Fix asm2wasm dead lock caused by empty modules.

This commit fixes an asm2wasm dead lock when asm2wasm is compiling an
empty module, i.e. a module without any functions.

Without this commit, worker threads are likely to leave `workerMain()`
and decrease `liveWorkers` early.  Consequently, `waitUntilAllReady()`
will never observe `liveWorkers == numWorkers`.

6 files changed, 29 insertions, 1 deletions

diff --git a/src/wasm-module-building.h b/src/wasm-module-building.h
index cb72fef3f..ead074991 100644
--- a/src/wasm-module-building.h
+++ b/src/wasm-module-building.h
@@ -86,7 +86,15 @@ class OptimizingIncrementalModuleBuilder {
 public:
   // numFunctions must be equal to the number of functions allocated, or higher. Knowing
   // this bounds helps avoid locking.
-  OptimizingIncrementalModuleBuilder(Module* wasm, Index numFunctions) : wasm(wasm), numFunctions(numFunctions), nextFunction(0), finishing(false) {
+  OptimizingIncrementalModuleBuilder(Module* wasm, Index numFunctions)
+      : wasm(wasm), numFunctions(numFunctions), endMarker(nullptr), list(nullptr), nextFunction(0),
+        numWorkers(0), liveWorkers(0), activeWorkers(0), availableFuncs(0), finishedFuncs(0),
+        finishing(false) {
+    if (numFunctions == 0) {
+      // special case: no functions to be optimized.  Don't create any threads.
+      return;
+    }
+
     // prepare work list
     endMarker = new Function();
     list = new std::atomic<Function*>[numFunctions];
diff --git a/test/empty.asm.js b/test/empty.asm.js
new file mode 100644
index 000000000..94fa6bb38
--- /dev/null
+++ b/test/empty.asm.js
@@ -0,0 +1,4 @@
+function EmptyModule() {
+    'use asm';
+    return {};
+}
diff --git a/test/empty.fromasm b/test/empty.fromasm
new file mode 100644
index 000000000..939cbb87c
--- /dev/null
+++ b/test/empty.fromasm
@@ -0,0 +1,4 @@
+(module
+  (memory 256 256)
+  (export "memory" memory)
+)
diff --git a/test/empty.fromasm.imprecise b/test/empty.fromasm.imprecise
new file mode 100644
index 000000000..939cbb87c
--- /dev/null
+++ b/test/empty.fromasm.imprecise
@@ -0,0 +1,4 @@
+(module
+  (memory 256 256)
+  (export "memory" memory)
+)
diff --git a/test/empty.fromasm.imprecise.no-opts b/test/empty.fromasm.imprecise.no-opts
new file mode 100644
index 000000000..939cbb87c
--- /dev/null
+++ b/test/empty.fromasm.imprecise.no-opts
@@ -0,0 +1,4 @@
+(module
+  (memory 256 256)
+  (export "memory" memory)
+)
diff --git a/test/empty.fromasm.no-opts b/test/empty.fromasm.no-opts
new file mode 100644
index 000000000..939cbb87c
--- /dev/null
+++ b/test/empty.fromasm.no-opts
@@ -0,0 +1,4 @@
+(module
+  (memory 256 256)
+  (export "memory" memory)
+)