Do not generate TC Egress program if PSA Egress pipeline is empty (p4…

…lang#3349)

* Do not generate TC Egress program if PSA Egress pipeline is empty

backends/ebpf/psa/README.md

@@ -32,7 +32,8 @@ The TC-based design of PSA for eBPF is depicted in Figure below.
   The role of Traffic Manager is to redirect traffic between the Ingress (TC) and Egress (TC).
   It is also responsible for packet replication via clone sessions or multicast groups and sending packet to CPU.
 - `tc-egress` - The PSA Egress pipeline (composed of Parser, Control block and Deparser) is attached to the TC Egress hook. As there is no
-  XDP hook in the Egress path, the use of TC is mandatory for the egress processing.
+  XDP hook in the Egress path, the use of TC is mandatory for the egress processing. **Note!** If the PSA Egress pipeline is not used (i.e. it is left empty by a developer),
+  the PSA-eBPF compiler will not generate the TC Egress program. This brings a noticeable performance gain, if the egress pipeline is not used. 
 
 ## Packet paths
 

backends/ebpf/psa/ebpfPipeline.cpp

@@ -19,6 +19,32 @@ limitations under the License.
 
 namespace EBPF {
 
+bool EBPFPipeline::isEmpty() const {
+    // check if parser doesn't have any state
+    // Why 3? Parser will always have at least start, accept and reject states.
+    if (parser->parserBlock->container->states.size() > 3) {
+        return false;
+    }
+
+    auto startState = parser->parserBlock->container->states.at(0);
+    auto pathExpr = startState->selectExpression->to<IR::PathExpression>()->path;
+    if (!startState->components.empty() || pathExpr->name.name != IR::ParserState::accept) {
+        return false;
+    }
+
+    // check if control is empty
+    if (!control->controlBlock->container->body->components.empty()) {
+        return false;
+    }
+
+    // check if deparser doesn't emit anything
+    if (!deparser->controlBlock->container->body->components.empty()) {
+        return false;
+    }
+
+    return true;
+}
+
 void EBPFPipeline::emitLocalVariables(CodeBuilder* builder) {
     builder->emitIndent();
     builder->appendFormat("unsigned %s = 0;", offsetVar.c_str());

backends/ebpf/psa/ebpfPipeline.h

@@ -71,6 +71,10 @@ class EBPFPipeline : public EBPFProgram {
         oneKey = EBPFModel::reserved("one");
     }
 
+    /* Check if pipeline does any processing.
+     * Return false if not. */
+    bool isEmpty() const;
+
     virtual cstring dropReturnCode() {
         if (sectionName.startsWith("xdp")) {
             return "XDP_DROP";

backends/ebpf/psa/ebpfPsaGen.cpp

@@ -383,7 +383,10 @@ void PSAArchTC::emit(CodeBuilder *builder) const {
     /*
      * 10. TC Egress program.
      */
-    egress->emit(builder);
+    if (!egress->isEmpty()) {
+        // Do not generate TC Egress program if PSA egress pipeline is not used (empty).
+        egress->emit(builder);
+    }
 
     builder->target->emitLicense(builder, ingress->license);
 }