Allow lookahead of structs with size of 0 (#4149)

Signed-off-by: Mihai Budiu <mbudiu@gmail.com>

Author: mihaibudiu

midend/expandLookahead.cpp

@@ -43,7 +43,7 @@ void DoExpandLookahead::expand(
         }
     } else if (type->is<IR::Type_Bits>() || type->is<IR::Type_Boolean>()) {
         unsigned size = type->width_bits();
-        BUG_CHECK(size > 0, "%1%: unexpected size %2%", type, size);
+        if (size == 0) return;
         const IR::Expression *expression =
             new IR::Slice(bitvector->clone(), *offset - 1, *offset - size);
         auto tb = type->to<IR::Type_Bits>();

testdata/p4_16_samples/issue4143.p4

@@ -0,0 +1,19 @@
+#include <core.p4>
+
+parser Parser(packet_in pkt);
+package SimpleArch(Parser p);
+
+struct Foo {
+    bit<0> f0;
+}
+
+parser MyParser(packet_in packet) {
+   state start {
+       transition select(packet.lookahead<Foo>().f0) {
+           0: accept;
+           default: reject;
+       }
+   }
+}
+
+SimpleArch(MyParser()) main;

testdata/p4_16_samples_outputs/issue4143-first.p4

@@ -0,0 +1,18 @@
+#include <core.p4>
+
+parser Parser(packet_in pkt);
+package SimpleArch(Parser p);
+struct Foo {
+    bit<0> f0;
+}
+
+parser MyParser(packet_in packet) {
+    state start {
+        transition select((packet.lookahead<Foo>()).f0) {
+            0: accept;
+            default: reject;
+        }
+    }
+}
+
+SimpleArch(MyParser()) main;

testdata/p4_16_samples_outputs/issue4143-frontend.p4

@@ -0,0 +1,22 @@
+#include <core.p4>
+
+parser Parser(packet_in pkt);
+package SimpleArch(Parser p);
+struct Foo {
+    bit<0> f0;
+}
+
+parser MyParser(packet_in packet) {
+    @name("MyParser.tmp") bit<0> tmp;
+    @name("MyParser.tmp_0") Foo tmp_0;
+    state start {
+        tmp_0 = packet.lookahead<Foo>();
+        tmp = tmp_0.f0;
+        transition select(tmp) {
+            0: accept;
+            default: reject;
+        }
+    }
+}
+
+SimpleArch(MyParser()) main;

testdata/p4_16_samples_outputs/issue4143-midend.p4

@@ -0,0 +1,20 @@
+#include <core.p4>
+
+parser Parser(packet_in pkt);
+package SimpleArch(Parser p);
+struct Foo {
+    bit<0> f0;
+}
+
+parser MyParser(packet_in packet) {
+    @name("MyParser.tmp_0") Foo tmp_0;
+    state start {
+        packet.lookahead<bit<0>>();
+        transition select(tmp_0.f0) {
+            0: accept;
+            default: reject;
+        }
+    }
+}
+
+SimpleArch(MyParser()) main;

testdata/p4_16_samples_outputs/issue4143.p4

@@ -0,0 +1,18 @@
+#include <core.p4>
+
+parser Parser(packet_in pkt);
+package SimpleArch(Parser p);
+struct Foo {
+    bit<0> f0;
+}
+
+parser MyParser(packet_in packet) {
+    state start {
+        transition select((packet.lookahead<Foo>()).f0) {
+            0: accept;
+            default: reject;
+        }
+    }
+}
+
+SimpleArch(MyParser()) main;