[FunC] Forbid impure operations inside pure functions

In stdlib, all existing pure functions are asm-implemented.
But since we introduced a `pure` keyword applicable to user-defined functions,
we need to check that they won't have any side effects
(exceptions, globals modification, etc.)

crypto/func/abscode.cpp

@@ -223,15 +223,6 @@ void VarDescrList::show(std::ostream& os) const {
   os << " ]\n";
 }
 
-void Op::flags_set_clear(int set, int clear) {
-  flags = (flags | set) & ~clear;
-  for (auto& op : block0) {
-    op.flags_set_clear(set, clear);
-  }
-  for (auto& op : block1) {
-    op.flags_set_clear(set, clear);
-  }
-}
 void Op::split_vars(const std::vector<TmpVar>& vars) {
   split_var_list(left, vars);
   split_var_list(right, vars);
@@ -296,7 +287,7 @@ void Op::show(std::ostream& os, const std::vector<TmpVar>& vars, std::string pfx
   if (noreturn()) {
     dis += "<noret> ";
   }
-  if (!is_pure()) {
+  if (impure()) {
     dis += "<impure> ";
   }
   switch (cl) {
@@ -469,12 +460,6 @@ void Op::show_block(std::ostream& os, const Op* block, const std::vector<TmpVar>
   os << pfx << "}";
 }
 
-void CodeBlob::flags_set_clear(int set, int clear) {
-  for (auto& op : ops) {
-    op.flags_set_clear(set, clear);
-  }
-}
-
 std::ostream& operator<<(std::ostream& os, const CodeBlob& code) {
   code.print(os);
   return os;

crypto/func/analyzer.cpp

@@ -360,10 +360,10 @@ bool Op::compute_used_vars(const CodeBlob& code, bool edit) {
     case _Tuple:
     case _UnTuple: {
       // left = EXEC right;
-      if (!next_var_info.count_used(left) && is_pure()) {
+      if (!next_var_info.count_used(left) && !impure()) {
         // all variables in `left` are not needed
         if (edit) {
-          disable();
+          set_disabled();
         }
         return std_compute_used_vars(true);
       }
@@ -372,7 +372,7 @@ bool Op::compute_used_vars(const CodeBlob& code, bool edit) {
     case _SetGlob: {
       // GLOB = right
       if (right.empty() && edit) {
-        disable();
+        set_disabled();
       }
       return std_compute_used_vars(right.empty());
     }
@@ -399,7 +399,7 @@ bool Op::compute_used_vars(const CodeBlob& code, bool edit) {
       }
       if (!cnt && edit) {
         // all variables in `left` are not needed
-        disable();
+        set_disabled();
       }
       return set_var_info(std::move(new_var_info));
     }
@@ -860,15 +860,45 @@ VarDescrList Op::fwd_analyze(VarDescrList values) {
   }
 }
 
-bool Op::set_noreturn(bool nr) {
+void Op::set_disabled(bool flag) {
-  if (nr) {
+  if (flag) {
+    flags |= _Disabled;
+  } else {
+    flags &= ~_Disabled;
+  }
+}
+
+
+bool Op::set_noreturn(bool flag) {
+  if (flag) {
     flags |= _NoReturn;
   } else {
     flags &= ~_NoReturn;
   }
-  return nr;
+  return flag;
 }
 
+void Op::set_impure(const CodeBlob &code) {
+  // todo calling this function with `code` is a bad design (flags are assigned after Op is constructed)
+  // later it's better to check this somewhere in code.emplace_back()
+  if (code.flags & CodeBlob::_ForbidImpure) {
+    throw src::ParseError(where, "An impure operation in a pure function");
+  }
+  flags |= _Impure;
+}
+
+void Op::set_impure(const CodeBlob &code, bool flag) {
+  if (flag) {
+    if (code.flags & CodeBlob::_ForbidImpure) {
+      throw src::ParseError(where, "An impure operation in a pure function");
+    }
+    flags |= _Impure;
+  } else {
+    flags &= ~_Impure;
+  }
+}
+
+
 bool Op::mark_noreturn() {
   switch (cl) {
     case _Nop:
@@ -888,13 +918,14 @@ bool Op::mark_noreturn() {
     case _Call:
       return set_noreturn(next->mark_noreturn());
     case _Return:
-      return set_noreturn(true);
+      return set_noreturn();
     case _If:
     case _TryCatch:
+      // note, that & | (not && ||) here and below is mandatory to invoke both left and right calls
       return set_noreturn((block0->mark_noreturn() & (block1 && block1->mark_noreturn())) | next->mark_noreturn());
     case _Again:
       block0->mark_noreturn();
-      return set_noreturn(true);
+      return set_noreturn();
     case _Until:
       return set_noreturn(block0->mark_noreturn() | next->mark_noreturn());
     case _While:

crypto/func/auto-tests/tests/invalid-pure-1.fc

@@ -0,0 +1,18 @@
+int f_impure();
+
+int f_pure() pure {
+    return f_impure();
+}
+
+int main() {
+    return f_pure();
+}
+
+{-
+@compilation_should_fail
+@stderr
+"""
+An impure operation in a pure function
+return f_impure();
+"""
+-}

crypto/func/auto-tests/tests/invalid-pure-2.fc

@@ -0,0 +1,25 @@
+builder begin_cell() pure asm "NEWC";
+
+global int g;
+
+(builder) f_pure() pure {
+    var g;  // strange, but this doesn't make a variable local, it still refers to a global one
+    builder b = begin_cell();
+    g = g + 1;
+    return b;
+}
+
+int main() {
+    g = 0;
+    f_pure();
+    return g;
+}
+
+{-
+@compilation_should_fail
+@stderr
+"""
+An impure operation in a pure function
+g = g + 1;
+"""
+-}

crypto/func/auto-tests/tests/invalid-pure-3.fc

@@ -0,0 +1,23 @@
+(int, int, int, int) compute_data_size?(cell c, int max_cells) pure asm "CDATASIZEQ NULLSWAPIFNOT2 NULLSWAPIFNOT";
+builder begin_cell() pure asm "NEWC";
+cell end_cell(builder b) pure asm "ENDC";
+
+(int, int) validate_input(cell input) pure {
+    var (x, y, z, correct) = compute_data_size?(input, 10);
+    throw_unless(102, correct);
+}
+
+int main() pure {
+    cell c = begin_cell().end_cell();
+    validate_input(c);
+    return 0;
+}
+
+{-
+@compilation_should_fail
+@stderr
+"""
+An impure operation in a pure function
+throw_unless
+"""
+-}

crypto/func/auto-tests/tests/pure-functions.fc

@@ -0,0 +1,47 @@
+cell get_data() pure asm "c4 PUSH";
+slice begin_parse(cell c) pure asm "CTOS";
+builder begin_cell() pure asm "NEWC";
+cell end_cell(builder b) pure asm "ENDC";
+() set_data(cell c) asm "c4 POP";
+
+int f_pure2() pure;
+
+int f_pure1() pure {
+    return f_pure2();
+}
+
+int f_pure2() pure {
+    return 2;
+}
+
+(int, int) get_contract_data() pure {
+    cell c = get_data();
+    slice cs = c.begin_parse();
+    cs~load_bits(32);
+    int value = cs~load_uint(16);
+    return (1, value);
+}
+
+() save_contract_data(int value) {
+    builder b = begin_cell().store_int(1, 32).store_uint(value, 16);
+    set_data(b.end_cell());
+}
+
+int test1() pure method_id(101) {
+    return f_pure1();
+}
+
+int test2(int value) method_id(102) {
+    save_contract_data(value);
+    (_, var restored) = get_contract_data();
+    return restored;
+}
+
+() main() { return (); }
+
+{-
+
+TESTCASE | 101 |     | 2
+TESTCASE | 102 |  44 | 44
+
+-}

crypto/func/func.h

@@ -593,16 +593,19 @@ struct Op {
      SymDef* _fun = nullptr)
       : cl(_cl), flags(0), fun_ref(_fun), where(_where), left(std::move(_left)), right(std::move(_right)) {
   }
-  bool disabled() const {
+
-    return flags & _Disabled;
+  bool disabled() const { return flags & _Disabled; }
-  }
+  void set_disabled() { flags |= _Disabled; }
-  bool enabled() const {
+  void set_disabled(bool flag);
-    return !disabled();
+
-  }
+  bool noreturn() const { return flags & _NoReturn; }
-  void disable() {
+  bool set_noreturn() { flags |= _NoReturn; return true; }
-    flags |= _Disabled;
+  bool set_noreturn(bool flag);
-  }
+
-  void flags_set_clear(int set, int clear);
+  bool impure() const { return flags & _Impure; }
+  void set_impure(const CodeBlob &code);
+  void set_impure(const CodeBlob &code, bool flag);
+
   void show(std::ostream& os, const std::vector<TmpVar>& vars, std::string pfx = "", int mode = 0) const;
   void show_var_list(std::ostream& os, const std::vector<var_idx_t>& idx_list, const std::vector<TmpVar>& vars) const;
   void show_var_list(std::ostream& os, const std::vector<VarDescr>& list, const std::vector<TmpVar>& vars) const;
@@ -618,17 +621,10 @@ struct Op {
   bool set_var_info_except(VarDescrList&& new_var_info, const std::vector<var_idx_t>& var_list);
   void prepare_args(VarDescrList values);
   VarDescrList fwd_analyze(VarDescrList values);
-  bool set_noreturn(bool nr);
   bool mark_noreturn();
-  bool noreturn() const {
-    return flags & _NoReturn;
-  }
   bool is_empty() const {
     return cl == _Nop && !next;
   }
-  bool is_pure() const {
-    return !(flags & _Impure);
-  }
   bool generate_code_step(Stack& stack);
   void generate_code_all(Stack& stack);
   Op& last() {
@@ -689,7 +685,7 @@ typedef std::vector<FormalArg> FormalArgList;
 struct AsmOpList;
 
 struct CodeBlob {
-  enum { _AllowPostModification = 1, _ComputeAsmLtr = 2 };
+  enum { _AllowPostModification = 1, _ComputeAsmLtr = 2, _ForbidImpure = 4 };
   int var_cnt, in_var_cnt, op_cnt;
   TypeExpr* ret_type;
   std::string name;
@@ -733,7 +729,6 @@ struct CodeBlob {
     pop_cur();
   }
   void simplify_var_types();
-  void flags_set_clear(int set, int clear);
   void prune_unreachable_code();
   void fwd_analyze();
   void mark_noreturn();

crypto/func/gen-abscode.cpp

@@ -229,7 +229,7 @@ var_idx_t Expr::new_tmp(CodeBlob& code) const {
 void add_set_globs(CodeBlob& code, std::vector<std::pair<SymDef*, var_idx_t>>& globs, const SrcLocation& here) {
   for (const auto& p : globs) {
     auto& op = code.emplace_back(here, Op::_SetGlob, std::vector<var_idx_t>{}, std::vector<var_idx_t>{ p.second }, p.first);
-    op.flags |= Op::_Impure;
+    op.set_impure(code);
   }
 }
 
@@ -289,7 +289,7 @@ std::vector<var_idx_t> pre_compile_tensor(const std::vector<Expr *> args, CodeBl
       if (code.flags & CodeBlob::_AllowPostModification) {
         if (!lval_globs && (var.cls & TmpVar::_Named)) {
           Op *op = &code.emplace_back(nullptr, Op::_Let, std::vector<var_idx_t>(), std::vector<var_idx_t>());
-          op->flags |= Op::_Disabled;
+          op->set_disabled();
           var.on_modification.push_back([modified_vars, i, j, op, done = false](const SrcLocation &here) mutable {
             if (!done) {
               done = true;
@@ -319,7 +319,7 @@ std::vector<var_idx_t> pre_compile_tensor(const std::vector<Expr *> args, CodeBl
     var_idx_t v2 = code.create_tmp_var(code.vars[v].v_type, code.vars[v].where.get());
     m.op->left = {v2};
     m.op->right = {v};
-    m.op->flags &= ~Op::_Disabled;
+    m.op->set_disabled(false);
     v = v2;
   }
   std::vector<var_idx_t> res;
@@ -371,7 +371,7 @@ std::vector<var_idx_t> Expr::pre_compile(CodeBlob& code, std::vector<std::pair<S
       auto rvect = new_tmp_vect(code);
       auto& op = code.emplace_back(here, Op::_Call, rvect, res, applied_sym);
       if (flags & _IsImpure) {
-        op.flags |= Op::_Impure;
+        op.set_impure(code);
       }
       return rvect;
     }
@@ -389,7 +389,7 @@ std::vector<var_idx_t> Expr::pre_compile(CodeBlob& code, std::vector<std::pair<S
         auto rvect = new_tmp_vect(code);
         auto& op = code.emplace_back(here, Op::_Call, rvect, std::move(res), args[0]->sym);
         if (args[0]->flags & _IsImpure) {
-          op.flags |= Op::_Impure;
+          op.set_impure(code);
         }
         return rvect;
       } else {

crypto/func/parse-func.cpp

@@ -1216,7 +1216,7 @@ blk_fl::val parse_stmt(Lexer& lex, CodeBlob& code) {
   }
 }
 
-CodeBlob* parse_func_body(Lexer& lex, FormalArgList arg_list, TypeExpr* ret_type) {
+CodeBlob* parse_func_body(Lexer& lex, FormalArgList arg_list, TypeExpr* ret_type, bool marked_as_pure) {
   lex.expect('{');
   CodeBlob* blob = new CodeBlob{ret_type};
   if (pragma_allow_post_modification.enabled()) {
@@ -1225,6 +1225,9 @@ CodeBlob* parse_func_body(Lexer& lex, FormalArgList arg_list, TypeExpr* ret_type
   if (pragma_compute_asm_ltr.enabled()) {
     blob->flags |= CodeBlob::_ComputeAsmLtr;
   }
+  if (marked_as_pure) {
+    blob->flags |= CodeBlob::_ForbidImpure;
+  }
   blob->import_params(std::move(arg_list));
   blk_fl::val res = blk_fl::init;
   bool warned = false;
@@ -1604,7 +1607,10 @@ void parse_func_def(Lexer& lex) {
     if (func_sym_code->code) {
       lex.cur().error("redefinition of function `"s + func_name.str + "`");
     }
-    CodeBlob* code = parse_func_body(lex, arg_list, ret_type);
+    if (marked_as_pure && ret_type->get_width() == 0) {
+      lex.cur().error("a pure function should return something, otherwise it will be optimized out anyway");
+    }
+    CodeBlob* code = parse_func_body(lex, arg_list, ret_type, marked_as_pure);
     code->name = func_name.str;
     code->loc = loc;
     // code->print(std::cerr);  // !!!DEBUG!!!