[maglev] Support Switch without fallthrough

Bug: v8:7700
Change-Id: I7dfc1e87b57455cc5b4622a67ac7c27df216c195
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3823126Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82336}

src/maglev/maglev-graph-printer.cc

@@ -292,8 +292,10 @@ void MaglevPrintingVisitor::PreProcessGraph(
         const BasicBlockRef& target = node->Cast<Switch>()->targets()[i];
         AddTargetIfNotNext(targets_, target.block_ptr(), *(block_it + 1));
       }
-      BasicBlock* fallthrough_target = node->Cast<Switch>()->fallthrough();
-      AddTargetIfNotNext(targets_, fallthrough_target, *(block_it + 1));
+      if (node->Cast<Switch>()->has_fallthrough()) {
+        BasicBlock* fallthrough_target = node->Cast<Switch>()->fallthrough();
+        AddTargetIfNotNext(targets_, fallthrough_target, *(block_it + 1));
+      }
     }
   }
   DCHECK(std::all_of(targets_.begin(), targets_.end(),
@@ -534,11 +536,13 @@ void MaglevPrintingVisitor::Process(ControlNode* control_node,
                               &arrows_starting_here);
     }
 
-    BasicBlock* fallthrough_target =
-        control_node->Cast<Switch>()->fallthrough();
-    has_fallthrough |=
-        !AddTargetIfNotNext(targets_, fallthrough_target, state.next_block(),
-                            &arrows_starting_here);
+    if (control_node->Cast<Switch>()->has_fallthrough()) {
+      BasicBlock* fallthrough_target =
+          control_node->Cast<Switch>()->fallthrough();
+      has_fallthrough |=
+          !AddTargetIfNotNext(targets_, fallthrough_target, state.next_block(),
+                              &arrows_starting_here);
+    }
 
     PrintVerticalArrows(os_, targets_, arrows_starting_here);
     PrintPaddedId(os_, graph_labeller, max_node_id_, control_node, "─");

src/maglev/maglev-ir.cc

@@ -3060,7 +3060,11 @@ void Switch::GenerateCode(MaglevCodeGenState* code_gen_state,
   }
   __ Switch(kScratchRegister, ToRegister(value()), value_base(), labels.get(),
             size());
-  DCHECK_EQ(fallthrough(), state.next_block());
+  if (has_fallthrough()) {
+    DCHECK_EQ(fallthrough(), state.next_block());
+  } else {
+    __ Trap();
+  }
 }
 
 void Jump::AllocateVreg(MaglevVregAllocationState* vreg_state) {}

src/maglev/maglev-ir.h

@@ -3655,6 +3655,14 @@ class Deopt : public ControlNode {
 
 class Switch : public ConditionalControlNode {
  public:
+  explicit Switch(uint64_t bitfield, int value_base, BasicBlockRef* targets,
+                  int size)
+      : ConditionalControlNode(bitfield),
+        value_base_(value_base),
+        targets_(targets),
+        size_(size),
+        fallthrough_() {}
+
   explicit Switch(uint64_t bitfield, int value_base, BasicBlockRef* targets,
                   int size, BasicBlockRef* fallthrough)
       : ConditionalControlNode(bitfield),
@@ -3667,7 +3675,11 @@ class Switch : public ConditionalControlNode {
   const BasicBlockRef* targets() const { return targets_; }
   int size() const { return size_; }
 
-  BasicBlock* fallthrough() const { return fallthrough_.block_ptr(); }
+  bool has_fallthrough() const { return fallthrough_.has_value(); }
+  BasicBlock* fallthrough() const {
+    DCHECK(has_fallthrough());
+    return fallthrough_.value().block_ptr();
+  }
 
   Input& value() { return input(0); }
 
@@ -3679,7 +3691,7 @@ class Switch : public ConditionalControlNode {
   const int value_base_;
   const BasicBlockRef* targets_;
   const int size_;
-  BasicBlockRef fallthrough_;
+  base::Optional<BasicBlockRef> fallthrough_;
 };
 
 class BranchIfRootConstant