Fix bmv2 backend crash for unused action selectors

When an action selector is declared as a control local but never
referenced by a table, the compiler cannot figure out the selector
input, which means the action selector cannot be included in the bmv2
JSON.

backends/bmv2/jsonconverter.cpp

@@ -142,8 +142,10 @@ class SharedActionSelectorCheck : public Inspector {
     explicit SharedActionSelectorCheck(JsonConverter* converter) : converter(converter)
     { CHECK_NULL(converter); }
 
-    const Input &get_selector_input(const IR::Declaration_Instance* selector) const {
-        return selector_input_map.at(selector);
+    const Input *get_selector_input(const IR::Declaration_Instance* selector) const {
+        auto it = selector_input_map.find(selector);
+        if (it == selector_input_map.end()) return nullptr;  // selector never used
+        return &it->second;
     }
 
     bool preorder(const IR::P4Table* table) override {
@@ -2096,10 +2098,17 @@ Util::IJson* JsonConverter::convertControl(const IR::ControlBlock* block, cstrin
                         BUG_CHECK(hash->is<IR::Declaration_ID>(), "%1%: expected a member", hash);
                         auto algo = convertHashAlgorithm(hash->to<IR::Declaration_ID>()->name);
                         selector->emplace("algo", algo);
-                        const auto &input = selector_check.get_selector_input(
+                        auto input = selector_check.get_selector_input(
                             c->to<IR::Declaration_Instance>());
+                        if (input == nullptr) {
+                            // the selector is never used by any table, we cannot figure out its
+                            // input and therefore cannot include it in the JSON
+                            ::warning("Action selector '%1%' is never referenced by a table "
+                                      "and cannot be included in bmv2 JSON", c);
+                            continue;
+                        }
                         auto j_input = mkArrayField(selector, "input");
-                        for (auto expr : input) {
+                        for (auto expr : *input) {
                             auto jk = conv->convert(expr);
                             j_input->append(jk);
                         }

testdata/p4_16_samples/action_selector_unused-bmv2.p4

@@ -0,0 +1,52 @@
+#include <core.p4>
+#include <v1model.p4>
+
+struct H { };
+struct M {
+    bit<32> hash1;
+}
+
+parser ParserI(packet_in pk, out H hdr, inout M meta, inout standard_metadata_t
+    smeta) {
+    state start { transition accept; }
+}
+
+action empty() { }
+
+control IngressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+
+    action drop() { smeta.drop = 1; }
+
+    action_selector (HashAlgorithm.identity, 32w1024, 32w10) as;
+
+    table indirect_ws {
+        key = { meta.hash1 : selector; }
+        actions = { drop; NoAction; }
+        const default_action = NoAction();
+        implementation = as;
+    }
+
+    apply {
+        indirect_ws.apply();
+    }
+
+};
+
+control EgressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    apply { }
+};
+
+control DeparserI(packet_out pk, in H hdr) {
+    apply { }
+}
+
+control VerifyChecksumI(in H hdr, inout M meta) {
+    apply { }
+}
+
+control ComputeChecksumI(inout H hdr, inout M meta) {
+    apply { }
+}
+
+V1Switch(ParserI(), VerifyChecksumI(), IngressI(), EgressI(),
+         ComputeChecksumI(), DeparserI()) main;

testdata/p4_16_samples_outputs/action_selector_unused-bmv2-first.p4

@@ -0,0 +1,60 @@
+#include <core.p4>
+#include <v1model.p4>
+
+struct H {
+}
+
+struct M {
+    bit<32> hash1;
+}
+
+parser ParserI(packet_in pk, out H hdr, inout M meta, inout standard_metadata_t smeta) {
+    state start {
+        transition accept;
+    }
+}
+
+action empty() {
+}
+control IngressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    action drop() {
+        smeta.drop = 1w1;
+    }
+    action_selector(HashAlgorithm.identity, 32w1024, 32w10) as;
+    table indirect_ws {
+        key = {
+            meta.hash1: selector @name("meta.hash1") ;
+        }
+        actions = {
+            drop();
+            NoAction();
+        }
+        const default_action = NoAction();
+        implementation = as;
+    }
+    apply {
+        indirect_ws.apply();
+    }
+}
+
+control EgressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    apply {
+    }
+}
+
+control DeparserI(packet_out pk, in H hdr) {
+    apply {
+    }
+}
+
+control VerifyChecksumI(in H hdr, inout M meta) {
+    apply {
+    }
+}
+
+control ComputeChecksumI(inout H hdr, inout M meta) {
+    apply {
+    }
+}
+
+V1Switch<H, M>(ParserI(), VerifyChecksumI(), IngressI(), EgressI(), ComputeChecksumI(), DeparserI()) main;

testdata/p4_16_samples_outputs/action_selector_unused-bmv2-frontend.p4

@@ -0,0 +1,58 @@
+#include <core.p4>
+#include <v1model.p4>
+
+struct H {
+}
+
+struct M {
+    bit<32> hash1;
+}
+
+parser ParserI(packet_in pk, out H hdr, inout M meta, inout standard_metadata_t smeta) {
+    state start {
+        transition accept;
+    }
+}
+
+control IngressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    @name("drop") action drop_0() {
+        smeta.drop = 1w1;
+    }
+    @name("as") action_selector(HashAlgorithm.identity, 32w1024, 32w10) as_0;
+    @name("indirect_ws") table indirect_ws_0 {
+        key = {
+            meta.hash1: selector @name("meta.hash1") ;
+        }
+        actions = {
+            drop_0();
+            NoAction();
+        }
+        const default_action = NoAction();
+        implementation = as_0;
+    }
+    apply {
+        indirect_ws_0.apply();
+    }
+}
+
+control EgressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    apply {
+    }
+}
+
+control DeparserI(packet_out pk, in H hdr) {
+    apply {
+    }
+}
+
+control VerifyChecksumI(in H hdr, inout M meta) {
+    apply {
+    }
+}
+
+control ComputeChecksumI(inout H hdr, inout M meta) {
+    apply {
+    }
+}
+
+V1Switch<H, M>(ParserI(), VerifyChecksumI(), IngressI(), EgressI(), ComputeChecksumI(), DeparserI()) main;

testdata/p4_16_samples_outputs/action_selector_unused-bmv2-midend.p4

@@ -0,0 +1,60 @@
+#include <core.p4>
+#include <v1model.p4>
+
+struct H {
+}
+
+struct M {
+    bit<32> hash1;
+}
+
+parser ParserI(packet_in pk, out H hdr, inout M meta, inout standard_metadata_t smeta) {
+    state start {
+        transition accept;
+    }
+}
+
+control IngressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    @name("NoAction") action NoAction_0() {
+    }
+    @name("drop") action drop_0() {
+        smeta.drop = 1w1;
+    }
+    @name("as") action_selector(HashAlgorithm.identity, 32w1024, 32w10) as;
+    @name("indirect_ws") table indirect_ws {
+        key = {
+            meta.hash1: selector @name("meta.hash1") ;
+        }
+        actions = {
+            drop_0();
+            NoAction_0();
+        }
+        const default_action = NoAction_0();
+        implementation = as;
+    }
+    apply {
+        indirect_ws.apply();
+    }
+}
+
+control EgressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    apply {
+    }
+}
+
+control DeparserI(packet_out pk, in H hdr) {
+    apply {
+    }
+}
+
+control VerifyChecksumI(in H hdr, inout M meta) {
+    apply {
+    }
+}
+
+control ComputeChecksumI(inout H hdr, inout M meta) {
+    apply {
+    }
+}
+
+V1Switch<H, M>(ParserI(), VerifyChecksumI(), IngressI(), EgressI(), ComputeChecksumI(), DeparserI()) main;

testdata/p4_16_samples_outputs/action_selector_unused-bmv2.p4

@@ -0,0 +1,60 @@
+#include <core.p4>
+#include <v1model.p4>
+
+struct H {
+}
+
+struct M {
+    bit<32> hash1;
+}
+
+parser ParserI(packet_in pk, out H hdr, inout M meta, inout standard_metadata_t smeta) {
+    state start {
+        transition accept;
+    }
+}
+
+action empty() {
+}
+control IngressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    action drop() {
+        smeta.drop = 1;
+    }
+    action_selector(HashAlgorithm.identity, 32w1024, 32w10) as;
+    table indirect_ws {
+        key = {
+            meta.hash1: selector;
+        }
+        actions = {
+            drop;
+            NoAction;
+        }
+        const default_action = NoAction();
+        implementation = as;
+    }
+    apply {
+        indirect_ws.apply();
+    }
+}
+
+control EgressI(inout H hdr, inout M meta, inout standard_metadata_t smeta) {
+    apply {
+    }
+}
+
+control DeparserI(packet_out pk, in H hdr) {
+    apply {
+    }
+}
+
+control VerifyChecksumI(in H hdr, inout M meta) {
+    apply {
+    }
+}
+
+control ComputeChecksumI(inout H hdr, inout M meta) {
+    apply {
+    }
+}
+
+V1Switch(ParserI(), VerifyChecksumI(), IngressI(), EgressI(), ComputeChecksumI(), DeparserI()) main;