Re: Building Single Tree for a Specific Set of CFLAGS

[Christophe Lyon <christophe.lyon@arm.com>]

On 3/27/24 20:07, Joel Sherrill wrote:
> 
> 
> On Wed, Mar 27, 2024 at 3:53 AM Christophe Lyon via Gcc <gcc@gcc.gnu.org 
> <mailto:gcc@gcc.gnu.org>> wrote:
> 
>     Hi!
> 
>     On 3/26/24 22:52, Joel Sherrill via Gcc wrote:
>      > Hi
>      >
>      > For RTEMS, we normally build a multilib'ed gcc+newlib, but I have
>     a case
>      > where the CPU model is something not covered by our multilibs and
>     not one
>      > we are keen to add. I've looked around but not found anything
>     that makes me
>      > feel confident.
>      >
>      > What's the magic for building a gcc+newlib with a single set of
>     libraries
>      > that are built for a specific CPU CFLAGS?
>      >
>      > I am trying --disable-multlibs on the gcc configure and adding
>      > CFLAGS_FOR_TARGET to make.
>      >
>      > Advice appreciated.
>      >
> 
>     I would configure GCC with --disable-multilibs --with-cpu=XXX
>     --with-mode=XXX --with-float=XXX [maybe --with-fpu=XXX]
>     This way GCC defaults to what you want.
> 
> 
> Thanks. Is there any documentation or even a good example? I found
> --with-mode=[arm|thumb] but am having trouble mapping the others back
> to GCC options.

I don't know of any good doc/example.
I look in gcc/config.gcc to check what is supported.

> 
> I have this for CFLAGS_FOR_TARGET
> 
> "-mcpu=cortex-m7 -mthumb -mlittle-endian -mfloat-abi=hard 
> -mfpu=fpv5-sp-d16 -march=armv7e-m+fpv5"
> 
> I think that means...
> 
> --with-mode=thumb   for -mthumb
> --with-cpu=cortex-m7 for -mcortex-m7
> --with-float=hard         for -mfloat-abi=hard
> 
> That leaves a few options I don't know how to map.
> 
You can see that for arm:
supported_defaults="arch cpu float tune fpu abi mode tls"
so there's a --with-XXX for any of the above, meaning that there's no 
--with-endian (default endianness on arm is derived from the target 
triplet eg. armeb-* vs arm-*)

Also note that config.gcc checks that you don't provide both
--with-cpu and --with-arch
or --with-cpu and --with-tune

HTH,

Christophe

> --joel
> 
> 
>     Thanks,
> 
>     Christophe
> 
> 
>      > Thanks.
>      >
>      > --joel
>