Floating-Point

Floating-Point Intermediate Values

For floating-point operations and expression intermediate values, a greater precision can be used than the type of the expression. Only the minimum precision is set by the types of the operands, not the maximum. Implementation Note: On Intel x86 machines, for example, it is expected (but not required) that the intermediate calculations be done to the full 80 bits of precision implemented by the hardware.

Execution of floating-point expressions may yield a result of greater precision than dictated by the source.

Floating-Point Constant Folding

Regardless of the type of the operands, floating-point constant folding is done in real or greater precision. It is always done following IEEE-754 rules and round-to-nearest is used.

Floating-point constants are internally represented in the implementation in at least real precision, regardless of the constant's type. The extra precision is available for constant folding. Committing to the precision of the result is done as late as possible in the compilation process. For example:

const float f = 0.2f;
writeln(f - 0.2);

will print 0. A non-const static variable's value cannot be propagated at compile time, so:

static float f = 0.2f;
writeln(f - 0.2);

will print 2.98023e-09. Hex floating-point constants can also be used when specific floating-point bit patterns are needed that are unaffected by rounding. To find the hex value of 0.2f:

import std.stdio;

void main()
{
    writefln("%a", 0.2f);
}

which is 0x1.99999ap-3. Using the hex constant:

const float f = 0x1.99999ap-3f;
writeln(f - 0.2);

prints 2.98023e-09.

Different compiler settings, optimization settings, and inlining settings can affect opportunities for constant folding, therefore the results of floating-point calculations may differ depending on those settings.

Rounding Control

IEEE 754 floating-point arithmetic includes the ability to set 4 different rounding modes. These are accessible via the functions in core.stdc.fenv.

If the floating-point rounding mode is changed within a function, it must be restored before the function exits. If this rule is violated (for example, by the use of inline asm), the rounding mode used for subsequent calculations is undefined.

Exception Flags

IEEE 754 floating-point arithmetic can set several flags based on what happened with a computation:

FE_INVALID

FE_DENORMAL

FE_DIVBYZERO

FE_OVERFLOW

FE_UNDERFLOW

FE_INEXACT

These flags can be set/reset via the functions in core.stdc.fenv.

Floating-Point Transformations

An implementation may perform transformations on floating-point computations in order to reduce their strength.

Not all transformations are valid: The following transformations of floating-point expressions are not allowed because under IEEE rules they could produce different results.

Disallowed Floating-Point Transformations

transformation	comments
x + 0 → x	not valid if x is -0
x - 0 → x	not valid if x is ±0 and rounding is towards -∞
-x ↔ 0 - x	not valid if x is +0
x - x → 0	not valid if x is NaN or ±∞
x - y ↔ -(y - x)	not valid because (1-1=+0) whereas -(1-1)=-0
x * 0 → 0	not valid if x is NaN or ±∞
x / c ↔ x * (1/c)	valid if (1/c) yields an exact result
x != x → false	not valid if x is a NaN
x == x → true	not valid if x is a NaN
x !op y ↔ !(x op y)	not valid if x or y is a NaN

Of course, transformations that would alter side effects are also invalid.