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14.5: Constant Expression Types

The simplest RTL expressions are those that represent constant values.

(const_int i)
This type of expression represents the integer value i. i is customarily accessed with the macro INTVAL as in INTVAL (exp), which is equivalent to XWINT (exp, 0).

There is only one expression object for the integer value zero; it is the value of the variable const0_rtx. Likewise, the only expression for integer value one is found in const1_rtx, the only expression for integer value two is found in const2_rtx, and the only expression for integer value negative one is found in constm1_rtx. Any attempt to create an expression of code const_int and value zero, one, two or negative one will return const0_rtx, const1_rtx, const2_rtx or constm1_rtx as appropriate.

Similarly, there is only one object for the integer whose value is STORE_FLAG_VALUE. It is found in const_true_rtx. If STORE_FLAG_VALUE is one, const_true_rtx and const1_rtx will point to the same object. If STORE_FLAG_VALUE is -1, const_true_rtx and constm1_rtx will point to the same object.

(const_double:m addr i0 i1 ...)
Represents either a floating-point constant of mode m or an integer constant too large to fit into HOST_BITS_PER_WIDE_INT bits but small enough to fit within twice that number of bits (GNU CC does not provide a mechanism to represent even larger constants). In the latter case, m will be VOIDmode.

addr is used to contain the mem expression that corresponds to the location in memory that at which the constant can be found. If it has not been allocated a memory location, but is on the chain of all const_double expressions in this compilation (maintained using an undisplayed field), addr contains const0_rtx. If it is not on the chain, addr contains cc0_rtx. addr is customarily accessed with the macro CONST_DOUBLE_MEM and the chain field via CONST_DOUBLE_CHAIN.

If m is VOIDmode, the bits of the value are stored in i0 and i1. i0 is customarily accessed with the macro CONST_DOUBLE_LOW and i1 with CONST_DOUBLE_HIGH.

If the constant is floating point (regardless of its precision), then the number of integers used to store the value depends on the size of REAL_VALUE_TYPE (see Cross-compilation). The integers represent a floating point number, but not precisely in the target machine's or host machine's floating point format. To convert them to the precise bit pattern used by the target machine, use the macro REAL_VALUE_TO_TARGET_DOUBLE and friends (see Data Output).

The macro CONST0_RTX (mode) refers to an expression with value 0 in mode mode. If mode mode is of mode class MODE_INT, it returns const0_rtx. Otherwise, it returns a CONST_DOUBLE expression in mode mode. Similarly, the macro CONST1_RTX (mode) refers to an expression with value 1 in mode mode and similarly for CONST2_RTX.

(const_string str)
Represents a constant string with value str. Currently this is used only for insn attributes (see Insn Attributes) since constant strings in C are placed in memory.
(symbol_ref:mode symbol)
Represents the value of an assembler label for data. symbol is a string that describes the name of the assembler label. If it starts with a `*', the label is the rest of symbol not including the `*'. Otherwise, the label is symbol, usually prefixed with `_'.

The symbol_ref contains a mode, which is usually Pmode. Usually that is the only mode for which a symbol is directly valid.

(label_ref label)
Represents the value of an assembler label for code. It contains one operand, an expression, which must be a code_label that appears in the instruction sequence to identify the place where the label should go.

The reason for using a distinct expression type for code label references is so that jump optimization can distinguish them.

(const:m exp)
Represents a constant that is the result of an assembly-time arithmetic computation. The operand, exp, is an expression that contains only constants (const_int, symbol_ref and label_ref expressions) combined with plus and minus. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols.

m should be Pmode.

(high:m exp)
Represents the high-order bits of exp, usually a symbol_ref. The number of bits is machine-dependent and is normally the number of bits specified in an instruction that initializes the high order bits of a register. It is used with lo_sum to represent the typical two-instruction sequence used in RISC machines to reference a global memory location.

m should be Pmode.