### 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.

**REAL_VALUE_TYPE**

The C data type to be used to hold a floating point value in the target machine's format. Typically this is a

`struct`

containing an array of`HOST_WIDE_INT`

, but all code should treat it as an opaque quantity.

**REAL_VALUES_EQUAL**(

`REAL_VALUE_TYPE x, REAL_VALUE_TYPE y`)

Compares for equality the two values,

xandy. 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.

**REAL_VALUES_LESS**(

`REAL_VALUE_TYPE x, REAL_VALUE_TYPE y`)

Tests whether

xis less thany.

**REAL_VALUE_FIX**(

`REAL_VALUE_TYPE x`)

Truncates

xto a signed integer, rounding toward zero.

**REAL_VALUE_UNSIGNED_FIX**(

`REAL_VALUE_TYPE x`)

Truncates

xto an unsigned integer, rounding toward zero. Ifxis negative, returns zero.

**REAL_VALUE_ATOF**(

`const char *string, enum machine_mode mode`)

Converts

stringinto a floating point number in the target machine's representation for modemode. This routine can handle both decimal and hexadecimal floating point constants, using the syntax defined by the C language for both.

**REAL_VALUE_NEGATIVE**(

`REAL_VALUE_TYPE x`)

Returns 1 if

xis negative (including negative zero), 0 otherwise.

**REAL_VALUE_ISINF**(

`REAL_VALUE_TYPE x`)

Determines whether

xrepresents infinity (positive or negative).

**REAL_VALUE_ISNAN**(

`REAL_VALUE_TYPE x`)

Determines whether

xrepresents a “NaN” (not-a-number).

**REAL_ARITHMETIC**(

`REAL_VALUE_TYPE output, enum tree_code code, REAL_VALUE_TYPE x, REAL_VALUE_TYPE y`)

Calculates an arithmetic operation on the two floating point values

xandy, storing the result inoutput(which must be a variable).The operation to be performed is specified by

code. Only the following codes are supported:`PLUS_EXPR`

,`MINUS_EXPR`

,`MULT_EXPR`

,`RDIV_EXPR`

,`MAX_EXPR`

,`MIN_EXPR`

.If

`REAL_ARITHMETIC`

is 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.

**REAL_VALUE_NEGATE**(

`REAL_VALUE_TYPE x`)

Returns the negative of the floating point value

x.

**REAL_VALUE_TRUNCATE**(

`REAL_VALUE_TYPE mode, enum machine_mode x`)

Truncates the floating point value

xto fit inmode. 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 modemode.