Function Bodies - GNU Compiler Collection (GCC) Internals

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9.6.2 Function Bodies

A function that has a definition in the current translation unit will have a non-NULL DECL_INITIAL. However, back ends should not make use of the particular value given by DECL_INITIAL.

The DECL_SAVED_TREE macro will give the complete body of the function.

9.6.2.1 Statements

There are tree nodes corresponding to all of the source-level statement constructs, used within the C and C++ frontends. These are enumerated here, together with a list of the various macros that can be used to obtain information about them. There are a few macros that can be used with all statements:

STMT_IS_FULL_EXPR_P: In C++, statements normally constitute “full expressions”; temporaries created during a statement are destroyed when the statement is complete. However, G++ sometimes represents expressions by statements; these statements will not have STMT_IS_FULL_EXPR_P set. Temporaries created during such statements should be destroyed when the innermost enclosing statement with STMT_IS_FULL_EXPR_P set is exited.

Here is the list of the various statement nodes, and the macros used to access them. This documentation describes the use of these nodes in non-template functions (including instantiations of template functions). In template functions, the same nodes are used, but sometimes in slightly different ways.

Many of the statements have substatements. For example, a while loop will have a body, which is itself a statement. If the substatement is NULL_TREE, it is considered equivalent to a statement consisting of a single ;, i.e., an expression statement in which the expression has been omitted. A substatement may in fact be a list of statements, connected via their TREE_CHAINs. So, you should always process the statement tree by looping over substatements, like this:

     void process_stmt (stmt)
          tree stmt;
     {
       while (stmt)
         {
           switch (TREE_CODE (stmt))
             {
             case IF_STMT:
               process_stmt (THEN_CLAUSE (stmt));
               /* More processing here.  */
               break;
     
             ...
             }
     
           stmt = TREE_CHAIN (stmt);
         }
     }

In other words, while the then clause of an if statement in C++ can be only one statement (although that one statement may be a compound statement), the intermediate representation will sometimes use several statements chained together.

ASM_EXPR

Used to represent an inline assembly statement. For an inline assembly statement like:

          asm ("mov x, y");

The ASM_STRING macro will return a STRING_CST node for "mov x, y". If the original statement made use of the extended-assembly syntax, then ASM_OUTPUTS, ASM_INPUTS, and ASM_CLOBBERS will be the outputs, inputs, and clobbers for the statement, represented as STRING_CST nodes. The extended-assembly syntax looks like:

          asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

The first string is the ASM_STRING, containing the instruction template. The next two strings are the output and inputs, respectively; this statement has no clobbers. As this example indicates, “plain” assembly statements are merely a special case of extended assembly statements; they have no cv-qualifiers, outputs, inputs, or clobbers. All of the strings will be NUL-terminated, and will contain no embedded NUL-characters.

If the assembly statement is declared volatile, or if the statement was not an extended assembly statement, and is therefore implicitly volatile, then the predicate ASM_VOLATILE_P will hold of the ASM_EXPR. ASM_USES and ASM_LABEL are for CW assembly syntax only, providing REG_USE and label declaration information inside ASM_EXPR tree.

BREAK_STMT

Used to represent a break statement. There are no additional fields.

CASE_LABEL_EXPR

Use to represent a case label, range of case labels, or a default label. If CASE_LOW is NULL_TREE, then this is a default label. Otherwise, if CASE_HIGH is NULL_TREE, then this is an ordinary case label. In this case, CASE_LOW is an expression giving the value of the label. Both CASE_LOW and CASE_HIGH are INTEGER_CST nodes. These values will have the same type as the condition expression in the switch statement.

Otherwise, if both CASE_LOW and CASE_HIGH are defined, the statement is a range of case labels. Such statements originate with the extension that allows users to write things of the form:

          case 2 ... 5:

The first value will be CASE_LOW, while the second will be CASE_HIGH.

CLEANUP_STMT

Used to represent an action that should take place upon exit from the enclosing scope. Typically, these actions are calls to destructors for local objects, but back ends cannot rely on this fact. If these nodes are in fact representing such destructors, CLEANUP_DECL will be the VAR_DECL destroyed. Otherwise, CLEANUP_DECL will be NULL_TREE. In any case, the CLEANUP_EXPR is the expression to execute. The cleanups executed on exit from a scope should be run in the reverse order of the order in which the associated CLEANUP_STMTs were encountered.

CONTINUE_STMT

Used to represent a continue statement. There are no additional fields.

CTOR_STMT

Used to mark the beginning (if CTOR_BEGIN_P holds) or end (if CTOR_END_P holds of the main body of a constructor. See also SUBOBJECT for more information on how to use these nodes.

DECL_STMT

Used to represent a local declaration. The DECL_STMT_DECL macro can be used to obtain the entity declared. This declaration may be a LABEL_DECL, indicating that the label declared is a local label. (As an extension, GCC allows the declaration of labels with scope.) In C, this declaration may be a FUNCTION_DECL, indicating the use of the GCC nested function extension. For more information, see Functions.

DO_STMT

Used to represent a do loop. The body of the loop is given by DO_BODY while the termination condition for the loop is given by DO_COND. The condition for a do-statement is always an expression.

EMPTY_CLASS_EXPR

Used to represent a temporary object of a class with no data whose address is never taken. (All such objects are interchangeable.) The TREE_TYPE represents the type of the object.

EXPR_STMT

Used to represent an expression statement. Use EXPR_STMT_EXPR to obtain the expression.

FOR_STMT

Used to represent a for statement. The FOR_INIT_STMT is the initialization statement for the loop. The FOR_COND is the termination condition. The FOR_EXPR is the expression executed right before the FOR_COND on each loop iteration; often, this expression increments a counter. The body of the loop is given by FOR_BODY. Note that FOR_INIT_STMT and FOR_BODY return statements, while FOR_COND and FOR_EXPR return expressions.

GOTO_EXPR

Used to represent a goto statement. The GOTO_DESTINATION will usually be a LABEL_DECL. However, if the “computed goto” extension has been used, the GOTO_DESTINATION will be an arbitrary expression indicating the destination. This expression will always have pointer type.

HANDLER

Used to represent a C++ catch block. The HANDLER_TYPE is the type of exception that will be caught by this handler; it is equal (by pointer equality) to NULL if this handler is for all types. HANDLER_PARMS is the DECL_STMT for the catch parameter, and HANDLER_BODY is the code for the block itself.

IF_STMT

Used to represent an if statement. The IF_COND is the expression.

If the condition is a TREE_LIST, then the TREE_PURPOSE is a statement (usually a DECL_STMT). Each time the condition is evaluated, the statement should be executed. Then, the TREE_VALUE should be used as the conditional expression itself. This representation is used to handle C++ code like this:

          if (int i = 7) ...

where there is a new local variable (or variables) declared within the condition.

The THEN_CLAUSE represents the statement given by the then condition, while the ELSE_CLAUSE represents the statement given by the else condition.

LABEL_EXPR

Used to represent a label. The LABEL_DECL declared by this statement can be obtained with the LABEL_EXPR_LABEL macro. The IDENTIFIER_NODE giving the name of the label can be obtained from the LABEL_DECL with DECL_NAME.

RETURN_STMT

Used to represent a return statement. The RETURN_EXPR is the expression returned; it will be NULL_TREE if the statement was just

          return;

SUBOBJECT

In a constructor, these nodes are used to mark the point at which a subobject of this is fully constructed. If, after this point, an exception is thrown before a CTOR_STMT with CTOR_END_P set is encountered, the SUBOBJECT_CLEANUP must be executed. The cleanups must be executed in the reverse order in which they appear.

SWITCH_STMT

Used to represent a switch statement. The SWITCH_STMT_COND is the expression on which the switch is occurring. See the documentation for an IF_STMT for more information on the representation used for the condition. The SWITCH_STMT_BODY is the body of the switch statement. The SWITCH_STMT_TYPE is the original type of switch expression as given in the source, before any compiler conversions.

TRY_BLOCK

Used to represent a try block. The body of the try block is given by TRY_STMTS. Each of the catch blocks is a HANDLER node. The first handler is given by TRY_HANDLERS. Subsequent handlers are obtained by following the TREE_CHAIN link from one handler to the next. The body of the handler is given by HANDLER_BODY.

If CLEANUP_P holds of the TRY_BLOCK, then the TRY_HANDLERS will not be a HANDLER node. Instead, it will be an expression that should be executed if an exception is thrown in the try block. It must rethrow the exception after executing that code. And, if an exception is thrown while the expression is executing, terminate must be called.

USING_STMT

Used to represent a using directive. The namespace is given by USING_STMT_NAMESPACE, which will be a NAMESPACE_DECL. This node is needed inside template functions, to implement using directives during instantiation.

WHILE_STMT

Used to represent a while loop. The WHILE_COND is the termination condition for the loop. See the documentation for an IF_STMT for more information on the representation used for the condition.

The WHILE_BODY is the body of the loop.