Re: clear_bit_unlock_is_negative_byte

[Andreas Schwab <schwab@linux-m68k.org>]

On Jul 22 2023, Matthew Wilcox wrote:

> On Sat, Jul 22, 2023 at 08:24:34AM +0200, Andreas Schwab wrote:
>> On Jul 21 2023, Michael Schmitz wrote:
>> 
>> > static inline bool clear_bit_unlock_is_negative_byte(unsigned int nr,
>> >                 volatile unsigned long *p)
>> > {
>> >         unsigned char *cp = (unsigned char *) p;
>> >         char result;
>> >         char mask = 1 << nr;    /* nr guaranteed to be < 7 */
>> >
>> >         __asm__ __volatile__ ("eori.b %1, %2; smi %0"
>> >                 : "=d" (result)
>> >                 : "i" (mask), "o" (*(cp+3))
>> 
>> That should use "id" as constraint, so that the compiler can share the
>> constant with other insns.  Also, the third operand is modified, so it
>> needs to be marked as in/out.
>
> The "o" constraint is shared with bfset_mem_set_bit,
> bfclr_mem_test_and_clear_bit and a few other functions.  Should they
> all be changed?

They use a memory clobber, because the "o" constraint cannot accurately
describe the situation there.

-- 
Andreas Schwab, schwab@linux-m68k.org
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