[Parser] Parse start declarations (#6256)

Author: tlively

src/parser/contexts.h

@@ -589,6 +589,7 @@ struct ParseDeclsCtx : NullTypeParserCtx, NullInstrParserCtx {
   std::vector<DefPos> tableDefs;
   std::vector<DefPos> memoryDefs;
   std::vector<DefPos> globalDefs;
+  std::vector<DefPos> startDefs;
   std::vector<DefPos> elemDefs;
   std::vector<DefPos> dataDefs;
   std::vector<DefPos> tagDefs;
@@ -715,6 +716,14 @@ struct ParseDeclsCtx : NullTypeParserCtx, NullInstrParserCtx {
                      std::optional<ExprT>,
                      Index pos);
 
+  Result<> addStart(FuncIdxT, Index pos) {
+    if (!startDefs.empty()) {
+      return Err{"unexpected extra 'start' function"};
+    }
+    startDefs.push_back({{}, pos, 0});
+    return Ok{};
+  }
+
   Result<> addElem(Name, TableIdxT*, std::optional<ExprT>, ElemListT&&, Index);
 
   Result<> addDeclareElem(Name, ElemListT&&, Index) { return Ok{}; }
@@ -1325,6 +1334,11 @@ struct ParseDefsCtx : TypeParserCtx<ParseDefsCtx> {
                      std::optional<ExprT> exp,
                      Index);
 
+  Result<> addStart(Name name, Index pos) {
+    wasm.start = name;
+    return Ok{};
+  }
+
   Result<> addImplicitElems(Type type, std::vector<Expression*>&& elems);
 
   Result<> addDeclareElem(Name, std::vector<Expression*>&&, Index) {

src/parser/parsers.h

@@ -202,6 +202,7 @@ template<typename Ctx> MaybeResult<> table(Ctx&);
 template<typename Ctx> MaybeResult<> memory(Ctx&);
 template<typename Ctx> MaybeResult<> global(Ctx&);
 template<typename Ctx> MaybeResult<> export_(Ctx&);
+template<typename Ctx> MaybeResult<> start(Ctx&);
 template<typename Ctx> MaybeResult<typename Ctx::ExprT> maybeElemexpr(Ctx&);
 template<typename Ctx> Result<typename Ctx::ElemListT> elemlist(Ctx&, bool);
 template<typename Ctx> MaybeResult<> elem(Ctx&);
@@ -2648,6 +2649,23 @@ template<typename Ctx> MaybeResult<> export_(Ctx& ctx) {
   return Ok{};
 }
 
+// start ::= '(' 'start' funcidx ')'
+template<typename Ctx> MaybeResult<> start(Ctx& ctx) {
+  auto pos = ctx.in.getPos();
+  if (!ctx.in.takeSExprStart("start"sv)) {
+    return {};
+  }
+  auto func = funcidx(ctx);
+  CHECK_ERR(func);
+
+  CHECK_ERR(ctx.addStart(*func, pos));
+
+  if (!ctx.in.takeRParen()) {
+    return ctx.in.err("expected end of start declaration");
+  }
+  return Ok{};
+}
+
 // elemexpr ::= '(' 'item' expr ')' | '(' instr ')'
 template<typename Ctx>
 MaybeResult<typename Ctx::ExprT> maybeElemexpr(Ctx& ctx) {
@@ -2896,6 +2914,10 @@ template<typename Ctx> MaybeResult<> modulefield(Ctx& ctx) {
     CHECK_ERR(res);
     return Ok{};
   }
+  if (auto res = start(ctx)) {
+    CHECK_ERR(res);
+    return Ok{};
+  }
   if (auto res = elem(ctx)) {
     CHECK_ERR(res);
     return Ok{};

src/parser/wat-parser.cpp

@@ -171,6 +171,7 @@ Result<> parseModule(Module& wasm, std::string_view input) {
                      *typeIndices);
     CHECK_ERR(parseDefs(ctx, decls.tableDefs, table));
     CHECK_ERR(parseDefs(ctx, decls.globalDefs, global));
+    CHECK_ERR(parseDefs(ctx, decls.startDefs, start));
     CHECK_ERR(parseDefs(ctx, decls.elemDefs, elem));
     CHECK_ERR(parseDefs(ctx, decls.dataDefs, data));
 

test/lit/wat-kitchen-sink.wast

@@ -432,11 +432,15 @@
  (export "exported-global" (global $g1))
  (export "exported-tag" (tag 0))
 
+ ;; start function
+ ;; CHECK:      (export "exported-tag" (tag $imported))
+
+ ;; CHECK:      (start $return-none)
+ (start $return-none)
+
  ;; functions
  (func)
 
- ;; CHECK:      (export "exported-tag" (tag $imported))
-
  ;; CHECK:      (func $2 (type $void)
  ;; CHECK-NEXT:  (nop)
  ;; CHECK-NEXT: )