Using and Porting GNU CC
The simplest RTL expressions are those that represent constant values.
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
constm1_rtx as appropriate.
Similarly, there is only one object for the integer whose value is
STORE_FLAG_VALUE. It is found in
STORE_FLAG_VALUE is one,
const1_rtx will point to the same object. If
STORE_FLAG_VALUE is -1,
constm1_rtx will point to the same object.
(const_double:m addr i0 i1 ...)
HOST_BITS_PER_WIDE_INTbits 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
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
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
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).
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
*', the label is the rest of symbol not including the `
*'. Otherwise, the label is symbol, usually prefixed with `
symbol_ref contains a mode, which is usually
Usually that is the only mode for which a symbol is directly valid.
code_labelthat 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.
label_refexpressions) combined with
minus. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols.
m should be
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_sumto represent the typical two-instruction sequence used in RISC machines to reference a global memory location.
m should be