fix proper wasm emitting of untaken br_tables

Author: kripken

src/wasm/wasm-binary.cpp

@@ -666,6 +666,14 @@ void WasmBinaryWriter::visitSwitch(Switch *curr) {
     recurse(curr->value);
   }
   recurse(curr->condition);
+  if (!BranchUtils::isBranchTaken(curr)) {
+    // if the branch is not taken, then it's dangerous to emit it, as
+    // wasm type checking rules are stricter than ours - we tolerate
+    // an untaken branch to a target with a different value, but not
+    // wasm. so just don't emit it
+    o << int8_t(BinaryConsts::Unreachable);
+    return;
+  }
   o << int8_t(BinaryConsts::TableSwitch) << U32LEB(curr->targets.size());
   for (auto target : curr->targets) {
     o << U32LEB(getBreakIndex(target));
@@ -1796,7 +1804,7 @@ void WasmBinaryBuilder::processExpressions() { // until an end or else marker, o
 
 Expression* WasmBinaryBuilder::popExpression() {
   if (expressionStack.empty()) {
-    throw ParseException("attempted pop from empty stack");
+    throw ParseException("attempted pop from empty stack at " + std::to_string(pos));
   }
   auto ret = expressionStack.back();
   // to simulate the wasm polymorphic stack mode, leave a final

test/polymorphic_stack.wast

@@ -86,5 +86,15 @@
       )
     )
   )
+  (func $br_table_unreachable_to_also_unreachable (result i32)
+    (block $a (result i32)
+      (block $b
+        (br_table $a $b ;; seems to send a value, but is not taken
+          (unreachable)
+          (unreachable)
+        )
+      )
+    )
+  )
 )
 

test/polymorphic_stack.wast.from-wast

@@ -90,4 +90,14 @@
    )
   )
  )
+ (func $br_table_unreachable_to_also_unreachable (type $1) (result i32)
+  (block $a (result i32)
+   (block $b
+    (br_table $a $b
+     (unreachable)
+     (unreachable)
+    )
+   )
+  )
+ )
 )

test/polymorphic_stack.wast.fromBinary

@@ -125,5 +125,16 @@
    (unreachable)
   )
  )
+ (func $br_table_unreachable_to_also_unreachable (type $1) (result i32)
+  (block $label$0 (result i32)
+   (block $label$1
+    (unreachable)
+    (unreachable)
+    (unreachable)
+    (unreachable)
+   )
+   (unreachable)
+  )
+ )
 )
 

test/polymorphic_stack.wast.fromBinary.noDebugInfo

@@ -125,5 +125,16 @@
    (unreachable)
   )
  )
+ (func $6 (type $1) (result i32)
+  (block $label$0 (result i32)
+   (block $label$1
+    (unreachable)
+    (unreachable)
+    (unreachable)
+    (unreachable)
+   )
+   (unreachable)
+  )
+ )
 )