Re: [PATCH] powerpc: define __LITTLE_ENDIAN and __BIG_ENDIAN for math-emu

[David Laight]

On Wed, 20 May 2026 15:14:40 +0200
"Christophe Leroy (CS GROUP)" <chleroy@xxxxxxxxxx> wrote:

> Le 17/05/2026 à 15:54, David Laight a écrit :
> > On Sun, 17 May 2026 12:14:21 +0800
> > Mingcong Bai <jeffbai@xxxxxxx> wrote:
> >   
> >> Similar to commit b929926f01f2 ("sh: define __BIG_ENDIAN for math-emu"),
> >> define __LITTLE_ENDIAN and __BIG_ENDIAN as 0 to mitigate build-time
> >> warnings:
> >>
> >>    ./include/math-emu/double.h:59:21: error: ‘__BIG_ENDIAN’ is not defined, evaluates to ‘0’ [-Werror=undef]
> >>       59 | #if __BYTE_ORDER == __BIG_ENDIAN
> >>          |
> >>
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Fixes: 13da9e200fe4 ("Revert "endian: #define __BYTE_ORDER"")
> >> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> >> Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Foe-kbuild-all%2F202507301656.7FEX6J5W-lkp%40intel.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C08977974fb1c495e9bd508deb41bd275%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639146228768693730%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=4qGulR%2BL7i7inksEbEH9jNGZS8HG80uvm3I9IyYzZww%3D&reserved=0
> >> Signed-off-by: Mingcong Bai <jeffbai@xxxxxxx>
> >> ---
> >>   arch/powerpc/include/asm/sfp-machine.h | 4 +++-
> >>   1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/powerpc/include/asm/sfp-machine.h b/arch/powerpc/include/asm/sfp-machine.h
> >> index 8b957aabb826d..db8525605c026 100644
> >> --- a/arch/powerpc/include/asm/sfp-machine.h
> >> +++ b/arch/powerpc/include/asm/sfp-machine.h
> >> @@ -319,10 +319,12 @@
> >>   #define abort()								\
> >>   	return 0
> >>   
> >> -#ifdef __BIG_ENDIAN
> >> +#ifdef __BIG_ENDIAN__
> >>   #define __BYTE_ORDER __BIG_ENDIAN
> >> +#define __LITTLE_ENDIAN 0
> >>   #else
> >>   #define __BYTE_ORDER __LITTLE_ENDIAN
> >> +#define __BIG_ENDIAN 0
> >>   #endif  
> > 
> > I thought the expected/correct value for __BYTE_ORDER__ was either 1234 or 4321.
> > (apart from pdp11's 2143).  
> 
> That's the case, in include/linux/kconfig.h we have:
> 
> #ifdef CONFIG_CPU_BIG_ENDIAN
> #define __BIG_ENDIAN 4321
> #else
> #define __LITTLE_ENDIAN 1234
> #endif
> 
> But as far as I understand the problem is that math-emu expects 
> __BIG_ENDIAN to be defined at all time as it has tests like:
> 
> #if __BYTE_ORDER == __BIG_ENDIAN

The gcc docs have (https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html):
     
__BYTE_ORDER__
__ORDER_LITTLE_ENDIAN__
__ORDER_BIG_ENDIAN__
__ORDER_PDP_ENDIAN__

    __BYTE_ORDER__ is defined to one of the values __ORDER_LITTLE_ENDIAN__, __ORDER_BIG_ENDIAN__, or __ORDER_PDP_ENDIAN__ to reflect the layout of multi-byte and multi-word quantities in memory. If __BYTE_ORDER__ is equal to __ORDER_LITTLE_ENDIAN__ or __ORDER_BIG_ENDIAN__, then multi-byte and multi-word quantities are laid out identically: the byte (word) at the lowest address is the least significant or most significant byte (word) of the quantity, respectively. If __BYTE_ORDER__ is equal to __ORDER_PDP_ENDIAN__, then bytes in 16-bit words are laid out in a little-endian fashion, whereas the 16-bit subwords of a 32-bit quantity are laid out in big-endian fashion.

    You should use these macros for testing like this:

    /* Test for a little-endian machine */
    #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

The doc doesn't mention the value, but __ORDER_BIG_ENDIAN__ is 4321 (decimal).

So the math-emu code is neither following gcc's rules or the kernel ones.

Your change will break anything that currently does:
#ifdef __BIG_ENDIAN

Any change would have to be limited to code that is implementing math-emu.

-- David

> 
> Christophe
>