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July 24, 2006
Floating point trap handling -- is it possible?
I notice that the in the D docs, and in std.c.fenv, there is no mention 
of floating-point trap handling. Is it even feasible to handle traps in 
a portable manner?

Here's a theoretical example of overflow/underflow handling that would 
allow computation at maximum speed for normal arrays, and still yield 
correct results for (say) arr [] = [ real.max, real.max, real.min, 
real.min, 3.0 ];
which would ordinarily cause an overflow.

import mythical.floatTrap;

real product(real [] arr)
{
   int numOverflows=0;
   real prod = 1.0;

   floatTrap(OVERFLOW,  (real x) { prod = x; ++numOverflows; });
   floatTrap(UNDERFLOW, (real x) { prod = x; --numOverflows; });
   scope(exit) floatTrap(OVERFLOW, null); // restore previous trap state
   scope(exit) floatTrap(UNDERFLOW, null);

   foreach(x; arr) { prod*=x; }
   // now the total product is prod + pow(2, numOverflows*real.wrap)
   if (numOverflows==0) return prod;
   else {
    // do something sensible in this situation
   }
}

where
float.wrap = 192 = 0xC0
double.wrap = 1536 = 0x600
real.wrap = 0x6000

are intrinsic floating-point constant properties not currently supported 
by D (and which would be needed only if D could support traps).

I have no idea if this is feasible across the OSes supported by D; for 
example, it requires that the OS save the floating point exception 
handler states between threads. Walter once made a comment that 
signalling NaNs are poorly supported by OSes; is trap handling the reason?
July 24, 2006
Re: Floating point trap handling -- is it possible?
Don Clugston wrote:
> I notice that the in the D docs, and in std.c.fenv, there is no mention 
> of floating-point trap handling. Is it even feasible to handle traps in 
> a portable manner?

Not any better than you can in C.

> I have no idea if this is feasible across the OSes supported by D; for 
> example, it requires that the OS save the floating point exception 
> handler states between threads. Walter once made a comment that 
> signalling NaNs are poorly supported by OSes; is trap handling the reason?

I've generally just stuck with reading the sticky floating point 
exception flags.
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