IEEE Floating-Point

Sather - Language Specification
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# 14.5. IEEE Floating-Point

Sather attempts to conform to the IEEE 754-1985 specification for its floating point types. Unfortunately, many platforms make it difficult to do so. For example, underflow is often improperly implemented to flush to zero rather than use IEEE's gradual underflow. This happens because gradual underflow is a special case and can be quite slow if implemented using traps. When benchmarks include simulations which cause many underflows, marketing pressures make flush-to-zero the default.

There are many other problems. Microsoft's C and C++ compilers defeat the purpose of the invalid flag by using it exclusively to detect floating-point stack overflows, so programmers cannot use it. There is no portable C interface to IEEE exception flags and their behavior with respect to 'setjmp' is suspect. Threads packages often fail to address proper handling of IEEE exceptions and rounding modes.

Correct IEEE support from various platforms was the single worst porting headache of the Sather 1.0 compiler. In 1.1, we give up and make full IEEE compliance optional. Sather implementations are expected to conform to the spirit, if not the letter, of IEEE 754, although proper exceptions, extended types, underflow handling, and correct handling of positive and negative zero are specifically not required.

The Sather treatment of NaNs is particularly tricky; IEEE wants NaN to be neither equal nor unequal to anything else, including other NaNs. Because Sather defines 'x /= y' as 'x.is_eq(y).not' (See Syntactic sugar expressions), to get the IEEE notion of unequal is necessary to write 'x=x and y=y and x/=y'. Other comparison operators present similar difficulties.

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