[NFC + bugfix] Remove BreakTargetLocation from GUFA (#6956)

Before, a br would send its value to a BreakTargetLocation. That would then be
linked to the target:

{ br's value } => BreakTargetLocation(target name) => { location of target }

This PR skips the middle:

{ br's value } => { location of target }

It just connects breaks directly to the targets. We can do that if we keep a
map of the targets as we go.

This is 2% faster as well as simplifies the code, as an NFC refactoring. But it
also fixes a bug: we have handling on ExpressionLocation that filters values as they
come in (they must accord with the expression's type). We were not doing
that on BreakTargetLocation, leading to an assert. Removing
BreakTargetLocation entirely is easier and better than adding filtering logic
for it.

Fixes #6955

src/ir/possible-contents.cpp

@@ -503,12 +503,30 @@ struct CollectedFuncInfo {
   std::unordered_map<Expression*, Expression*> childParents;
 };
 
+// Does a walk while maintaining a map of names of branch targets to those
+// expressions, so they can be found by their name.
+// TODO: can this replace ControlFlowWalker in other places?
+template<typename SubType, typename VisitorType = Visitor<SubType>>
+struct BreakTargetWalker : public PostWalker<SubType, VisitorType> {
+  std::unordered_map<Name, Expression*> breakTargets;
+
+  Expression* findBreakTarget(Name name) { return breakTargets[name]; }
+
+  static void scan(SubType* self, Expression** currp) {
+    auto* curr = *currp;
+    BranchUtils::operateOnScopeNameDefs(
+      curr, [&](Name name) { self->breakTargets[name] = curr; });
+
+    PostWalker<SubType, VisitorType>::scan(self, currp);
+  }
+};
+
 // Walk the wasm and find all the links we need to care about, and the locations
 // and roots related to them. This builds up a CollectedFuncInfo data structure.
 // After all InfoCollectors run, those data structures will be merged and the
 // main flow will begin.
 struct InfoCollector
-  : public PostWalker<InfoCollector, OverriddenVisitor<InfoCollector>> {
+  : public BreakTargetWalker<InfoCollector, OverriddenVisitor<InfoCollector>> {
   CollectedFuncInfo& info;
 
   InfoCollector(CollectedFuncInfo& info) : info(info) {}
@@ -553,9 +571,6 @@ struct InfoCollector
       return;
     }
 
-    // Values sent to breaks to this block must be received here.
-    handleBreakTarget(curr);
-
     // The final item in the block can flow a value to here as well.
     receiveChildValue(curr->list.back(), curr);
   }
@@ -1151,8 +1166,7 @@ struct InfoCollector
         for (Index i = 0; i < params.size(); i++) {
           if (isRelevant(params[i])) {
             info.links.push_back(
-              {TagLocation{tag, i},
-               BreakTargetLocation{getFunction(), target, i}});
+              {TagLocation{tag, i}, getBreakTargetLocation(target, i)});
           }
         }
 
@@ -1164,7 +1178,7 @@ struct InfoCollector
         addRoot(location,
                 PossibleContents::fromType(Type(HeapType::exn, NonNullable)));
         info.links.push_back(
-          {location, BreakTargetLocation{getFunction(), target, exnrefIndex}});
+          {location, getBreakTargetLocation(target, exnrefIndex)});
       }
     }
   }
@@ -1279,6 +1293,13 @@ struct InfoCollector
 
   // Helpers
 
+  // Returns the location of a break target by the name (e.g. returns the
+  // location of a block, if the name is the name of a block). Also receives the
+  // index in a tuple, if this is part of a tuple value.
+  Location getBreakTargetLocation(Name target, Index i) {
+    return ExpressionLocation{findBreakTarget(target), i};
+  }
+
   // Handles the value sent in a break instruction. Does not handle anything
   // else like the condition etc.
   void handleBreakValue(Expression* curr) {
@@ -1288,26 +1309,13 @@ struct InfoCollector
           for (Index i = 0; i < value->type.size(); i++) {
             // Breaks send the contents of the break value to the branch target
             // that the break goes to.
-            info.links.push_back(
-              {ExpressionLocation{value, i},
-               BreakTargetLocation{getFunction(), target, i}});
+            info.links.push_back({ExpressionLocation{value, i},
+                                  getBreakTargetLocation(target, i)});
           }
         }
       });
   }
 
-  // Handles receiving values from breaks at the target (as in a block).
-  void handleBreakTarget(Expression* curr) {
-    if (isRelevant(curr->type)) {
-      BranchUtils::operateOnScopeNameDefs(curr, [&](Name target) {
-        for (Index i = 0; i < curr->type.size(); i++) {
-          info.links.push_back({BreakTargetLocation{getFunction(), target, i},
-                                ExpressionLocation{curr, i}});
-        }
-      });
-    }
-  }
-
   // Connect a child's value to the parent, that is, all content in the child is
   // now considered possible in the parent as well.
   void receiveChildValue(Expression* child, Expression* parent) {
@@ -2400,16 +2408,16 @@ bool Flower::updateContents(LocationIndex locationIndex,
     }
   }
 
-  // After filtering we should always have more precise information than "many"
-  // - in the worst case, we can have the type declared in the wasm.
-  assert(!contents.isMany());
-
 #if defined(POSSIBLE_CONTENTS_DEBUG) && POSSIBLE_CONTENTS_DEBUG >= 2
   std::cout << "  updateContents has something new\n";
   contents.dump(std::cout, &wasm);
   std::cout << '\n';
 #endif
 
+  // After filtering we should always have more precise information than "many"
+  // - in the worst case, we can have the type declared in the wasm.
+  assert(!contents.isMany());
+
   // Add a work item if there isn't already.
   workQueue.insert(locationIndex);
 
@@ -2896,9 +2904,6 @@ void Flower::dump(Location location) {
               << '\n';
   } else if (auto* loc = std::get_if<GlobalLocation>(&location)) {
     std::cout << "  globalloc " << loc->name << '\n';
-  } else if (auto* loc = std::get_if<BreakTargetLocation>(&location)) {
-    std::cout << "  branchloc " << loc->func->name << " : " << loc->target
-              << " tupleIndex " << loc->tupleIndex << '\n';
   } else if (std::get_if<SignatureParamLocation>(&location)) {
     std::cout << "  sigparamloc " << '\n';
   } else if (std::get_if<SignatureResultLocation>(&location)) {

src/ir/possible-contents.h

@@ -402,21 +402,6 @@ struct ResultLocation {
   }
 };
 
-// The location of a break target in a function, identified by its name.
-struct BreakTargetLocation {
-  Function* func;
-  Name target;
-  // As in ExpressionLocation, the index inside the tuple, or 0 if not a tuple.
-  // That is, if the branch target has a tuple type, then each branch to that
-  // location sends a tuple, and we'll have a separate BreakTargetLocation for
-  // each, indexed by the index in the tuple that the branch sends.
-  Index tupleIndex;
-  bool operator==(const BreakTargetLocation& other) const {
-    return func == other.func && target == other.target &&
-           tupleIndex == other.tupleIndex;
-  }
-};
-
 // The location of a global in the module.
 struct GlobalLocation {
   Name name;
@@ -523,7 +508,6 @@ using Location = std::variant<ExpressionLocation,
                               ParamLocation,
                               LocalLocation,
                               ResultLocation,
-                              BreakTargetLocation,
                               GlobalLocation,
                               SignatureParamLocation,
                               SignatureResultLocation,
@@ -577,13 +561,6 @@ template<> struct hash<wasm::ResultLocation> {
   }
 };
 
-template<> struct hash<wasm::BreakTargetLocation> {
-  size_t operator()(const wasm::BreakTargetLocation& loc) const {
-    return std::hash<std::tuple<size_t, wasm::Name, wasm::Index>>{}(
-      {size_t(loc.func), loc.target, loc.tupleIndex});
-  }
-};
-
 template<> struct hash<wasm::GlobalLocation> {
   size_t operator()(const wasm::GlobalLocation& loc) const {
     return std::hash<wasm::Name>{}(loc.name);