On 5/31/2011 7:45 PM, Carl Banks wrote:
Fine, it wasn't "unheard of". I'm pretty sure the existence of a few
high end compiler/hardware combinations that supported traps doesn't
invalidate my basic point. NaN was needed because few systems had a
separate path to deal with exceptional situations like producing or
operating on something that isn't a number. When they did exist few
programmers used them. If floating-point were standardized today it
might not even have NaN (and definitely wouldn't support the
ridiculous NaN != NaN), because all modern systems can be expected to
support exceptions, and modern programmers can be expected to use
them.
Actually, it's the older architectures that support exact floating
point exceptions. x86 does, even in the superscalar and 64 bit variants.
But almost none of the RISC machines have exact floating point
exceptions. With all the parallelism inside FPUs today, it takes an
elaborate "retirement unit" to back out everything that happened
after the error and leave the CPU in a clean state. Most RISC
machines don't bother. ARM machines have both IEEE mode and
"run-fast mode"; the latter doesn't support FPU exceptions
but does support NaNs.
Many game machines and GPUs don't have full IEEE floating point.
Some don't have exceptions. Others don't have full INF/NaN semantics.
John Nagle
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