What is the minimum precision allowed for floats.

Décio Luiz Gazzoni Filho decio at decpp.net
Mon Sep 29 05:05:27 CEST 2008

On Sep 28, 2008, at 5:47 PM, Joel Kronander wrote:

> Hi
> I have a question regading the possible minimun precision for floats.
> For example is it possible to use a precision between native c float  
> and double
> ie precision digits between 6-15 for exmample.

I suppose you could do it, but why? First of all, you'd be moving from  
hardware-accelerated to software-emulated, so I would expect slowdowns  
on the order of 10x-100x. If you're doing it hoping to get a speedup,  
just trash the idea, it'll never ever work. If you want to save memory  
by, say, having a 48-bit float, and you don't care about the slowdown  
due to software emulation... well, first of all, GMP probably works  
with multiples of 32 or 64 bits, so 48 bits is out of the question,  
and besides, I believe it uses a whole limb (i.e. 32 or 64 bits) for  
the exponent and whatever is needed for the mantissa, so there'd be a  
further waste of memory if that's what you're concerned with. Maybe if  
you roll your own package.

I have no idea what you're working with, but have you considered fixed  
point rather than floating point? The speed hit shouldn't be too  
substantial, and you get the full word for the mantissa (in exchange  
for limited dynamic range).


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