### 9.8 Expressions

The internal representation for expressions is for the most part quite straightforward. However, there are a few facts that one must bear in mind. In particular, the expression “tree” is actually a directed acyclic graph. (For example there may be many references to the integer constant zero throughout the source program; many of these will be represented by the same expression node.) You should not rely on certain kinds of node being shared, nor should rely on certain kinds of nodes being unshared.

The following macros can be used with all expression nodes:

`TREE_TYPE`
  Returns the type of the expression. This value may not be precisely the same type that would be given the expression in the original program.

In what follows, some nodes that one might expect to always have type `bool` are documented to have either integral or boolean type. At some point in the future, the C front end may also make use of this same intermediate representation, and at this point these nodes will certainly have integral type. The previous sentence is not meant to imply that the C++ front end does not or will not give these nodes integral type.

Below, we list the various kinds of expression nodes. Except where noted otherwise, the operands to an expression are accessed using the `TREE_OPERAND` macro. For example, to access the first operand to a binary plus expression `expr`, use:

         TREE_OPERAND (expr, 0)

As this example indicates, the operands are zero-indexed.

All the expressions starting with `OMP_` represent directives and clauses used by the OpenMP API http://www.openmp.org/.

The table below begins with constants, moves on to unary expressions, then proceeds to binary expressions, and concludes with various other kinds of expressions:

`INTEGER_CST`
  These nodes represent integer constants. Note that the type of these constants is obtained with `TREE_TYPE`; they are not always of type `int`. In particular, `char` constants are represented with `INTEGER_CST` nodes. The value of the integer constant `e` is given by

                ((TREE_INT_CST_HIGH (e) << HOST_BITS_PER_WIDE_INT)
                + TREE_INST_CST_LOW (e))

  HOST_BITS_PER_WIDE_INT is at least thirty-two on all platforms. Both `TREE_INT_CST_HIGH` and `TREE_INT_CST_LOW` return a `HOST_WIDE_INT`. The value of an `INTEGER_CST` is interpreted as a signed or unsigned quantity depending on the type of the constant. In general, the expression given above will overflow, so it should not be used to calculate the value of the constant.

  The variable `integer_zero_node` is an integer constant with value zero. Similarly, `integer_one_node` is an integer constant with value one. The `size_zero_node` and `size_one_node` variables are analogous, but have type `size_t` rather than `int`.

  The function `tree_int_cst_lt` is a predicate which holds if its first argument is less than its second. Both constants are assumed to have the same signedness (i.e., either both should be signed or both should be unsigned.) The full width of the constant is used when doing the comparison; the usual rules about promotions and conversions are ignored. Similarly, `tree_int_cst_equal` holds if the two constants are equal. The `tree_int_cst_sgn` function returns the sign of a constant. The value is `1`, `0`, or `-1` according on whether the constant is greater than, equal to, or less than zero. Again, the signedness of the constant's type is taken into account; an unsigned constant is never less than zero, no matter what its bit-pattern.

`REAL_CST`
  FIXME: Talk about how to obtain representations of this constant, do comparisons, and so forth.
`COMPLEX_CST`
  These nodes are used to represent complex number constants, that is a `__complex__` whose parts are constant nodes. The `TREE_REALPART` and `TREE_IMAGPART` return the real and the imaginary parts respectively.
`VECTOR_CST`
  These nodes are used to represent vector constants, whose parts are constant nodes. Each individual constant node is either an integer or a double constant node. The first operand is a `TREE_LIST` of the constant nodes and is accessed through `TREE_VECTOR_CST_ELTS`.
`STRING_CST`
  These nodes represent string-constants. The `TREE_STRING_LENGTH` returns the length of the string, as an `int`. The `TREE_STRING_POINTER` is a `char*` containing the string itself. The string may not be `NUL`-terminated, and it may contain embedded `NUL` characters. Therefore, the `TREE_STRING_LENGTH` includes the trailing `NUL` if it is present.

  For wide string constants, the `TREE_STRING_LENGTH` is the number of bytes in the string, and the `TREE_STRING_POINTER` points to an array of the bytes of the string, as represented on the target system (that is, as integers in the target endianness). Wide and non-wide string constants are distinguished only by the `TREE_TYPE` of the `STRING_CST`.

  FIXME: The formats of string constants are not well-defined when the target system bytes are not the same width as host system bytes.

`PTRMEM_CST`
  These nodes are used to represent pointer-to-member constants. The `PTRMEM_CST_CLASS` is the class type (either a `RECORD_TYPE` or `UNION_TYPE` within which the pointer points), and the `PTRMEM_CST_MEMBER` is the declaration for the pointed to object. Note that the `DECL_CONTEXT` for the `PTRMEM_CST_MEMBER` is in general different from the `PTRMEM_CST_CLASS`. For example, given:

                struct B { int i; };
                struct D : public B {};
                int D::*dp = &D::i;

  The `PTRMEM_CST_CLASS` for `&D::i` is `D`, even though the `DECL_CONTEXT` for the `PTRMEM_CST_MEMBER` is `B`, since `B::i` is a member of `B`, not `D`.

`VAR_DECL`
  These nodes represent variables, including static data members. For more information, see Declarations.
`NEGATE_EXPR`
  These nodes represent unary negation of the single operand, for both integer and floating-point types. The type of negation can be determined by looking at the type of the expression.

  The behavior of this operation on signed arithmetic overflow is controlled by the `flag_wrapv` and `flag_trapv` variables.

`ABS_EXPR`
  These nodes represent the absolute value of the single operand, for both integer and floating-point types. This is typically used to implement the `abs`, `labs` and `llabs` builtins for integer types, and the `fabs`, `fabsf` and `fabsl` builtins for floating point types. The type of abs operation can be determined by looking at the type of the expression.

  This node is not used for complex types. To represent the modulus or complex abs of a complex value, use the `BUILT_IN_CABS`, `BUILT_IN_CABSF` or `BUILT_IN_CABSL` builtins, as used to implement the C99 `cabs`, `cabsf` and `cabsl` built-in functions.

`BIT_NOT_EXPR`
  These nodes represent bitwise complement, and will always have integral type. The only operand is the value to be complemented.
`TRUTH_NOT_EXPR`
  These nodes represent logical negation, and will always have integral (or boolean) type. The operand is the value being negated. The type of the operand and that of the result are always of `BOOLEAN_TYPE` or `INTEGER_TYPE`.
`PREDECREMENT_EXPR`
`PREINCREMENT_EXPR`
`POSTDECREMENT_EXPR`
`POSTINCREMENT_EXPR`
  These nodes represent increment and decrement expressions. The value of the single operand is computed, and the operand incremented or decremented. In the case of `PREDECREMENT_EXPR` and `PREINCREMENT_EXPR`, the value of the expression is the value resulting after the increment or decrement; in the case of `POSTDECREMENT_EXPR` and `POSTINCREMENT_EXPR` is the value before the increment or decrement occurs. The type of the operand, like that of the result, will be either integral, boolean, or floating-point.
`ADDR_EXPR`
  These nodes are used to represent the address of an object. (These expressions will always have pointer or reference type.) The operand may be another expression, or it may be a declaration.

  As an extension, GCC allows users to take the address of a label. In this case, the operand of the `ADDR_EXPR` will be a `LABEL_DECL`. The type of such an expression is `void*`.

  If the object addressed is not an lvalue, a temporary is created, and the address of the temporary is used.

`INDIRECT_REF`
  These nodes are used to represent the object pointed to by a pointer. The operand is the pointer being dereferenced; it will always have pointer or reference type.
`FIX_TRUNC_EXPR`
  These nodes represent conversion of a floating-point value to an integer. The single operand will have a floating-point type, while the complete expression will have an integral (or boolean) type. The operand is rounded towards zero.
`FLOAT_EXPR`
  These nodes represent conversion of an integral (or boolean) value to a floating-point value. The single operand will have integral type, while the complete expression will have a floating-point type.

  FIXME: How is the operand supposed to be rounded? Is this dependent on `-mieee`?

`COMPLEX_EXPR`
  These nodes are used to represent complex numbers constructed from two expressions of the same (integer or real) type. The first operand is the real part and the second operand is the imaginary part.
`CONJ_EXPR`
  These nodes represent the conjugate of their operand.
`REALPART_EXPR`
`IMAGPART_EXPR`
  These nodes represent respectively the real and the imaginary parts of complex numbers (their sole argument).
`NON_LVALUE_EXPR`
  These nodes indicate that their one and only operand is not an lvalue. A back end can treat these identically to the single operand.
`NOP_EXPR`
  These nodes are used to represent conversions that do not require any code-generation. For example, conversion of a `char*` to an `int*` does not require any code be generated; such a conversion is represented by a `NOP_EXPR`. The single operand is the expression to be converted. The conversion from a pointer to a reference is also represented with a `NOP_EXPR`.
`CONVERT_EXPR`
  These nodes are similar to `NOP_EXPR`s, but are used in those situations where code may need to be generated. For example, if an `int*` is converted to an `int` code may need to be generated on some platforms. These nodes are never used for C++-specific conversions, like conversions between pointers to different classes in an inheritance hierarchy. Any adjustments that need to be made in such cases are always indicated explicitly. Similarly, a user-defined conversion is never represented by a `CONVERT_EXPR`; instead, the function calls are made explicit.
`THROW_EXPR`
  These nodes represent `throw` expressions. The single operand is an expression for the code that should be executed to throw the exception. However, there is one implicit action not represented in that expression; namely the call to `__throw`. This function takes no arguments. If `setjmp`/`longjmp` exceptions are used, the function `__sjthrow` is called instead. The normal GCC back end uses the function `emit_throw` to generate this code; you can examine this function to see what needs to be done.
`LSHIFT_EXPR`
`RSHIFT_EXPR`
  These nodes represent left and right shifts, respectively. The first operand is the value to shift; it will always be of integral type. The second operand is an expression for the number of bits by which to shift. Right shift should be treated as arithmetic, i.e., the high-order bits should be zero-filled when the expression has unsigned type and filled with the sign bit when the expression has signed type. Note that the result is undefined if the second operand is larger than or equal to the first operand's type size.
`BIT_IOR_EXPR`
`BIT_XOR_EXPR`
`BIT_AND_EXPR`
  These nodes represent bitwise inclusive or, bitwise exclusive or, and bitwise and, respectively. Both operands will always have integral type.
`TRUTH_ANDIF_EXPR`
`TRUTH_ORIF_EXPR`
  These nodes represent logical and and logical or, respectively. These operators are not strict; i.e., the second operand is evaluated only if the value of the expression is not determined by evaluation of the first operand. The type of the operands and that of the result are always of `BOOLEAN_TYPE` or `INTEGER_TYPE`.
`TRUTH_AND_EXPR`
`TRUTH_OR_EXPR`
`TRUTH_XOR_EXPR`
  These nodes represent logical and, logical or, and logical exclusive or. They are strict; both arguments are always evaluated. There are no corresponding operators in C or C++, but the front end will sometimes generate these expressions anyhow, if it can tell that strictness does not matter. The type of the operands and that of the result are always of `BOOLEAN_TYPE` or `INTEGER_TYPE`.
`PLUS_EXPR`
`MINUS_EXPR`
`MULT_EXPR`
  These nodes represent various binary arithmetic operations. Respectively, these operations are addition, subtraction (of the second operand from the first) and multiplication. Their operands may have either integral or floating type, but there will never be case in which one operand is of floating type and the other is of integral type.

  The behavior of these operations on signed arithmetic overflow is controlled by the `flag_wrapv` and `flag_trapv` variables.

`RDIV_EXPR`
  This node represents a floating point division operation.
`TRUNC_DIV_EXPR`
`FLOOR_DIV_EXPR`
`CEIL_DIV_EXPR`
`ROUND_DIV_EXPR`
  These nodes represent integer division operations that return an integer result. `TRUNC_DIV_EXPR` rounds towards zero, `FLOOR_DIV_EXPR` rounds towards negative infinity, `CEIL_DIV_EXPR` rounds towards positive infinity and `ROUND_DIV_EXPR` rounds to the closest integer. Integer division in C and C++ is truncating, i.e. `TRUNC_DIV_EXPR`.

  The behavior of these operations on signed arithmetic overflow, when dividing the minimum signed integer by minus one, is controlled by the `flag_wrapv` and `flag_trapv` variables.

`TRUNC_MOD_EXPR`
`FLOOR_MOD_EXPR`
`CEIL_MOD_EXPR`
`ROUND_MOD_EXPR`
  These nodes represent the integer remainder or modulus operation. The integer modulus of two operands `a` and `b` is defined as `a - (a/b)*b` where the division calculated using the corresponding division operator. Hence for `TRUNC_MOD_EXPR` this definition assumes division using truncation towards zero, i.e. `TRUNC_DIV_EXPR`. Integer remainder in C and C++ uses truncating division, i.e. `TRUNC_MOD_EXPR`.
`EXACT_DIV_EXPR`
  The `EXACT_DIV_EXPR` code is used to represent integer divisions where the numerator is known to be an exact multiple of the denominator. This allows the backend to choose between the faster of `TRUNC_DIV_EXPR`, `CEIL_DIV_EXPR` and `FLOOR_DIV_EXPR` for the current target.
`ARRAY_REF`
  These nodes represent array accesses. The first operand is the array; the second is the index. To calculate the address of the memory accessed, you must scale the index by the size of the type of the array elements. The type of these expressions must be the type of a component of the array. The third and fourth operands are used after gimplification to represent the lower bound and component size but should not be used directly; call `array_ref_low_bound` and `array_ref_element_size` instead.
`ARRAY_RANGE_REF`
  These nodes represent access to a range (or “slice”) of an array. The operands are the same as that for `ARRAY_REF` and have the same meanings. The type of these expressions must be an array whose component type is the same as that of the first operand. The range of that array type determines the amount of data these expressions access.
`TARGET_MEM_REF`
  These nodes represent memory accesses whose address directly map to an addressing mode of the target architecture. The first argument is `TMR_SYMBOL` and must be a `VAR_DECL` of an object with a fixed address. The second argument is `TMR_BASE` and the third one is `TMR_INDEX`. The fourth argument is `TMR_STEP` and must be an `INTEGER_CST`. The fifth argument is `TMR_OFFSET` and must be an `INTEGER_CST`. Any of the arguments may be NULL if the appropriate component does not appear in the address. Address of the `TARGET_MEM_REF` is determined in the following way.

                &TMR_SYMBOL + TMR_BASE + TMR_INDEX * TMR_STEP + TMR_OFFSET

  The sixth argument is the reference to the original memory access, which is preserved for the purposes of the RTL alias analysis. The seventh argument is a tag representing the results of tree level alias analysis.

`LT_EXPR`
`LE_EXPR`
`GT_EXPR`
`GE_EXPR`
`EQ_EXPR`
`NE_EXPR`
  These nodes represent the less than, less than or equal to, greater than, greater than or equal to, equal, and not equal comparison operators. The first and second operand with either be both of integral type or both of floating type. The result type of these expressions will always be of integral or boolean type. These operations return the result type's zero value for false, and the result type's one value for true.

  For floating point comparisons, if we honor IEEE NaNs and either operand is NaN, then `NE_EXPR` always returns true and the remaining operators always return false. On some targets, comparisons against an IEEE NaN, other than equality and inequality, may generate a floating point exception.

`ORDERED_EXPR`
`UNORDERED_EXPR`
  These nodes represent non-trapping ordered and unordered comparison operators. These operations take two floating point operands and determine whether they are ordered or unordered relative to each other. If either operand is an IEEE NaN, their comparison is defined to be unordered, otherwise the comparison is defined to be ordered. The result type of these expressions will always be of integral or boolean type. These operations return the result type's zero value for false, and the result type's one value for true.
`UNLT_EXPR`
`UNLE_EXPR`
`UNGT_EXPR`
`UNGE_EXPR`
`UNEQ_EXPR`
`LTGT_EXPR`
  These nodes represent the unordered comparison operators. These operations take two floating point operands and determine whether the operands are unordered or are less than, less than or equal to, greater than, greater than or equal to, or equal respectively. For example, `UNLT_EXPR` returns true if either operand is an IEEE NaN or the first operand is less than the second. With the possible exception of `LTGT_EXPR`, all of these operations are guaranteed not to generate a floating point exception. The result type of these expressions will always be of integral or boolean type. These operations return the result type's zero value for false, and the result type's one value for true.
`MODIFY_EXPR`
  These nodes represent assignment. The left-hand side is the first operand; the right-hand side is the second operand. The left-hand side will be a `VAR_DECL`, `INDIRECT_REF`, `COMPONENT_REF`, or other lvalue.

  These nodes are used to represent not only assignment with ‘`=`’ but also compound assignments (like ‘`+=`’), by reduction to ‘`=`’ assignment. In other words, the representation for ‘`i += 3`’ looks just like that for ‘`i = i + 3`’.

`INIT_EXPR`
  These nodes are just like `MODIFY_EXPR`, but are used only when a variable is initialized, rather than assigned to subsequently. This means that we can assume that the target of the initialization is not used in computing its own value; any reference to the lhs in computing the rhs is undefined.
`COMPONENT_REF`
  These nodes represent non-static data member accesses. The first operand is the object (rather than a pointer to it); the second operand is the `FIELD_DECL` for the data member. The third operand represents the byte offset of the field, but should not be used directly; call `component_ref_field_offset` instead.
`COMPOUND_EXPR`
  These nodes represent comma-expressions. The first operand is an expression whose value is computed and thrown away prior to the evaluation of the second operand. The value of the entire expression is the value of the second operand.
`COND_EXPR`
  These nodes represent `?:` expressions. The first operand is of boolean or integral type. If it evaluates to a nonzero value, the second operand should be evaluated, and returned as the value of the expression. Otherwise, the third operand is evaluated, and returned as the value of the expression.

  The second operand must have the same type as the entire expression, unless it unconditionally throws an exception or calls a noreturn function, in which case it should have void type. The same constraints apply to the third operand. This allows array bounds checks to be represented conveniently as `(i >= 0 && i < 10) ? i : abort()`.

  As a GNU extension, the C language front-ends allow the second operand of the `?:` operator may be omitted in the source. For example, `x ? : 3` is equivalent to `x ? x : 3`, assuming that `x` is an expression without side-effects. In the tree representation, however, the second operand is always present, possibly protected by `SAVE_EXPR` if the first argument does cause side-effects.

`CALL_EXPR`
  These nodes are used to represent calls to functions, including non-static member functions. The first operand is a pointer to the function to call; it is always an expression whose type is a `POINTER_TYPE`. The second argument is a `TREE_LIST`. The arguments to the call appear left-to-right in the list. The `TREE_VALUE` of each list node contains the expression corresponding to that argument. (The value of `TREE_PURPOSE` for these nodes is unspecified, and should be ignored.) For non-static member functions, there will be an operand corresponding to the `this` pointer. There will always be expressions corresponding to all of the arguments, even if the function is declared with default arguments and some arguments are not explicitly provided at the call sites.
`STMT_EXPR`
  These nodes are used to represent GCC's statement-expression extension. The statement-expression extension allows code like this:

                int f() { return ({ int j; j = 3; j + 7; }); }

  In other words, an sequence of statements may occur where a single expression would normally appear. The `STMT_EXPR` node represents such an expression. The `STMT_EXPR_STMT` gives the statement contained in the expression. The value of the expression is the value of the last sub-statement in the body. More precisely, the value is the value computed by the last statement nested inside `BIND_EXPR`, `TRY_FINALLY_EXPR`, or `TRY_CATCH_EXPR`. For example, in:

                ({ 3; })

  the value is `3` while in:

                ({ if (x) { 3; } })

  there is no value. If the `STMT_EXPR` does not yield a value, it's type will be `void`.

`BIND_EXPR`
  These nodes represent local blocks. The first operand is a list of variables, connected via their `TREE_CHAIN` field. These will never require cleanups. The scope of these variables is just the body of the `BIND_EXPR`. The body of the `BIND_EXPR` is the second operand.
`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`.
`CLEANUP_POINT_EXPR`
  These nodes represent full-expressions. The single operand is an expression to evaluate. Any destructor calls engendered by the creation of temporaries during the evaluation of that expression should be performed immediately after the expression is evaluated.
`CONSTRUCTOR`
  These nodes represent the brace-enclosed initializers for a structure or array. The first operand is reserved for use by the back end. The second operand is a `TREE_LIST`. If the `TREE_TYPE` of the `CONSTRUCTOR` is a `RECORD_TYPE` or `UNION_TYPE`, then the `TREE_PURPOSE` of each node in the `TREE_LIST` will be a `FIELD_DECL` and the `TREE_VALUE` of each node will be the expression used to initialize that field.

  If the `TREE_TYPE` of the `CONSTRUCTOR` is an `ARRAY_TYPE`, then the `TREE_PURPOSE` of each element in the `TREE_LIST` will be an `INTEGER_CST` or a `RANGE_EXPR` of two `INTEGER_CST`s. A single `INTEGER_CST` indicates which element of the array (indexed from zero) is being assigned to. A `RANGE_EXPR` indicates an inclusive range of elements to initialize. In both cases the `TREE_VALUE` is the corresponding initializer. It is re-evaluated for each element of a `RANGE_EXPR`. If the `TREE_PURPOSE` is `NULL_TREE`, then the initializer is for the next available array element.

  In the front end, you should not depend on the fields appearing in any particular order. However, in the middle end, fields must appear in declaration order. You should not assume that all fields will be represented. Unrepresented fields will be set to zero.

`COMPOUND_LITERAL_EXPR`
  These nodes represent ISO C99 compound literals. The `COMPOUND_LITERAL_EXPR_DECL_STMT` is a `DECL_STMT` containing an anonymous `VAR_DECL` for the unnamed object represented by the compound literal; the `DECL_INITIAL` of that `VAR_DECL` is a `CONSTRUCTOR` representing the brace-enclosed list of initializers in the compound literal. That anonymous `VAR_DECL` can also be accessed directly by the `COMPOUND_LITERAL_EXPR_DECL` macro.
`SAVE_EXPR`
  A `SAVE_EXPR` represents an expression (possibly involving side-effects) that is used more than once. The side-effects should occur only the first time the expression is evaluated. Subsequent uses should just reuse the computed value. The first operand to the `SAVE_EXPR` is the expression to evaluate. The side-effects should be executed where the `SAVE_EXPR` is first encountered in a depth-first preorder traversal of the expression tree.
`TARGET_EXPR`
  A `TARGET_EXPR` represents a temporary object. The first operand is a `VAR_DECL` for the temporary variable. The second operand is the initializer for the temporary. The initializer is evaluated and, if non-void, copied (bitwise) into the temporary. If the initializer is void, that means that it will perform the initialization itself.

  Often, a `TARGET_EXPR` occurs on the right-hand side of an assignment, or as the second operand to a comma-expression which is itself the right-hand side of an assignment, etc. In this case, we say that the `TARGET_EXPR` is “normal”; otherwise, we say it is “orphaned”. For a normal `TARGET_EXPR` the temporary variable should be treated as an alias for the left-hand side of the assignment, rather than as a new temporary variable.

  The third operand to the `TARGET_EXPR`, if present, is a cleanup-expression (i.e., destructor call) for the temporary. If this expression is orphaned, then this expression must be executed when the statement containing this expression is complete. These cleanups must always be executed in the order opposite to that in which they were encountered. Note that if a temporary is created on one branch of a conditional operator (i.e., in the second or third operand to a `COND_EXPR`), the cleanup must be run only if that branch is actually executed.

  See `STMT_IS_FULL_EXPR_P` for more information about running these cleanups.

`AGGR_INIT_EXPR`
  An `AGGR_INIT_EXPR` represents the initialization as the return value of a function call, or as the result of a constructor. An `AGGR_INIT_EXPR` will only appear as a full-expression, or as the second operand of a `TARGET_EXPR`. The first operand to the `AGGR_INIT_EXPR` is the address of a function to call, just as in a `CALL_EXPR`. The second operand are the arguments to pass that function, as a `TREE_LIST`, again in a manner similar to that of a `CALL_EXPR`.

  If `AGGR_INIT_VIA_CTOR_P` holds of the `AGGR_INIT_EXPR`, then the initialization is via a constructor call. The address of the third operand of the `AGGR_INIT_EXPR`, which is always a `VAR_DECL`, is taken, and this value replaces the first argument in the argument list.

  In either case, the expression is void.

`VA_ARG_EXPR`
  This node is used to implement support for the C/C++ variable argument-list mechanism. It represents expressions like `va_arg (ap, type)`. Its `TREE_TYPE` yields the tree representation for `type` and its sole argument yields the representation for `ap`.
`OMP_PARALLEL`
  Represents `#pragma omp parallel [clause1 ... clauseN]`. It has four operands:

  Operand `OMP_PARALLEL_BODY` is valid while in GENERIC and High GIMPLE forms. It contains the body of code to be executed by all the threads. During GIMPLE lowering, this operand becomes `NULL` and the body is emitted linearly after `OMP_PARALLEL`.

  Operand `OMP_PARALLEL_CLAUSES` is the list of clauses associated with the directive.

  Operand `OMP_PARALLEL_FN` is created by `pass_lower_omp`, it contains the `FUNCTION_DECL` for the function that will contain the body of the parallel region.

  Operand `OMP_PARALLEL_DATA_ARG` is also created by `pass_lower_omp`. If there are shared variables to be communicated to the children threads, this operand will contain the `VAR_DECL` that contains all the shared values and variables.

`OMP_FOR`
  Represents `#pragma omp for [clause1 ... clauseN]`. It has 5 operands:

  Operand `OMP_FOR_BODY` contains the loop body.

  Operand `OMP_FOR_CLAUSES` is the list of clauses associated with the directive.

  Operand `OMP_FOR_INIT` is the loop initialization code of the form `VAR = N1`.

  Operand `OMP_FOR_COND` is the loop conditional expression of the form `VAR {<,>,<=,>=} N2`.

  Operand `OMP_FOR_INCR` is the loop index increment of the form `VAR {+=,-=} INCR`.

  Operand `OMP_FOR_PRE_BODY` contains side-effect code from operands `OMP_FOR_INIT`, `OMP_FOR_COND` and `OMP_FOR_INC`. These side-effects are part of the `OMP_FOR` block but must be evaluated before the start of loop body.

  The loop index variable `VAR` must be a signed integer variable, which is implicitly private to each thread. Bounds `N1` and `N2` and the increment expression `INCR` are required to be loop invariant integer expressions that are evaluated without any synchronization. The evaluation order, frequency of evaluation and side-effects are unspecified by the standard.

`OMP_SECTIONS`
  Represents `#pragma omp sections [clause1 ... clauseN]`.

  Operand `OMP_SECTIONS_BODY` contains the sections body, which in turn contains a set of `OMP_SECTION` nodes for each of the concurrent sections delimited by `#pragma omp section`.

  Operand `OMP_SECTIONS_CLAUSES` is the list of clauses associated with the directive.

`OMP_SECTION`
  Section delimiter for `OMP_SECTIONS`.
`OMP_SINGLE`
  Represents `#pragma omp single`.

  Operand `OMP_SINGLE_BODY` contains the body of code to be executed by a single thread.

  Operand `OMP_SINGLE_CLAUSES` is the list of clauses associated with the directive.

`OMP_MASTER`
  Represents `#pragma omp master`.

  Operand `OMP_MASTER_BODY` contains the body of code to be executed by the master thread.

`OMP_ORDERED`
  Represents `#pragma omp ordered`.

  Operand `OMP_ORDERED_BODY` contains the body of code to be executed in the sequential order dictated by the loop index variable.

`OMP_CRITICAL`
  Represents `#pragma omp critical [name]`.

  Operand `OMP_CRITICAL_BODY` is the critical section.

  Operand `OMP_CRITICAL_NAME` is an optional identifier to label the critical section.

`OMP_RETURN`
  This does not represent any OpenMP directive, it is an artificial marker to indicate the end of the body of an OpenMP. It is used by the flow graph (`tree-cfg.c`) and OpenMP region building code (`omp-low.c`).
`OMP_CONTINUE`
  Similarly, this instruction does not represent an OpenMP directive, it is used by `OMP_FOR` and `OMP_SECTIONS` to mark the place where the code needs to loop to the next iteration (in the case of `OMP_FOR`) or the next section (in the case of `OMP_SECTIONS`).

  In some cases, `OMP_CONTINUE` is placed right before `OMP_RETURN`. But if there are cleanups that need to occur right after the looping body, it will be emitted between `OMP_CONTINUE` and `OMP_RETURN`.

`OMP_ATOMIC`
  Represents `#pragma omp atomic`.

  Operand 0 is the address at which the atomic operation is to be performed.

  Operand 1 is the expression to evaluate. The gimplifier tries three alternative code generation strategies. Whenever possible, an atomic update built-in is used. If that fails, a compare-and-swap loop is attempted. If that also fails, a regular critical section around the expression is used.

`OMP_CLAUSE`
  Represents clauses associated with one of the `OMP_` directives. Clauses are represented by separate sub-codes defined in `tree.h`. Clauses codes can be one of: `OMP_CLAUSE_PRIVATE`, `OMP_CLAUSE_SHARED`, `OMP_CLAUSE_FIRSTPRIVATE`, `OMP_CLAUSE_LASTPRIVATE`, `OMP_CLAUSE_COPYIN`, `OMP_CLAUSE_COPYPRIVATE`, `OMP_CLAUSE_IF`, `OMP_CLAUSE_NUM_THREADS`, `OMP_CLAUSE_SCHEDULE`, `OMP_CLAUSE_NOWAIT`, `OMP_CLAUSE_ORDERED`, `OMP_CLAUSE_DEFAULT`, and `OMP_CLAUSE_REDUCTION`. Each code represents the corresponding OpenMP clause.

  Clauses associated with the same directive are chained together via `OMP_CLAUSE_CHAIN`. Those clauses that accept a list of variables are restricted to exactly one, accessed with `OMP_CLAUSE_VAR`. Therefore, multiple variables under the same clause `C` need to be represented as multiple `C` clauses chained together. This facilitates adding new clauses during compilation.