17.23 Cross Compilation and Floating Point
While all modern machines use twos-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, all floating point constants must be represented in the target machine's format. Therefore, the cross compiler cannot safely use the host machine's floating point arithmetic; it must emulate the target's arithmetic. To ensure consistency, GCC always uses emulation to work with floating point values, even when the host and target floating point formats are identical.
The following macros are provided by real.h for the compiler to use. All parts of the compiler which generate or optimize floating-point calculations must use these macros. They may evaluate their operands more than once, so operands must not have side effects.
The C data type to be used to hold a floating point value in the target machine's format. Typically this is a
structcontaining an array of
HOST_WIDE_INT, but all code should treat it as an opaque quantity.
Compares for equality the two values, x and y. If the target floating point format supports negative zeroes and/or NaNs, REAL_VALUES_EQUAL (-0.0, 0.0) is true, and REAL_VALUES_EQUAL (NaN, NaN) is false.
Tests whether x is less than y.
Truncates x to a signed integer, rounding toward zero.
Truncates x to an unsigned integer, rounding toward zero. If x is negative, returns zero.
Converts string into a floating point number in the target machine's representation for mode mode. This routine can handle both decimal and hexadecimal floating point constants, using the syntax defined by the C language for both.
Returns 1 if x is negative (including negative zero), 0 otherwise.
Determines whether x represents infinity (positive or negative).
Determines whether x represents a “NaN” (not-a-number).
Calculates an arithmetic operation on the two floating point values x and y, storing the result in output (which must be a variable).
The operation to be performed is specified by code. Only the following codes are supported:
REAL_ARITHMETICis asked to evaluate division by zero and the target's floating point format cannot represent infinity, it will call
abort. Callers should check for this situation first, using
MODE_HAS_INFINITIES. See Storage Layout.
Returns the negative of the floating point value x.
Truncates the floating point value x to fit in mode. The return value is still a full-size
REAL_VALUE_TYPE, but it has an appropriate bit pattern to be output as a floating constant whose precision accords with mode mode.
Converts a floating point value x into a double-precision integer which is then stored into low and high. If the value is not integral, it is truncated.