Add SIMPLICITY_FALLTHROUGH macro and enable -Wimplicit-fallthrough in CI

.github/workflows/ci.yml

@@ -11,6 +11,12 @@ jobs:
         with:
             ignore_words_file: spelling.ignore.txt
             skip: alectryon
+  clang-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build and test C library with clang (-Wall -Werror)
+        run: CC=clang CFLAGS="-Wimplicit-fallthrough -Wconditional-uninitialized" make -C C -j$(nproc) check doCheck=1
   build:
     strategy:
       fail-fast: false

C/dag.c

@@ -116,6 +116,7 @@ sha256_midstate simplicity_computeWordCMR(const bitstring* value, size_t n) {
      case 0: i = getBit(value, 0); break;
      case 1: i = 2 + ((1U * getBit(value, 0) << 1) | getBit(value, 1)); break;
      case 2: i = 6 + ((1U * getBit(value, 0) << 3) | (1U * getBit(value, 1) << 2) | (1U * getBit(value, 2) << 1) | getBit(value, 3)); break;
+     default: SIMPLICITY_UNREACHABLE;
     }
     memcpy(stack_ptr, &word_cmr[i], sizeof(uint32_t[8]));
   } else {
@@ -174,14 +175,15 @@ void simplicity_computeCommitmentMerkleRoot(dag_node* dag, const uint_fast32_t i
    case PAIR:
     memcpy(block + j, dag[dag[i].child[1]].cmr.s, sizeof(uint32_t[8]));
     j = 0;
-    /*@fallthrough@*/
+    SIMPLICITY_FALLTHROUGH;
    case DISCONNECT: /* Only the first child is used in the CMR. */
    case INJL:
    case INJR:
    case TAKE:
    case DROP:
     memcpy(block + j, dag[dag[i].child[0]].cmr.s, sizeof(uint32_t[8]));
     simplicity_sha256_compression(dag[i].cmr.s, block);
+    SIMPLICITY_FALLTHROUGH;
    case IDEN:
    case UNIT:
    case WITNESS:
@@ -224,13 +226,14 @@ static void computeIdentityHashRoots(sha256_midstate* ihr, const dag_node* dag,
      case DISCONNECT:
       memcpy(block + j, ihr[dag[i].child[1]].s, sizeof(uint32_t[8]));
       j = 0;
-      /*@fallthrough@*/
+      SIMPLICITY_FALLTHROUGH;
      case INJL:
      case INJR:
      case TAKE:
      case DROP:
       memcpy(block + j, ihr[dag[i].child[0]].s, sizeof(uint32_t[8]));
       simplicity_sha256_compression(ihr[i].s, block);
+      SIMPLICITY_FALLTHROUGH;
      case IDEN:
      case UNIT:
      case HIDDEN:
@@ -420,6 +423,7 @@ simplicity_err simplicity_verifyCanonicalOrder(dag_node* dag, const uint_fast32_
         continue;
       }
       if (bottom == child) bottom++;
+      SIMPLICITY_FALLTHROUGH;
      case IDEN:
      case UNIT:
      case WITNESS:
@@ -444,6 +448,7 @@ simplicity_err simplicity_verifyCanonicalOrder(dag_node* dag, const uint_fast32_
         continue;
       }
       if (bottom == child) bottom++;
+      SIMPLICITY_FALLTHROUGH;
      case INJL:
      case INJR:
      case TAKE:

C/eval.c

@@ -462,7 +462,7 @@ static simplicity_err runTCO(evalState state, call* stack, const dag_node* dag,
       skip(state.activeWriteFrame, pad( INJR == dag[pc].tag
                                  , type_dag[INJ_B(dag, type_dag, pc)].bitSize
                                  , type_dag[INJ_C(dag, type_dag, pc)].bitSize));
-      /*@fallthrough@*/
+      SIMPLICITY_FALLTHROUGH;
      case TAKE:
       simplicity_debug_assert(calling);
       /* TAIL_CALL(dag[pc].child[0], SAME_TCO); */
@@ -496,7 +496,7 @@ static simplicity_err runTCO(evalState state, call* stack, const dag_node* dag,
       } else {
         writeValue(state.activeWriteFrame, &dag[pc].compactValue, dag[pc].targetType, type_dag);
       }
-      /*@fallthrough@*/
+      SIMPLICITY_FALLTHROUGH;
      case UNIT:
       simplicity_debug_assert(calling);
       if (get_tco_flag(&stack[pc])) {

C/simplicity_assert.h

@@ -34,4 +34,18 @@
 #  define SIMPLICITY_UNREACHABLE assert(NULL == "SIMPLICITY_UNCREACHABLE was reached")
 #endif
 
+/* Defines a FALLTHROUGH macro to annotate intentional switch fallthroughs, silencing -Wimplicit-fallthrough
+ * warnings on compilers that support the 'fallthrough' attribute.
+ * No-op on compilers (e.g. MSVC) that don't support it.
+ */
+#if defined(__has_attribute)
+#  if __has_attribute(fallthrough)
+#    define SIMPLICITY_FALLTHROUGH __attribute__((fallthrough))
+#  endif
+#endif
+
+#ifndef SIMPLICITY_FALLTHROUGH
+#  define SIMPLICITY_FALLTHROUGH ((void)0)
+#endif
+
 #endif /* SIMPLICITY_SIMPLICITY_ASSERT_H */