Messages in this thread | | | Date | Thu, 16 Jan 2020 17:21:23 -0800 | From | Matthew Wilcox <> | Subject | Re: [PATCH] fs: optimise kiocb_set_rw_flags() |
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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;
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