p4lang · mihaibudiu · Jun 25, 2022 · May 24, 2022 · Jun 3, 2022 · Jun 9, 2022
diff --git a/frontends/CMakeLists.txt b/frontends/CMakeLists.txt
@@ -44,6 +44,7 @@ set (P4_FRONTEND_SRCS
   p4/parserCallGraph.cpp
   p4/parserControlFlow.cpp
   p4/reassociation.cpp
+  p4/redundantParsers.cpp
   p4/removeParameters.cpp
   p4/removeReturns.cpp
   p4/reservedWords.cpp
@@ -114,6 +115,7 @@ set (P4_FRONTEND_HDRS
   p4/parserCallGraph.h
   p4/parserControlFlow.h
   p4/reassociation.h
+  p4/redundantParsers.h
   p4/removeParameters.h
   p4/removeReturns.h
   p4/reservedWords.h

diff --git a/frontends/p4/frontend.cpp b/frontends/p4/frontend.cpp
@@ -48,6 +48,7 @@ limitations under the License.
 #include "parseAnnotations.h"
 #include "parserControlFlow.h"
 #include "reassociation.h"
+#include "redundantParsers.h"
 #include "removeParameters.h"
 #include "removeReturns.h"
 #include "resetHeaders.h"
@@ -218,6 +219,7 @@ const IR::P4Program *FrontEnd::run(const CompilerOptions &options, const IR::P4P
         new RemoveDontcareArgs(&refMap, &typeMap),
         new MoveConstructors(&refMap),
         new RemoveAllUnusedDeclarations(&refMap),
+        new RemoveRedundantParsers(&refMap, &typeMap),
         new ClearTypeMap(&typeMap),
         evaluator,
         new Inline(&refMap, &typeMap, evaluator, options.optimizeParserInlining),

diff --git a/frontends/p4/redundantParsers.cpp b/frontends/p4/redundantParsers.cpp
@@ -0,0 +1,59 @@
+/*
+Copyright 2022 Intel Corporation
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#include "redundantParsers.h"
+
+#include "frontends/common/resolveReferences/referenceMap.h"
+#include "frontends/p4/methodInstance.h"
+
+namespace P4 {
+bool FindRedundantParsers::preorder(const IR::P4Parser *parser) {
+    for (const IR::ParserState *state : parser->states) {
+        if (state->name != IR::ParserState::start) {
+            continue;
+        }
+        const auto *pathExpr = state->selectExpression->to<IR::PathExpression>();
+        if (!pathExpr ||
+            pathExpr->path->name != IR::ParserState::accept ||
+            !state->components.empty()) {
+            continue;
+        }
+        LOG4("Found redundant parser " << parser->name);
+        redundantParsers.insert(parser);
+    }
+    return false;
+}
+
+const IR::Node *EliminateSubparserCalls::postorder(IR::MethodCallStatement *mcs) {
+    auto mi = MethodInstance::resolve(mcs->methodCall, refMap, typeMap, true);
+    if (!mi->isApply()) return mcs;
+
+    auto apply = mi->to<ApplyMethod>()->applyObject;
+    auto parser = apply->to<IR::Type_Parser>();
+    if (!parser) return mcs;
+
+    auto declInstance = mi->object->to<IR::Declaration_Instance>();
+    if (!declInstance) return mcs;
+
+    auto decl = refMap->getDeclaration(declInstance->type->to<IR::Type_Name>()->path);
+    auto p4parser = decl->to<IR::P4Parser>();
+    if (!p4parser || !redundantParsers.count(p4parser)) return mcs;
+
+    LOG4("Removing apply call to redundant parser " << parser->getName()
+         << ": " << *mcs);
+    return nullptr;
+}
+}
diff --git a/frontends/p4/redundantParsers.h b/frontends/p4/redundantParsers.h
@@ -0,0 +1,67 @@
+/*
+Copyright 2022 Intel Corporation
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#ifndef FRONTENDS_P4_REDUNDANTPARSERS_H_
+#define FRONTENDS_P4_REDUNDANTPARSERS_H_
+
+#include "ir/ir.h"
+#include "frontends/p4/typeChecking/typeChecker.h"
+
+namespace P4 {
+
+/** Find parsers that have an unconditional "accept" in their start
+ *  state, and put them in redundantParsers.
+ */
+class FindRedundantParsers : public Inspector {
+    std::set<const IR::P4Parser *> &redundantParsers;
+    bool preorder(const IR::P4Parser *parser) override;
+ public:
+    explicit FindRedundantParsers(std::set<const IR::P4Parser *> &redundantParsers)
+        : redundantParsers(redundantParsers) { }
+};
+
+/** Find .apply() calls on parsers that are on redundantParsers, and
+ *  eliminate them.
+ */
+class EliminateSubparserCalls : public Transform {
+    const std::set<const IR::P4Parser *> &redundantParsers;
+    ReferenceMap *refMap;
+    TypeMap *typeMap;
+    const IR::Node *postorder(IR::MethodCallStatement *methodCallStmt) override;
+ public:
+    EliminateSubparserCalls(const std::set<const IR::P4Parser *> &redundantParsers,
+                            ReferenceMap *refMap,
+                            TypeMap *typeMap)
+        : redundantParsers(redundantParsers), refMap(refMap), typeMap(typeMap)
+    { }
+};
+
+class RemoveRedundantParsers : public PassManager {
+    std::set<const IR::P4Parser *> redundantParsers;
+ public:
+    RemoveRedundantParsers(ReferenceMap *refMap, TypeMap *typeMap)
+        : PassManager {
+                new TypeChecking(refMap, typeMap, true),
+                new FindRedundantParsers(redundantParsers),
+                new EliminateSubparserCalls(redundantParsers, refMap, typeMap)
+        } {
+        setName("RemoveRedundantParsers");
+    }
+};
+
+}
+
+#endif  /* FRONTENDS_P4_REDUNDANTPARSERS_H_ */
diff --git a/testdata/p4_16_samples_outputs/parser-arg-frontend.p4 b/testdata/p4_16_samples_outputs/parser-arg-frontend.p4
@@ -4,27 +4,14 @@ parser Parser();
 package Package(Parser p1, Parser p2);
 parser Parser1_0() {
     state start {
-        transition Inside_start;
-    }
-    state Inside_start {
-        transition start_0;
-    }
-    state start_0 {
         transition accept;
     }
 }
 
 parser Parser2_0() {
     state start {
-        transition Inside_start_0;
-    }
-    state Inside_start_0 {
-        transition start_1;
-    }
-    state start_1 {
         transition accept;
     }
 }
 
 Package(Parser1_0(), Parser2_0()) main;
-