Re: [linux-next:master 3203/9634] arch/powerpc/include/asm/bitops.h:126:26: error: constraint 'n' expects an integer constant expression

[Andrea Righi <arighi@nvidia.com>]

Hi Christophe,

On Sun, Mar 09, 2025 at 07:48:56PM +0100, Christophe Leroy wrote:
...
> #define DEFINE_CLROP(fn, prefix)					\
> static inline void fn(unsigned long mask, volatile unsigned long *_p)	\
> {									\
> 	unsigned long old;						\
> 	unsigned long *p = (unsigned long *)_p;				\
> 									\
> 	if (IS_ENABLED(CONFIG_PPC32) &&					\
> 	    __builtin_constant_p(mask) && is_rlwinm_mask_valid(~mask)) {\
> 		asm volatile (						\
> 			prefix						\
> 		"1:"	"lwarx	%0,0,%3\n"				\
> 			"rlwinm	%0,%0,0,%2\n"				\
> 			"stwcx.	%0,0,%3\n"				\
> 			"bne- 1b\n"					\
> 			: "=&r" (old), "+m" (*p)			\
> 			: "n" (~mask), "r" (p)				\
> 			: "cc", "memory");				\
> 	} else {							\
> 		asm volatile (						\
> 			prefix						\
> 		"1:"	PPC_LLARX "%0,0,%3,0\n"				\
> 			"andc %0,%0,%2\n"				\
> 			PPC_STLCX "%0,0,%3\n"				\
> 			"bne- 1b\n"					\
> 			: "=&r" (old), "+m" (*p)			\
> 			: "r" (mask), "r" (p)				\
> 			: "cc", "memory");				\
> 	}								\
> }
> 
> 
> clang says "constraint 'n' expects an integer constant expression" ?
> 
> The only place I see 'n' constraint in that macro is the '"n" (~mask)' in
> the if __builtin_constant_p(mask) branch. As far as I know if mask is a
> constant (~mask) is also a constant ?

Right, that's my understanding as well.

> 
> Must be a clang bug ?

Possibly...

> 
> Maybe try with __builtin_constant_p(~mask) instead of
> __builtin_constant_p(mask)

I tried using ~mask, and even tried to use just mask throughout the
entire macro (although the code would be incorrect), but I still get the
same build error.

The only way that I found to fix this is by adding
!IS_ENABLED(CONFIG_CC_IS_CLANG) to the if, which isn't really elegant, but
maybe it's a reasonable workaround? I'll also open a clang issue in the
meantime.

Thanks,
-Andrea