GNU Compiler Collection (GCC) Internals: Basic Statements

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#### 11.7.1 Basic Statements

`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`.

`DECL_EXPR`

  Used to represent a local declaration. The `DECL_EXPR_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.

`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`.

`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.

`RETURN_EXPR`

  Used to represent a `return` statement. Operand 0 represents the value to return. It should either be the `RESULT_DECL` for the containing function, or a `MODIFY_EXPR` or `INIT_EXPR` setting the function’s `RESULT_DECL`. It will be `NULL_TREE` if the statement was just

      return;

`LOOP_EXPR`

  These nodes represent “infinite” loops. The `LOOP_EXPR_BODY` represents the body of the loop. It should be executed forever, unless an `EXIT_EXPR` is encountered.

`EXIT_EXPR`

  These nodes represent conditional exits from the nearest enclosing `LOOP_EXPR`. The single operand is the condition; if it is nonzero, then the loop should be exited. An `EXIT_EXPR` will only appear within a `LOOP_EXPR`.

`SWITCH_EXPR`

  Used to represent a `switch` statement. The `SWITCH_COND` is the expression on which the switch is occurring. The `SWITCH_BODY` is the body of the switch statement. `SWITCH_ALL_CASES_P` is true if the switch includes a default label or the case label ranges cover all possible values of the condition expression.

  Note that `TREE_TYPE` for a `SWITCH_EXPR` represents the original type of switch expression as given in the source, before any compiler conversions, instead of the type of the switch expression itself (which is not meaningful).

`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`.

`DEBUG_BEGIN_STMT`

  Marks the beginning of a source statement, for purposes of debug information generation.

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