Fix a code-folding fuzz bug (WebAssembly#1282)

* fix a code-folding bug where when merging function-level tails, we moved code out of where it could reach a break target - we must not move code if it has a break target not enclosed in itself. the EffectAnalyzer already had the functionality for that, move the code around a little there to make that clearer too

src/ir/effects.h

@@ -62,6 +62,9 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer> {
   bool hasSideEffects() { return hasGlobalSideEffects() || localsWritten.size() > 0 || branches || implicitTrap; }
   bool hasAnything() { return branches || calls || accessesLocal() || readsMemory || writesMemory || accessesGlobal() || implicitTrap || isAtomic; }
 
+  // check if we break to anything external from ourselves
+  bool hasExternalBreakTargets() { return !breakNames.empty(); }
+
   // checks if these effects would invalidate another set (e.g., if we write, we invalidate someone that reads, they can't be moved past us)
   bool invalidates(EffectAnalyzer& other) {
     if (branches || other.branches

src/passes/CodeFolding.cpp

@@ -61,6 +61,7 @@
 #include "wasm-builder.h"
 #include "ir/utils.h"
 #include "ir/branch-utils.h"
+#include "ir/effects.h"
 #include "ir/label-utils.h"
 
 namespace wasm {
@@ -474,9 +475,18 @@ struct CodeFolding : public WalkerPass<ControlFlowWalker<CodeFolding>> {
     };
     // let's see if we can merge deeper than num, to num + 1
     auto next = tails;
-    // remove tails that are too short
+    // remove tails that are too short, or that we hit an item we can't handle
     next.erase(std::remove_if(next.begin(), next.end(), [&](Tail& tail) {
-      return effectiveSize(tail) < num + 1;
+      if (effectiveSize(tail) < num + 1) return true;
+      auto* newItem = getItem(tail, num);
+      // ignore tails that break to outside blocks. we want to move code to
+      // the very outermost position, so such code cannot be moved
+      // TODO: this should not be a problem in *non*-terminating tails,
+      //       but double-verify that
+      if (EffectAnalyzer(getPassOptions(), newItem).hasExternalBreakTargets()) {
+        return true;
+      }
+      return false;
     }), next.end());
     // if we have enough to investigate, do so
     if (next.size() >= 2) {

test/passes/code-folding.txt

@@ -49,4 +49,61 @@
    (f32.const 0)
   )
  )
+ (func $break-target-outside-of-return-merged-code (; 4 ;) (type $1)
+  (block $label$A
+   (if
+    (unreachable)
+    (block $block
+     (block $block0
+      (block $label$B
+       (if
+        (unreachable)
+        (br_table $label$A $label$B
+         (unreachable)
+        )
+       )
+      )
+      (return)
+     )
+    )
+    (block $block2
+     (block $label$C
+      (if
+       (unreachable)
+       (br_table $label$A $label$C
+        (unreachable)
+       )
+      )
+     )
+     (return)
+    )
+   )
+  )
+ )
+ (func $break-target-inside-all-good (; 5 ;) (type $1)
+  (block $folding-inner0
+   (block $label$A
+    (if
+     (unreachable)
+     (block $block
+      (block $block4
+       (br $folding-inner0)
+      )
+     )
+     (block $block6
+      (br $folding-inner0)
+     )
+    )
+   )
+  )
+  (block $label$B
+   (if
+    (unreachable)
+    (br_table $label$B $label$B
+     (unreachable)
+    )
+   )
+  )
+  (return)
+ )
 )

test/passes/code-folding.wast

@@ -52,5 +52,67 @@
    )
   )
  )
+ (func $break-target-outside-of-return-merged-code
+  (block $label$A
+   (if
+    (unreachable)
+    (block
+     (block
+      (block $label$B
+       (if
+        (unreachable)
+        (br_table $label$A $label$B
+         (unreachable)
+        )
+       )
+      )
+      (return)
+     )
+    )
+    (block
+     (block $label$C
+      (if
+       (unreachable)
+       (br_table $label$A $label$C ;; this all looks mergeable, but $label$A is outside
+        (unreachable)
+       )
+      )
+     )
+     (return)
+    )
+   )
+  )
+ )
+ (func $break-target-inside-all-good
+  (block $label$A
+   (if
+    (unreachable)
+    (block
+     (block
+      (block $label$B
+       (if
+        (unreachable)
+        (br_table $label$B $label$B
+         (unreachable)
+        )
+       )
+      )
+      (return)
+     )
+    )
+    (block
+     (block $label$C
+      (if
+       (unreachable)
+       (br_table $label$C $label$C ;; this all looks mergeable, and is, B ~~ C
+        (unreachable)
+       )
+      )
+     )
+     (return)
+    )
+   )
+  )
+ )
 )