Minimum floating-point requirements

[Rich Felker]

On Mon, Feb 17, 2014 at 03:59:04PM -0600, Steve Munroe wrote:
> > Such as...? Basically everywhere of modern relevance it's either
> > equivalent to double or it's ld80 (which is a conforming IEEE extended
> > precision type).
> >
> If ld80 is IEEE it is after the fact. Otherwise why do we have:
> 
> -ffloat-store
>       Do not store floating-point variables in registers, and inhibit
>       other options that might change whether a floating-point value is
>       taken from a register or memory.
> 
> 
>       This option prevents undesirable excess precision on machines such as
>       the 68000 where the floating registers (of the 68881) keep more
>       precision than a double is supposed to have. Similarly for the x86
>       architecture. For most programs, the excess precision does only good,
>       but a few programs rely on the precise definition of IEEE floating
>       point. Use -ffloat-store for such programs, after modifying them to
>       store all pertinent intermediate computations into variables.

The description of this option is outdated. It does not give
conforming behavior; it actually makes the behavior worse by
introducing double-rounding in places it shouldn't happen. The modern
replacement is -fexcess-precision=standard, which is default with
-std=c* and gives the ISO C mandated behavior for FLT_EVAL_METHOD!=0.

> And why do I keep find packages with -ffloat-store forced on? This is bad
> because PowerPC always had IEEE compliant double and this really hurts
> performance.

Ignorance most likely. The incorrect description above does not help.

Rich