21.19 Cross Compilation and Floating Point

While all modern machines use 2's complement representation for integers, there are a variety of representations for floating point numbers. This means that in a cross-compiler the representation of floating point numbers in the compiled program may be different from that used in the machine doing the compilation.

Because different representation systems may offer different amounts of range and precision, the cross compiler cannot safely use the host machine's floating point arithmetic. Therefore, floating point constants must be represented in the target machine's format. This means that the cross compiler cannot use atof to parse a floating point constant; it must have its own special routine to use instead. Also, constant folding must emulate the target machine's arithmetic (or must not be done at all).

The macros in the following table should be defined only if you are cross compiling between different floating point formats.

Otherwise, don't define them. Then default definitions will be set up which use double as the data type, == to test for equality, etc.

You don't need to worry about how many times you use an operand of any of these macros. The compiler never uses operands which have side effects.

REAL_VALUE_TYPE

A macro for the C data type to be used to hold a floating point value in the target machine's format. Typically this would be a struct containing an array of int.

REAL_VALUES_EQUAL (x, y)

A macro for a C expression which compares for equality the two values, x and y, both of type REAL_VALUE_TYPE.

REAL_VALUES_LESS (x, y)

A macro for a C expression which tests whether x is less than y, both values being of type REAL_VALUE_TYPE and interpreted as floating point numbers in the target machine's representation.

REAL_VALUE_LDEXP (x, scale)

A macro for a C expression which performs the standard library function ldexp, but using the target machine's floating point representation. Both x and the value of the expression have type REAL_VALUE_TYPE. The second argument, scale, is an integer.

REAL_VALUE_FIX (x)

A macro whose definition is a C expression to convert the target-machine floating point value x to a signed integer. x has type REAL_VALUE_TYPE.

REAL_VALUE_UNSIGNED_FIX (x)

A macro whose definition is a C expression to convert the target-machine floating point value x to an unsigned integer. x has type REAL_VALUE_TYPE.

REAL_VALUE_RNDZINT (x)

A macro whose definition is a C expression to round the target-machine floating point value x towards zero to an integer value (but still as a floating point number). x has type REAL_VALUE_TYPE, and so does the value.

REAL_VALUE_UNSIGNED_RNDZINT (x)

A macro whose definition is a C expression to round the target-machine floating point value x towards zero to an unsigned integer value (but still represented as a floating point number). x has type REAL_VALUE_TYPE, and so does the value.

REAL_VALUE_ATOF (string, mode)

A macro for a C expression which converts string, an expression of type char *, into a floating point number in the target machine's representation for mode mode. The value has type REAL_VALUE_TYPE.

REAL_INFINITY

Define this macro if infinity is a possible floating point value, and therefore division by 0 is legitimate.

REAL_VALUE_ISINF (x)

A macro for a C expression which determines whether x, a floating point value, is infinity. The value has type int. By default, this is defined to call isinf.

REAL_VALUE_ISNAN (x)

A macro for a C expression which determines whether x, a floating point value, is a "nan" (not-a-number). The value has type int. By default, this is defined to call isnan.

Define the following additional macros if you want to make floating point constant folding work while cross compiling. If you don't define them, cross compilation is still possible, but constant folding will not happen for floating point values.

REAL_ARITHMETIC (output, code, x, y)

A macro for a C statement which calculates an arithmetic operation of the two floating point values x and y, both of type REAL_VALUE_TYPE in the target machine's representation, to produce a result of the same type and representation which is stored in output (which will be a variable).

The operation to be performed is specified by code, a tree code which will always be one of the following: PLUS_EXPR, MINUS_EXPR, MULT_EXPR, RDIV_EXPR, MAX_EXPR, MIN_EXPR.

The expansion of this macro is responsible for checking for overflow. If overflow happens, the macro expansion should execute the statement return 0;, which indicates the inability to perform the arithmetic operation requested.

REAL_VALUE_NEGATE (x)

A macro for a C expression which returns the negative of the floating point value x. Both x and the value of the expression have type REAL_VALUE_TYPE and are in the target machine's floating point representation.

There is no way for this macro to report overflow, since overflow can't happen in the negation operation.

REAL_VALUE_TRUNCATE (mode, x)

A macro for a C expression which converts the floating point value x to mode mode.

Both x and the value of the expression are in the target machine's floating point representation and have type REAL_VALUE_TYPE. However, the value should have an appropriate bit pattern to be output properly as a floating constant whose precision accords with mode mode.

There is no way for this macro to report overflow.

REAL_VALUE_TO_INT (low, high, x)

A macro for a C expression which converts a floating point value x into a double-precision integer which is then stored into low and high, two variables of type int.

REAL_VALUE_FROM_INT (x, low, high, mode)

A macro for a C expression which converts a double-precision integer found in low and high, two variables of type int, into a floating point value which is then stored into x. The value is in the target machine's representation for mode mode and has the type REAL_VALUE_TYPE.