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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: is double slower?
Date: Tue, 5 Nov 2024 09:14:15 +0100
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On 04/11/2024 08:53, fir wrote:
> float takes less space and when you keep arrays of floats for sure float
> is better (less spase and uses less memory bandwidth so i guess floats
> can be as twice faster in some aspects)
> 

Certainly if you have a lot of them, then the memory bandwidth and cache 
it rate can make floats faster than doubles.

> but when you do calculations on local variables not floats do the double 
> is slower?

I assume that for the calculations in question, the accuracy and range 
of float is enough - otherwise the answer is obviously use doubles.


This is going to depend on the cpu, the type of instructions, the source 
code in question, the compiler and the options.  So there is no single 
easy answer.

You can, as Bonita suggested, look up instruction timing information at 
agner.org for the cpu you are using (assuming it's an x86 device) to get 
some idea of any fundamental differences in timings.  Usually for modern 
"big" processors, basic operations such as addition and multiplication 
are single cycle or faster (i.e., multiple instructions can be done in 
parallel) for float and double.  But division, square root, and other 
more complex operations can take a lot longer with doubles.

Next, consider if you can be using vector or SIMD operations.  On some 
devices, you can do that with floats but not doubles - and even if you 
can use doubles, you can usually run floats at twice the rate.


In the source code, remember it is very easy to accidentally promote to 
double when writing in C.  If you want to stick to floats, make sure you 
don't use double-precision constants - a missing "f" suffix can change a 
whole expression into double calculations.  Remember that it takes time 
to convert between float and double.


Then look at your compiler flags - these can make a big difference to 
the speed of floating point code.  I'm giving gcc flags, because those 
are the ones I know - if you are using another compiler, look at the 
details of its flags.

Obviously you want optimisation enabled if speed is relevant - -O2 is a 
good start.  Make sure you are optimising for the cpu(s) you are using - 
"-march=native" is good for local programs, but you will want something 
more specific if the binary needs to run on a variety of machines.  The 
closer you are to the exact cpu model, the better the code scheduling 
and instruction choice can be.

Look closely at "-ffast-math" in the gcc manual.  If that is suitable 
for your code (and it often is), it can make a huge difference to 
floating point intensive code.  If it is unsuitable because you have 
infinities, or need deterministic control of things like associativity, 
it will make your results wrong.

"-Wdouble-promotion" can be helpful to spot accidental use of doubles in 
what you think is a float expression.  "-Wfloat-equal" is a good idea, 
especially if you are mixing floats and doubles.  "-Wfloat-conversion" 
will warn about implicit conversions from doubles to floats (or to 
integers).