C++ front-end: support constexpr

Mark `constexpr` symbols as macros and use direct replacement
(non-function symbols) or in-place evaluation (uses of `constexpr`
function symbols).

Author: tautschnig

regression/cpp/constexpr1/main.cpp

@@ -14,6 +14,18 @@ constexpr int some_other_value =
 
 static_assert(some_other_value == 2, "some_other_value == 2");
 
+constexpr int some_function2(int a)
+{
+  int b;
+  a = a + 1;
+  b = a;
+  return b + 1;
+}
+
+constexpr int some_other_value2 = some_function2(1);
+
+static_assert(some_other_value2 == 3, "some_other_value == 2");
+
 int main()
 {
 }

regression/cpp/constexpr1/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.cpp
 -std=c++11
 ^EXIT=0$

src/cpp/cpp_convert_type.cpp

@@ -63,8 +63,6 @@ void cpp_convert_typet::read_rec(const typet &type)
     ++char16_t_count;
   else if(type.id()==ID_char32_t)
     ++char32_t_count;
-  else if(type.id()==ID_constexpr)
-    c_qualifiers.is_constant = true;
   else if(type.id()==ID_function_type)
   {
     read_function_type(type);

src/cpp/cpp_declarator_converter.cpp

@@ -456,7 +456,8 @@ symbolt &cpp_declarator_convertert::convert_new_symbol(
   symbol.is_weak = storage_spec.is_weak();
   symbol.module=cpp_typecheck.module;
   symbol.is_type=is_typedef;
-  symbol.is_macro=is_typedef && !is_template_parameter;
+  symbol.is_macro =
+    (is_typedef && !is_template_parameter) || storage_spec.is_constexpr();
   symbol.pretty_name=pretty_name;
 
   if(is_code && !symbol.is_type)
@@ -493,7 +494,7 @@ symbolt &cpp_declarator_convertert::convert_new_symbol(
       storage_spec.is_thread_local();
 
     symbol.is_file_local =
-      symbol.is_macro ||
+      (symbol.is_macro && !storage_spec.is_constexpr()) ||
       (!cpp_typecheck.cpp_scopes.current_scope().is_global_scope() &&
        !storage_spec.is_extern()) ||
       (cpp_typecheck.cpp_scopes.current_scope().is_global_scope() &&
@@ -553,10 +554,14 @@ symbolt &cpp_declarator_convertert::convert_new_symbol(
   // do the value
   if(!new_symbol->is_type)
   {
-    if(is_code && declarator.type().id()!=ID_template)
-      cpp_typecheck.add_method_body(new_symbol);
-
-    if(!is_code)
+    if(is_code)
+    {
+      if(new_symbol->is_macro)
+        cpp_typecheck.convert_function(*new_symbol);
+      else if(declarator.type().id() != ID_template)
+        cpp_typecheck.add_method_body(new_symbol);
+    }
+    else
       cpp_typecheck.convert_initializer(*new_symbol);
   }
 

src/cpp/cpp_storage_spec.cpp

@@ -33,4 +33,6 @@ void cpp_storage_spect::read(const typet &type)
     set_asm();
   else if(type.id() == ID_weak)
     set_weak();
+  else if(type.id() == ID_constexpr)
+    set_constexpr();
 }

src/cpp/cpp_storage_spec.h

@@ -47,6 +47,10 @@ class cpp_storage_spect:public irept
   {
     return get_bool(ID_weak);
   }
+  bool is_constexpr() const
+  {
+    return get_bool(ID_constexpr);
+  }
 
   void set_static()       { set(ID_static, true); }
   void set_extern()       { set(ID_extern, true); }
@@ -59,11 +63,16 @@ class cpp_storage_spect:public irept
   {
     set(ID_weak, true);
   }
+  void set_constexpr()
+  {
+    set(ID_constexpr, true);
+  }
 
   bool is_empty() const
   {
     return !is_static() && !is_extern() && !is_auto() && !is_register() &&
-           !is_mutable() && !is_thread_local() && !is_asm() && !is_weak();
+           !is_mutable() && !is_thread_local() && !is_asm() && !is_weak() &&
+           !is_constexpr();
   }
 
   cpp_storage_spect &operator|=(const cpp_storage_spect &other)
@@ -84,6 +93,8 @@ class cpp_storage_spect:public irept
       set_asm();
     if(other.is_weak())
       set_weak();
+    if(other.is_constexpr())
+      set_constexpr();
 
     return *this;
   }

src/cpp/cpp_typecheck_expr.cpp

@@ -22,6 +22,7 @@ Author: Daniel Kroening, kroening@cs.cmu.edu
 #include <util/mathematical_types.h>
 #include <util/pointer_expr.h>
 #include <util/pointer_offset_size.h>
+#include <util/replace_symbol.h>
 #include <util/symbol_table_base.h>
 
 #include <ansi-c/c_qualifiers.h>
@@ -1820,6 +1821,71 @@ void cpp_typecheckt::typecheck_side_effect_function_call(
 
   add_implicit_dereference(expr);
 
+  if(auto sym_expr = expr_try_dynamic_cast<symbol_exprt>(expr.function()))
+  {
+    const auto &symbol = lookup(sym_expr->get_identifier());
+    if(symbol.is_macro)
+    {
+      // constexpr functions evaluated using a mini interpreter
+      const auto &code_type = to_code_type(symbol.type);
+      // PRECONDITION(code_type.return_type().id() != ID_empty);
+      PRECONDITION(expr.arguments().size() == code_type.parameters().size());
+      replace_symbolt value_map;
+      auto param_it = code_type.parameters().begin();
+      for(const auto &arg : expr.arguments())
+      {
+        value_map.insert(
+          symbol_exprt{param_it->get_identifier(), param_it->type()},
+          typecast_exprt::conditional_cast(arg, param_it->type()));
+        ++param_it;
+      }
+      const auto &block = to_code_block(to_code(symbol.value));
+      for(const auto &stmt : block.statements())
+      {
+        if(
+          auto return_stmt = expr_try_dynamic_cast<code_frontend_returnt>(stmt))
+        {
+          PRECONDITION(return_stmt->has_return_value());
+          exprt tmp = return_stmt->return_value();
+          value_map.replace(tmp);
+          expr.swap(tmp);
+          return;
+        }
+        else if(auto expr_stmt = expr_try_dynamic_cast<code_expressiont>(stmt))
+        {
+          // C++14 and later only
+          if(
+            auto assign = expr_try_dynamic_cast<side_effect_expr_assignt>(
+              expr_stmt->expression()))
+          {
+            PRECONDITION(assign->lhs().id() == ID_symbol);
+            exprt rhs = assign->rhs();
+            value_map.replace(rhs);
+            value_map.set(to_symbol_expr(assign->lhs()), rhs);
+          }
+          else
+            UNIMPLEMENTED_FEATURE(
+              "constexpr with " + expr_stmt->expression().pretty());
+        }
+        else if(stmt.get_statement() == ID_decl_block)
+        {
+          // C++14 and later only
+          for(const auto &expect_decl : stmt.operands())
+          {
+            PRECONDITION(to_code(expect_decl).get_statement() == ID_decl);
+            PRECONDITION(!to_code_frontend_decl(to_code(expect_decl))
+                            .initial_value()
+                            .has_value());
+          }
+        }
+        else
+        {
+          UNIMPLEMENTED_FEATURE("constexpr with " + stmt.pretty());
+        }
+      }
+    }
+  }
+
   // we will deal with some 'special' functions here
   exprt tmp=do_special_functions(expr);
   if(tmp.is_not_nil())

src/cpp/cpp_typecheck_resolve.cpp

@@ -359,8 +359,16 @@ exprt cpp_typecheck_resolvet::convert_identifier(
     }
     else if(symbol.is_macro)
     {
-      e=symbol.value;
-      PRECONDITION(e.is_not_nil());
+      if(symbol.type.id() == ID_code)
+      {
+        // constexpr function
+        e = cpp_symbol_expr(symbol);
+      }
+      else
+      {
+        e = symbol.value;
+        PRECONDITION(e.is_not_nil());
+      }
     }
     else
     {

src/cpp/parse.cpp

@@ -764,11 +764,10 @@ bool Parser::isTypeSpecifier()
 {
   int t=lex.LookAhead(0);
 
-  return is_identifier(t) || t == TOK_SCOPE || t == TOK_CONSTEXPR ||
-         t == TOK_CONST || t == TOK_VOLATILE || t == TOK_RESTRICT ||
-         t == TOK_CHAR || t == TOK_INT || t == TOK_SHORT || t == TOK_LONG ||
-         t == TOK_CHAR16_T || t == TOK_CHAR32_T || t == TOK_WCHAR_T ||
-         t == TOK_COMPLEX // new !!!
+  return is_identifier(t) || t == TOK_SCOPE || t == TOK_CONST ||
+         t == TOK_VOLATILE || t == TOK_RESTRICT || t == TOK_CHAR ||
+         t == TOK_INT || t == TOK_SHORT || t == TOK_LONG || t == TOK_CHAR16_T ||
+         t == TOK_CHAR32_T || t == TOK_WCHAR_T || t == TOK_COMPLEX // new !!!
          || t == TOK_SIGNED || t == TOK_UNSIGNED || t == TOK_FLOAT ||
          t == TOK_DOUBLE || t == TOK_INT8 || t == TOK_INT16 || t == TOK_INT32 ||
          t == TOK_INT64 || t == TOK_GCC_INT128 || t == TOK_PTR32 ||
@@ -2017,7 +2016,7 @@ bool Parser::optMemberSpec(cpp_member_spect &member_spec)
 
 /*
   storage.spec : STATIC | EXTERN | AUTO | REGISTER | MUTABLE | ASM |
-                 THREAD_LOCAL
+                 THREAD_LOCAL | CONSTEXPR
 */
 bool Parser::optStorageSpec(cpp_storage_spect &storage_spec)
 {
@@ -2026,7 +2025,7 @@ bool Parser::optStorageSpec(cpp_storage_spect &storage_spec)
   if(
     t == TOK_STATIC || t == TOK_EXTERN || (t == TOK_AUTO && !cpp11) ||
     t == TOK_REGISTER || t == TOK_MUTABLE || t == TOK_GCC_ASM ||
-    t == TOK_THREAD_LOCAL)
+    t == TOK_THREAD_LOCAL || t == TOK_CONSTEXPR)
   {
     cpp_tokent tk;
     lex.get_token(tk);
@@ -2040,6 +2039,9 @@ bool Parser::optStorageSpec(cpp_storage_spect &storage_spec)
     case TOK_MUTABLE: storage_spec.set_mutable(); break;
     case TOK_GCC_ASM: storage_spec.set_asm(); break;
     case TOK_THREAD_LOCAL: storage_spec.set_thread_local(); break;
+    case TOK_CONSTEXPR:
+      storage_spec.set_constexpr();
+      break;
     default: UNREACHABLE;
     }
 
@@ -2050,30 +2052,24 @@ bool Parser::optStorageSpec(cpp_storage_spect &storage_spec)
 }
 
 /*
-  cv.qualify : (CONSTEXPR | CONST | VOLATILE | RESTRICT)+
+  cv.qualify : (CONST | VOLATILE | RESTRICT)+
 */
 bool Parser::optCvQualify(typet &cv)
 {
   for(;;)
   {
     int t=lex.LookAhead(0);
-    if(t==TOK_CONSTEXPR ||
-       t==TOK_CONST || t==TOK_VOLATILE || t==TOK_RESTRICT ||
-       t==TOK_PTR32 || t==TOK_PTR64 ||
-       t==TOK_GCC_ATTRIBUTE || t==TOK_GCC_ASM)
+    if(
+      t == TOK_CONST || t == TOK_VOLATILE || t == TOK_RESTRICT ||
+      t == TOK_PTR32 || t == TOK_PTR64 || t == TOK_GCC_ATTRIBUTE ||
+      t == TOK_GCC_ASM)
     {
       cpp_tokent tk;
       lex.get_token(tk);
       typet p;
 
       switch(t)
       {
-      case TOK_CONSTEXPR:
-        p=typet(ID_constexpr);
-        set_location(p, tk);
-        merge_types(p, cv);
-        break;
-
       case TOK_CONST:
         p=typet(ID_const);
         set_location(p, tk);