUnvisited;
     MachineRepresentation representation_ =
         MachineRepresentation::kNone;             // Output representation.
     Truncation truncation_ = Truncation::None();  // Information about uses.
+    bool weakened_ = false;
 
     Type restriction_type_ = Type::Any();
     Type feedback_type_;
-    bool weakened_ = false;
   };
 
   RepresentationSelector(JSGraph* jsgraph, JSHeapBroker* broker, Zone* zone,