Fix task calls as fork statements (#4055)

Before this patch, calling tasks directly under forks would result in each
statement of these tasks being executed concurrently. This was due to Verilator
inlining tasks most of the time. Such inlined tasks' statements would simply
replace the original call, and there would be no indication that these used to
be grouped together. Ultimately resulting in `V3Timing` treating each statement
as a separate process.

The solution is simply to wrap each fork sub-statement in a begin in `V3Begin`
(except for the ones that are begins, as that would be pointless). `V3Begin` is
already aware of forks, and is supposed to avoid issues like this one, so it
seems like a natural fit. This also protects us from similar bugs, i.e. if some
statement gets replaced or expanded into multiple statements.

Signed-off-by: Krzysztof Bieganski <[email protected]>

src/V3Begin.cpp

@@ -144,9 +144,20 @@ private:
 
     // VISITORS
     void visit(AstFork* nodep) override {
-        // Keep this begin to group its statements together
+        // Keep begins in forks to group their statements together
         VL_RESTORER(m_keepBegins);
         m_keepBegins = true;
+        // If a statement is not a begin, wrap it in a begin. This fixes an issue when the
+        // statement is a task call that gets inlined later (or any other statement that gets
+        // replaced with multiple statements)
+        for (AstNode* stmtp = nodep->stmtsp(); stmtp; stmtp = stmtp->nextp()) {
+            if (!VN_IS(stmtp, Begin)) {
+                AstBegin* const beginp = new AstBegin{stmtp->fileline(), "", nullptr};
+                stmtp->replaceWith(beginp);
+                beginp->addStmtsp(stmtp);
+                stmtp = beginp;
+            }
+        }
         dotNames(nodep, "__FORK__");
         nodep->name("");
     }