On 17/01/2020 04:21, Matthew Wilcox wrote:
> On Fri, Jan 17, 2020 at 04:16:41AM +0300, Pavel Begunkov wrote:
>> kiocb_set_rw_flags() generates a poor code with several memory writes
>> and a lot of jumps. Help compilers to optimise it.
>>
>> Tested with gcc 9.2 on x64-86, and as a result, it its output now is a
>> plain code without jumps accumulating in a register before a memory
>> write.
>
> Nice!
>
>> static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>> {
>> + int kiocb_flags = 0;
>> +
>> if (unlikely(flags & ~RWF_SUPPORTED))
>> return -EOPNOTSUPP;
>>
>> if (flags & RWF_NOWAIT) {
>> if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
>> return -EOPNOTSUPP;
>> - ki->ki_flags |= IOCB_NOWAIT;
>> + kiocb_flags |= IOCB_NOWAIT;
>> }
>> if (flags & RWF_HIPRI)
>> - ki->ki_flags |= IOCB_HIPRI;
>> + kiocb_flags |= IOCB_HIPRI;
>> if (flags & RWF_DSYNC)
>> - ki->ki_flags |= IOCB_DSYNC;
>> + kiocb_flags |= IOCB_DSYNC;
>> if (flags & RWF_SYNC)
>> - ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
>> + kiocb_flags |= (IOCB_DSYNC | IOCB_SYNC);
>> if (flags & RWF_APPEND)
>> - ki->ki_flags |= IOCB_APPEND;
>> + kiocb_flags |= IOCB_APPEND;
>> +
>> + if (kiocb_flags)
>> + ki->ki_flags |= kiocb_flags;
>> return 0;
>> }
>
> Might it generate even better code to do ...
Good idea, thanks! I'll resend
>
> int kiocb_flags = 0;
>
> + if (!flags)
> + return 0;
> if (unlikely(flags & ~RWF_SUPPORTED))
> return -EOPNOTSUPP;
>
> ...
>
> - if (kiocb_flags)
> - ki->ki_flags |= kiocb_flags;
> + ki->ki_flags |= kiocb_flags;
>
--
Pavel Begunkov
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