Re: [PATCHv2 8/10] xfs: avoid repeated pointer dereferences

Alex Elder
2010-04-14T21:00:48Z

On Mon, 2010-04-12 at 16:56 +1000, Dave Chinner wrote:
> On Fri, Apr 09, 2010 at 05:30:42PM -0500, Alex Elder wrote:
> > In xlog_find_cycle_start() use a local variable for some repeated
> > operations rather than constantly updating the memory location
> > whose address is passed in.
>
> Won't the compiler optimise that out for us? i.e. does the dissassembly
> of the function look any better before and after this change?

I doubt it *can* optimize that. Doing so would change
the way the function interacts semantically with the
pointed-to memory. Still, I performed a quick check
to be sure (on an x86_64) and the compiled code really
does de-reference the pointer for both reads from and
writes to memory.

> > Signed-off-by: Alex Elder <aelder@sgi.com>
> >
> > ---
> > fs/xfs/xfs_log_recover.c | 83 ++++++++++++++++++++++++-----------------------
> > fs/xfs/xfs_log_recover.c | 26 ++++++++++++++------------
> > 1 file changed, 14 insertions(+), 12 deletions(-)
> >
> > Index: b/fs/xfs/xfs_log_recover.c
> > ===================================================================
> > --- a/fs/xfs/xfs_log_recover.c
> > +++ b/fs/xfs/xfs_log_recover.c
> > @@ -354,26 +354,28 @@ xlog_find_cycle_start(
> > {
> > xfs_caddr_t offset;
> > xfs_daddr_t mid_blk;
> > + xfs_daddr_t end_blk;
> > uint mid_cycle;
> > int error;
> >
> > - mid_blk = BLK_AVG(first_blk, *last_blk);
> > - while (mid_blk != first_blk && mid_blk != *last_blk) {
> > + ASSERT(last_blk != NULL);
> > + end_blk = *last_blk;
>
> FWIW, there is no need for that ASSERT there - the machine will
> panic on the very next line, anyway....

Agreed. I make a habit of stating assumptions about
passed-in arguments via assertions. Not a big deal,
I'll take it out.

Still soliciting a reviewed-by on this one.

-Alex