Hard float/soft float

interndan · 07-31-2014, 02:14 PM

I'm curious as to what all would be involved in building Slackware Arm with hard float. Is it a matter of re-compiling the kernel, or is there more involved. If more than just the kernel, what all would have to be rebuilt? I might be interested in building hard float enabled for the Pi.

smallpond · 07-31-2014, 03:14 PM

The kernel does not use floats. Saving the state on interrupts would kill performance. User space can do what it wants on a program-by-program basis.

louigi600 · 08-01-2014, 04:35 AM

It is possible to have a hardfloat enabled kernel and run a softfloat userland that will just never ask the kernel to do hardfloat calculations ... to put it in short you need both kernel and userland to be compiled to use hardfloat.
There is a hibrid solution as suggested in othr posts bu gus3:

Quote:

Originally Posted by gus3

There is also a third option for ARM: "softfp", which passes FP arguments on the stack, conforming to "soft" ABI, but generates instructions for using the VFP/Neon instruction within a function. It can provide a code speedup, as well as decreasing a process's RSS. It's even possible to rebuild Slackware's Glibc to use softfp for /lib/libm.*. I have a HOWTO explaining this, but bear in mind it is definitely not for the faint-hearted. I can't post a URL, but you can do a Google search for "slackware arm rebuild glibc vfp" and it will be at the top of the results.

drmozes · 08-01-2014, 05:29 AM

Quote:

Originally Posted by louigi600

It is possible to have a hardfloat enabled kernel and run a softfloat userland that will just never ask the kernel to do hardfloat calculations ... to put it in short you need both kernel and userland to be compiled to use hardfloat.
There is a hibrid solution as suggested in othr posts bu gus3:

The Kernel is not connected to the Hard float or Soft float - this is just userland stuff.
I could take the same 'armv7' kernel in Slackware ARM and use it with a hard float userland.

The soft float vs hard float question has been answered many times already. I might collate the info at some point and put it into a sticky thread.
http://www.linuxquestions.org/questi...archid=6749461

interndan · 08-01-2014, 09:43 AM

I am working on a project with my Raspberry Pi and am looking for ways to optimize its performance. All my searches suggest compiling for hard float will do that. Obviously glibc will need to be rebuilt, but the question is what else will need rebuilding. Leaving aside the kernel ( I will take your word Stuart that is not needed) what else will have to be rebuilt. If the answer is "everything" then perhaps I should start over and install a distro that is already built for hard float. I would rather not do that since Slackware has always been my distro of choice.

By the way Stuart the link you provided just takes me to a page that says 'sorry no matches'.

Thanks Dan

drmozes · 08-01-2014, 10:23 AM

Quote:

Originally Posted by interndan

I am working on a project with my Raspberry Pi and am looking for ways to optimize its performance. All my searches suggest compiling for hard float will do that. Obviously glibc will need to be rebuilt, but the question is what else will need rebuilding. Leaving aside the kernel ( I will take your word Stuart that is not needed) what else will have to be rebuilt. If the answer is "everything" then perhaps I should start over and install a distro that is already built for hard float. I would rather not do that since Slackware has always been my distro of choice.

By the way Stuart the link you provided just takes me to a page that says 'sorry no matches'.

Thanks Dan

It seems that the search is time-bound.

I've just posted a summary of hf/sf as a new thread which I'll get made 'sticky' later.