extend ActionSynthesisPolicy (#1689)

- add a query to specify when statements should be combined into a
  single action, or made into separate actions.
- add Context argument to query functions.

Author: ChrisDodd

backends/bmv2/common/backend.h

@@ -82,7 +82,7 @@ class SkipControls : public P4::ActionSynthesisPolicy {
 
  public:
     explicit SkipControls(const std::set<cstring> *skip) : skip(skip) { CHECK_NULL(skip); }
-    bool convert(const IR::P4Control* control) const {
+    bool convert(const Visitor::Context *, const IR::P4Control* control) override {
         if (skip->find(control->name) != skip->end())
             return false;
         return true;

backends/p4test/midend.cpp

@@ -58,7 +58,7 @@ class SkipControls : public P4::ActionSynthesisPolicy {
 
  public:
     explicit SkipControls(const std::set<cstring> *skip) : skip(skip) { CHECK_NULL(skip); }
-    bool convert(const IR::P4Control* control) const override {
+    bool convert(const Visitor::Context *, const IR::P4Control* control) override {
         if (skip->find(control->name) != skip->end())
             return false;
         return true;

midend/actionSynthesis.cpp

@@ -113,7 +113,7 @@ bool DoSynthesizeActions::mustMove(const IR::AssignmentStatement *assign) {
 const IR::Node* DoSynthesizeActions::preorder(IR::P4Control* control) {
     actions.clear();
     changes = false;
-    if (policy != nullptr && !policy->convert(control))
+    if (policy != nullptr && !policy->convert(getContext(), control))
         prune();  // skip this one
     return control;
 }
@@ -132,10 +132,20 @@ const IR::Node* DoSynthesizeActions::preorder(IR::BlockStatement* statement) {
     for (auto c : statement->components) {
         if (c->is<IR::AssignmentStatement>()) {
             if (mustMove(c->to<IR::AssignmentStatement>())) {
+                if (policy && !actbody->components.empty() &&
+                    !policy->can_combine(getContext(), actbody, c)) {
+                    auto action = createAction(actbody);
+                    left->push_back(action);
+                    actbody = new IR::BlockStatement; }
                 actbody->push_back(c);
                 continue; }
         } else if (c->is<IR::MethodCallStatement>()) {
             if (mustMove(c->to<IR::MethodCallStatement>())) {
+                if (policy && !actbody->components.empty() &&
+                    !policy->can_combine(getContext(), actbody, c)) {
+                    auto action = createAction(actbody);
+                    left->push_back(action);
+                    actbody = new IR::BlockStatement; }
                 actbody->push_back(c);
                 continue;
             }

midend/actionSynthesis.h

@@ -34,7 +34,21 @@ class ActionSynthesisPolicy {
        If the policy returns true the control block is processed,
        otherwise it is left unchanged.
     */
-    virtual bool convert(const IR::P4Control* control) const = 0;
+    virtual bool convert(const Visitor::Context *ctxt, const IR::P4Control* control) = 0;
+
+    /**
+       Called for each statement that may be put into an action when there are preceeding
+       statements already put into an action --
+        @param ctxt context of the code being processsed (control and parents)
+        @param blk  previous statement(s) being put into an action
+        @param stmt statement to be added to the block for a single action
+        @returns
+            true  statement should be added to the same action
+            false statement should start a new action
+    */
+    virtual bool can_combine(const Visitor::Context *, const IR::BlockStatement *,
+                             const IR::StatOrDecl *) {
+        return true; }
 };
 
 /**