Disable threading in simd_op_check to address Issue #3669

test/correctness/simd_op_check.cpp

@@ -2476,6 +2476,20 @@ int main(int argc, char **argv) {
         num_threads = 1;
     }
 
+    // TODO: multithreading here is the cause of https://github.com/halide/Halide/issues/3669;
+    // the fundamental issue is that we make one set of ImageParams to construct many
+    // Exprs, then realize those Exprs on arbitrary threads; it is known that sharing
+    // one Func across multiple threads is not guaranteed to be safe, and indeed, TSAN
+    // reports data races, of which some are likely 'benign' (e.g. Function.freeze) but others
+    // are highly suspect (e.g. Function.lock_loop_levels). Since multithreading here
+    // was added just to avoid having this test be the last to finish, the expedient 'fix'
+    // for now is to remove the multithreading. A proper fix could be made by restructuring this
+    // test so that every Expr constructed for testing was guaranteed to share no Funcs
+    // (Function.deep_copy() perhaps). Of course, it would also be desirable to allow Funcs, Exprs, etc
+    // to be usable across multiple threads, but that is a major undertaking that is
+    // definitely not worthwhile for present Halide usage patterns.
+    num_threads = 1;
+
     if (argc > 2) {
         // Don't forget: if you want to run the standard tests to a specific output
         // directory, you'll need to invoke with the first arg enclosed