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I have just heard that the iphone cannot do double natively thereby making them much slower that regular float.
Is this true? Evidence?
I am very interested in the issue because my program needs high precision calculations, and I will have to compromise on speed.
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Swift enforces you to use Doubles instead of Floats. You should use Float only if you want to sacrifice precision to save memory. Also, if you are working with types that require Float, then you have to use a Float.
A float has 7 decimal digits of precision and occupies 32 bits . A double is a 64-bit IEEE 754 double-precision floating-point number. 1 bit for the sign, 11 bits for the exponent, and 52 bits for the value. A double has 15 decimal digits of precision and occupies a total of 64 bits .
Use float or double ? The precision of a floating point value indicates how many digits the value can have after the decimal point. The precision of float is only six or seven decimal digits, while double variables have a precision of about 15 digits. Therefore it is safer to use double for most calculations.
Difference in Precision (Accuracy) float and double both have varying capacities when it comes to the number of decimal digits they can hold. float can hold up to 7 decimal digits accurately while double can hold up to 15.
The iPhone can do both single and double precision arithmetic in hardware. On the 1176 (original iPhone and iPhone3G), they operate at approximately the same speed, though you can fit more single-precision data in the caches. On the Cortex-A8 (iPhone3GS, iPhone4 and iPad), single-precision arithmetic is done on the NEON unit instead of VFP, and is substantially faster.
Make sure to turn off thumb mode in your compile settings for armv6 if you are doing intensive floating-point computation.
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This slide show gives insight in why there isn't good floating point and why there is (the vector floating point unit). Apparently, it is important you check the "thumb mode" which influences whether or not floating point support is on. This is not always an improvement. It shows how to find the right instructions in the assembly code.
It also depends on what version of the phone you want to run your code. The most recent one seems "more capable" in doing floating point math.
EDIT: here's an interesting read on floating point optimizations on the ARM with VFP and NEON SSE.
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