Does GCC use single-precision FP hardware on Cortex-M4 parts?

On Wednesday, September 28, 2016 at 8:50:31 AM UTC-4, Anders....@kapsi.spam.stop.fi.invalid wrote:
> Tim Wescott <tim@seemywebsite.com> wrote:
> > On Sat, 24 Sep 2016 09:19:58 -0700, Dave Nadler wrote:
> 
> >> The PIC32MZ EF series parts have hardware double precision;
> >> presumably GCC uses this for double?
> > I would check.  You may have to direct the compiler to do so.
> 
> By default Microchip's XC32 compiler uses 32-bit doubles. There is a 
> compiler switch for 64-bit doubles, but there have been several reports 
> on their forums about the wrong libraries being linked in or other such 
> nonsense.

Yes, this is lovely stuff:
http://www.microchip.com/forums/m940391.aspx

Thanks for the heads-up on -fno-short-double.
I expected this for PIC24, but a bit surprising for PIC32?
For some of our applications, 23 bits of precision doesn't do it...

Thanks Anders,
Best Regards, Dave

On Sunday, September 25, 2016 at 6:29:40 AM UTC-4, David Brown wrote:
> Yes, you need to tell gcc about the processor you are using:
> 
> -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16
> 
> You may also want to add "-fsingle-precision-constant" which makes gcc 
> treat floating point constants as single precision, so that "x * 2.5" is 
> done as single precision rather than double precision (without having to 
> write "x * 2.5f").
> 
> And you /definitely/ want to have "-Wdouble-promotion" to warn about any 
> implicit promotions to double that might have occurred accidentally. 
> (You can always cast to double, or assign values to double variables, if 
> you really want slow but accurate doubles in the code.)
> 
> 
> As for the maths library, "sinf", "cosf", and friends are standard 
> functions for single-precision versions of the trigonometric functions. 
>   But remember that there is no such thing as the "gcc library" - gcc 
> can come with a variety of libraries.  Some of these will have good 
> implementations of "sinf" and friends, done using only single-precision 
> floating point in order to be fast on a the M4F and similar processors. 
>   Others simply convert their argument to double, call "sin", then 
> convert the result back to single.  If you need to use standard library 
> maths functions, and you need the speed of single precision, then check 
> the details for the libraries available in the particular toolchain you 
> have.
> 
> Of course, when you need fast trig functions, it is usually best to have 
> tables, interpolations, or other approximations that give you the 
> required accuracy far faster than any IEEE-compliant standard library.
> 
> 
> > (In C++ one _could_ have a smart math library that would see the type of
> > the argument and call the correct library function -- I don't believe
> > that's done, and I could see it causing all sorts of trouble when things
> > get inadvertently up-cast to double as you maintain the code.)
> 
> If you are going for C++, you can easily create a wrapper around "float" 
> that does not have any implicit conversions to double so that you can be 
> sure to avoid accidents.
> 
> Or you can use the gcc flags I mentioned above :-)

Thanks David for your (as usual) clear explanation,
Best Regards, Dave

On Sunday, September 25, 2016 at 4:19:20 AM UTC-4, John Devereux wrote:
> FYI the latest M7 processors from ST have double-precision floating
> point e.g. STM32F769
> 
> <http://www.st.com/content/st_com/en/products/microcontrollers/stm32-32-bit-arm-cortex-mcus/stm32f7-series.html?querycriteria=productId=SS1858>

Thanks John, we'll add the ST part to the list...