GNU Compiler Collection (GCC) Internals: Unary and Binary Expressions

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10.6.3 Unary and Binary Expressions

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.

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_EXPRs, 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.

FIXED_CONVERT_EXPR

These nodes are used to represent conversions that involve fixed-point values. For example, from a fixed-point value to another fixed-point value, from an integer to a fixed-point value, from a fixed-point value to an integer, from a floating-point value to a fixed-point value, or from a fixed-point value to a floating-point value.

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. Unlike most nodes, these can have a vector as first operand and a scalar as second operand.

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.

POINTER_PLUS_EXPR

This node represents pointer arithmetic. The first operand is always a pointer/reference type. The second operand is always an unsigned integer type compatible with sizetype. This is the only binary arithmetic operand that can operate on pointer types.

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.

MULT_HIGHPART_EXPR

This node represents the “high-part” of a widening multiplication. For an integral type with b bits of precision, the result is the most significant b bits of the full 2b product.

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.

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 operands will either be both of integral type, both of floating type or both of vector type. The result type of these expressions will always be of integral, boolean or signed integral vector type. These operations return the result type’s zero value for false, the result type’s one value for true, and a vector whose elements are zero (false) or minus one (true) for vectors.

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.

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. CALL_EXPRs are implemented as expression nodes with a variable number of operands. Rather than using TREE_OPERAND to extract them, it is preferable to use the specialized accessor macros and functions that operate specifically on CALL_EXPR nodes.

CALL_EXPR_FN returns a pointer to the function to call; it is always an expression whose type is a POINTER_TYPE.

The number of arguments to the call is returned by call_expr_nargs, while the arguments themselves can be accessed with the CALL_EXPR_ARG macro. The arguments are zero-indexed and numbered left-to-right. You can iterate over the arguments using FOR_EACH_CALL_EXPR_ARG, as in:

tree call, arg;
call_expr_arg_iterator iter;
FOR_EACH_CALL_EXPR_ARG (arg, iter, call)
  /* arg is bound to successive arguments of call.  */
  …;

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.

CALL_EXPRs also have a CALL_EXPR_STATIC_CHAIN operand that is used to implement nested functions. This operand is otherwise null.

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 an array. They contain a sequence of component values made out of a vector of constructor_elt, which is a (INDEX, VALUE) pair.

If the TREE_TYPE of the CONSTRUCTOR is a RECORD_TYPE, UNION_TYPE or QUAL_UNION_TYPE then the INDEX of each node in the sequence will be a FIELD_DECL and the VALUE will be the expression used to initialize that field.

If the TREE_TYPE of the CONSTRUCTOR is an ARRAY_TYPE, then the INDEX of each node in the sequence will be an INTEGER_CST or a RANGE_EXPR of two INTEGER_CSTs. A single INTEGER_CST indicates which element of the array is being assigned to. A RANGE_EXPR indicates an inclusive range of elements to initialize. In both cases the VALUE is the corresponding initializer. It is re-evaluated for each element of a RANGE_EXPR. If the INDEX 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 cleared (zeroed), unless the CONSTRUCTOR_NO_CLEARING flag is set, in which case their value becomes undefined.

COMPOUND_LITERAL_EXPR

These nodes represent ISO C99 compound literals. The COMPOUND_LITERAL_EXPR_DECL_EXPR is a DECL_EXPR 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.

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.

ANNOTATE_EXPR

This node is used to attach markers to an expression. The first operand is the annotated expression, the second is an INTEGER_CST with a value from enum annot_expr_kind.

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