Re: mcpu vs march

[Channagoud <ckadabi at codeaurora dot org>]

On 8/9/2013 9:59 AM, Kyrylo Tkachov wrote:
> > On 8/9/2013 4:32 AM, Kyrylo Tkachov wrote:
> > > > Hi,
> > > > I wanted to understand the difference of using march vs mcpu as
> > argument
> > > > to gcc.
> > > > I am compiling my code for a cortex-A8 CPU,  what would be better to
> > use:
> > > > -mcpu=cortex-a8 or -march=armv7-a
> > > -mcpu=cortex-a8 will perform specific optimisations for the Cortex-A8
> > such as
> > > instruction scheduling and will produce better performing code on that
> > core.
> > > -march=armv7-a just selects the ARMv7-a architecture which tells the
> > compiler
> > > that it can use the instructions in ARMv7-a, but it will not perform
> > any
> > > core-specific performance tuning.
> > >
> > > If you know that you'll be running your code on a Cortex-A8 you should
> > specify
> > > -mcpu=cortex-a8, it also automatically implies -march=armv7-a.
> > >
> > > For more information on the options available for ARM you can look at:
> > > http://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html
> > >
> > > HTH,
> > > Kyrill
> > >
> > > > Thanks,
> > > > Channa
> > Hi Kyrill,
> > Thanks for the info.
> >
> > Yes thats understood, but my concern is with respect to differences in
> > instructions when the code is compiled. For eg: if I use armv7-a as -
> > march & my cpu is cortex-a5 would it be possible that the code would
> > compile fine but might give undefined instruction at some point if the
> > compiler generated code has some instruction which is not supported on
> > cortex-a5?
> march=armv7-a will generate code that will run fine on all cores implementing
> ARMv7-a.
> Cortex-A5 is an ARMv7-a core, so with -march=armv7-a the code should not
> produce any undefined instruction exceptions on it.
>
> That being said, keep in mind that there is also the -mfpu option that
> controls the floating point/SIMD instructions that can be generated.
> If, for example, you compile with -mfpu=neon-vfpv4 but the core you're running
> on does not have a NEON unit, then you would get an exception if your code
> contained any NEON instructions.
>
> Note that the neither the -mcpu or -march options automatically set -mfpu,
> that's something that you have to set yourself.
>
> Kyrill
Hi Kyrill,

But what if there are certain CPU specific features?
For eg: cortex -a5 could have some features which are not supported on 
cortexa-8. And if I compile the code using -march=armv7-a ,compiler 
would compile it fine for both cortex-a8 & cortex-a5. But the code might 
have some issues running on cortex-a8 right?

Thanks,
Channa