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 ...
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;
