[torque] make CPP generation deterministic by sorting changed variables

I suspect that the non-deterministic order of changed variables in the
generated Label constructor calls is what causes non-deterministic
builds, since this is the only change I observed locally in the
generated .cc files.

Drive-by cleanup: follow style-guide by avoiding mutable ref parameter.

Bug: chromium:867308
Change-Id: I137af359df570ee11b95cb620ace179bf93481c5
Reviewed-on: https://chromium-review.googlesource.com/1148729Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54687}

src/torque/implementation-visitor.cc

@@ -1232,7 +1232,7 @@ Callable* ImplementationVisitor::LookupCall(const std::string& name,
 }
 
 void ImplementationVisitor::GetFlattenedStructsVars(
-    const Variable* base, std::set<const Variable*>& vars) {
+    const Variable* base, std::set<const Variable*>* vars) {
   const Type* type = base->type();
   if (base->IsConst()) return;
   if (type->IsStructType()) {
@@ -1243,7 +1243,7 @@ void ImplementationVisitor::GetFlattenedStructsVars(
           Variable::cast(declarations()->LookupValue(field_var_name)), vars);
     }
   } else {
-    vars.insert(base);
+    vars->insert(base);
   }
 }
 
@@ -1253,11 +1253,18 @@ void ImplementationVisitor::GenerateChangedVarsFromControlSplit(AstNode* node) {
           node, declarations()->GetCurrentSpecializationTypeNamesVector());
   std::set<const Variable*> flattened_vars;
   for (auto v : changed_vars) {
-    GetFlattenedStructsVars(v, flattened_vars);
+    GetFlattenedStructsVars(v, &flattened_vars);
   }
+  std::vector<const Variable*> flattened_vars_sorted(flattened_vars.begin(),
+                                                     flattened_vars.end());
+  auto compare_variables = [](const Variable* a, const Variable* b) {
+    return a->value() < b->value();
+  };
+  std::sort(flattened_vars_sorted.begin(), flattened_vars_sorted.end(),
+            compare_variables);
   source_out() << "{";
-  PrintCommaSeparatedList(source_out(), flattened_vars,
-                          [&](const Variable* v) { return v->value(); });
+  PrintCommaSeparatedList(source_out(), flattened_vars_sorted,
+                          [](const Variable* v) { return v->value(); });
   source_out() << "}";
 }
 

src/torque/implementation-visitor.h

@@ -173,7 +173,7 @@ class ImplementationVisitor : public FileVisitor {
   bool GenerateChangedVarFromControlSplit(const Variable* v, bool first = true);
 
   void GetFlattenedStructsVars(const Variable* base,
-                               std::set<const Variable*>& vars);
+                               std::set<const Variable*>* vars);
 
   void GenerateChangedVarsFromControlSplit(AstNode* node);