re-do

src/passes/Heap2Local.cpp

@@ -170,8 +170,16 @@ namespace {
 // Core analysis that provides an escapes() method to check if an allocation
 // escapes in a way that prevents optimizing it away as described above. It also
 // stashes information about the relevant expressions as it goes, which helps
-// optimization later (|reached|).
+// optimization later (|seen| and |reached|).
 struct EscapeAnalyzer {
+  // All the expressions that have already been seen by the optimizer, see the
+  // comment above on exclusivity: once we have seen something when analyzing
+  // one allocation, if we reach it again then we can exit early since seeing it
+  // a second time proves we lost exclusivity. We must track this across
+  // multiple instances of EscapeAnalyzer as each handles a particular
+  // allocation.
+  std::unordered_set<Expression*>& seen;
+
   // To find what escapes, we need to follow where values flow, both up to
   // parents, and via branches, and through locals.
   // TODO: for efficiency, only scan reference types in LocalGraph
@@ -182,12 +190,13 @@ struct EscapeAnalyzer {
   const PassOptions& passOptions;
   Module& wasm;
 
-  EscapeAnalyzer(const LocalGraph& localGraph,
+  EscapeAnalyzer(std::unordered_set<Expression*>& seen,
+                 const LocalGraph& localGraph,
                  const Parents& parents,
                  const BranchUtils::BranchTargets& branchTargets,
                  const PassOptions& passOptions,
                  Module& wasm)
-    : localGraph(localGraph), parents(parents),
+    : seen(seen), localGraph(localGraph), parents(parents),
       branchTargets(branchTargets), passOptions(passOptions), wasm(wasm) {}
 
   // We must track all the local.sets that write the allocation, to verify
@@ -252,6 +261,28 @@ struct EscapeAnalyzer {
       assert(interaction == ParentChildInteraction::FullyConsumes ||
              interaction == ParentChildInteraction::Flows);
 
+      // If we've already seen an expression, stop since we cannot optimize
+      // things that overlap in any way (see the notes on exclusivity, above).
+      // Note that we use a nonrepeating queue here, so we already do not visit
+      // the same thing more than once; what this check does is verify we don't
+      // look at something that another allocation reached, which would be in a
+      // different call to this function and use a different queue (any overlap
+      // between calls would prove non-exclusivity).
+      //
+      // It is ok, however, to see a parent more than once, if the allocation
+      // flows to it from two children, as is the case for ref.eq. Likewise, for
+      // struct.set, where we also have different considerations for the two
+      // children (the reference does not escape, but the value does), and in
+      // that case there may be different allocations for the children.
+      if (interaction == ParentChildInteraction::Flows) {
+        auto seenBefore = !seen.emplace(parent).second;
+        auto reachedInThisAllocation = reached.count(parent) > 0;
+        if (seenBefore && !reachedInThisAllocation) {
+        // XXX struct.set can be from different allocations!
+          return true;
+        }
+      }
+
       // We can proceed, as the parent interacts with us properly, and we are
       // the only allocation to get here.
 
@@ -1031,6 +1062,10 @@ struct Heap2Local {
     // flow to.
     localGraph.computeSetInfluences();
 
+    // All the expressions we have already looked at. We use this to avoid
+    // repeated work, see above.
+    std::unordered_set<Expression*> seen;
+
     // Find all the relevant allocations in the function: StructNew, ArrayNew,
     // ArrayNewFixed.
     struct AllocationFinder : public PostWalker<AllocationFinder> {
@@ -1090,7 +1125,7 @@ struct Heap2Local {
       }
 
       EscapeAnalyzer analyzer(
-        localGraph, parents, branchTargets, passOptions, wasm);
+        seen, localGraph, parents, branchTargets, passOptions, wasm);
       if (!analyzer.escapes(allocation)) {
         // Convert the allocation and all its uses into a struct. Then convert
         // the struct into locals.
@@ -1110,7 +1145,7 @@ struct Heap2Local {
       // Check for escaping, noting relevant information as we go. If this does
       // not escape, optimize it into locals.
       EscapeAnalyzer analyzer(
-        localGraph, parents, branchTargets, passOptions, wasm);
+        seen, localGraph, parents, branchTargets, passOptions, wasm);
       if (!analyzer.escapes(allocation)) {
         Struct2Local(allocation, analyzer, func, wasm);
       }