Re: [PATCH] tools/nolibc: MIPS: fix clobbers of 'lo' and 'hi' registers on different ISAs

[Maciej W. Rozycki]

On Wed, 18 Feb 2026, Thomas Weißschuh wrote:

> Earlier MIPS64 ISAs still provide the 'lo' and 'hi' special registers.
> These are clobbered by system calls and need to be marked as such to
> avoid miscompilations.

 Correct.

> Also 32-bit ISAs from r6 on do not define the 'lo' and 'hi' registers.

 There's no MIPS ISA beyond R6 (nanoMIPS is a completely different ISA).  
Also MIPS64r6 is the same in this respect as MIPS32r6 is.

> diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
> index a72506ceec6b..210bd907b51f 100644
> --- a/tools/include/nolibc/arch-mips.h
> +++ b/tools/include/nolibc/arch-mips.h
> @@ -41,23 +41,44 @@
>  
>  #if defined(_ABIO32)
>  
> -#define _NOLIBC_SYSCALL_CLOBBERLIST \
> -	"memory", "cc", "at", "v1", "hi", "lo", \
> -	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
>  #define _NOLIBC_SYSCALL_STACK_RESERVE "addiu $sp, $sp, -32\n"
>  #define _NOLIBC_SYSCALL_STACK_UNRESERVE "addiu $sp, $sp, 32\n"
>  
>  #else /* _ABIN32 || _ABI64 */
>  
> +#define _NOLIBC_SYSCALL_STACK_RESERVE
> +#define _NOLIBC_SYSCALL_STACK_UNRESERVE
> +
> +#endif /* _ABIO32 */
> +
> +
> +#if defined(_ABIO32) && __mips_isa_rev >= 6
> +
> +#define _NOLIBC_SYSCALL_CLOBBERLIST \
> +	"memory", "cc", "at", "v1", \
> +	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
> +
> +#elif defined(_ABIO32) && __mips_isa_rev < 6
> +
> +#define _NOLIBC_SYSCALL_CLOBBERLIST \
> +	"memory", "cc", "at", "v1", "hi", "lo", \
> +	"t0", "t1", "t2", "t3", "t4", "t5", "t6", "t7", "t8", "t9"
> +
> +#elif __mips_isa_rev >= 6 /* _ABIN32 || _ABI64 */
> +
>  /* binutils, GCC and clang disagree about register aliases, use numbers instead. */
>  #define _NOLIBC_SYSCALL_CLOBBERLIST \
>  	"memory", "cc", "at", "v1", \
>  	"10", "11", "12", "13", "14", "15", "24", "25"
>  
> -#define _NOLIBC_SYSCALL_STACK_RESERVE
> -#define _NOLIBC_SYSCALL_STACK_UNRESERVE
> +#else /* __mips_is_rev < 6 && (_ABIN32 || _ABI64) */

 s/__mips_is_rev/__mips_isa_rev/

> +
> +#define _NOLIBC_SYSCALL_CLOBBERLIST \
> +	"memory", "cc", "at", "v1", "hi", "lo", \
> +	"10", "11", "12", "13", "14", "15", "24", "25"
> +
> +#endif /* __mips_isa_rev and ABI */
>  
> -#endif /* _ABIO32 */

 I think this would be more readable/maintainable if you did something 
such as:

#if __mips_isa_rev < 6
#define _NOLIBC_SYSCALL_CLOBBER_HI "hi"
#define _NOLIBC_SYSCALL_CLOBBER_LO "lo"
#else
#define _NOLIBC_SYSCALL_CLOBBER_HI "$0"
#define _NOLIBC_SYSCALL_CLOBBER_LO "$0"
#endif

and then used the macros rather than duplicating the clobber lists.  Or 
you could reuse the approach taken with VDSO_SYSCALL_CLOBBERS, perhaps a 
bit messy, also when it comes to the name of the macro.