Re: [PATCH] powerpc/atomic: Use YZ constraints for DS-form instructions

[Segher Boessenkool]

Hi!

On Mon, Sep 16, 2024 at 10:05:10PM +1000, Michael Ellerman wrote:
> The 'ld' and 'std' instructions require a 4-byte aligned displacement
> because they are DS-form instructions. But the "m" asm constraint
> doesn't enforce that.
> 
> That can lead to build errors if the compiler chooses a non-aligned
> displacement, as seen with GCC 14:
> 
>   /tmp/ccuSzwiR.s: Assembler messages:
>   /tmp/ccuSzwiR.s:2579: Error: operand out of domain (39 is not a multiple of 4)
>   make[5]: *** [scripts/Makefile.build:229: net/core/page_pool.o] Error 1
> 
> Dumping the generated assembler shows:
> 
>   ld 8,39(8)       # MEM[(const struct atomic64_t *)_29].counter, t
> 
> Use the YZ constraints to tell the compiler either to generate a DS-form
> displacement, or use an X-form instruction, either of which prevents the
> build error.

Great explanation text, a perfect commit!  :-)

> See commit 2d43cc701b96 ("powerpc/uaccess: Fix build errors seen with
> GCC 13/14") for more details on the constraint letters.
> 
> Fixes: 9f0cbea0d8cc ("[POWERPC] Implement atomic{, 64}_{read, write}() without volatile")
> Cc: stable@xxxxxxxxxxxxxxx # v2.6.24+
> Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20240913125302.0a06b4c7@xxxxxxxxxxxxxxxx
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

Reviewed-By: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>


Segher