Re: [PATCH] create_delta_index: simplify condition always evaluating to true

["Philip Oakley" <philipoakley@iee.org>]

From: "Stefan Beller" <stefanbeller@googlemail.com>
> The code sequence  ' (1u << i) < hsize && i < 31 ' is a multi step
> process, whose first step requires that 'i' is already less that 31,
> otherwise the result (1u << i)  is undefined (and  'undef_val < hsize'
> can therefore be assumed to be 'false'), and so the later test  i < 31
> can always be optimized away as dead code ('i' is already less than 
> 31,
> or the short circuit 'and' applies).
>
> So we need to get rid of that code. One way would be to exchange the
> order of the conditions, so the expression 'i < 31 && (1u << i) < 
> hsize'
> would remove that optimized unstable code already.
>
> However when checking the previous lines in that function, we can 
> deduce
> that 'hsize' must always be smaller than (1u<<31), since 506049c7df2c6
> (fix >4GiB source delta assertion failure), because 'entries' is
> capped at an upper bound of 0xfffffffeU, so 'hsize' contains a maximum
> value of 0x3fffffff, which is smaller than (1u<<31), so the value of
> 'i' will never be larger than 31 and we can remove that condition
> entirely.
>
> Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
> Acked-by: Nicolas Pitre <nico@fluxnic.net>
> ---

Acked-by: Philip Oakley philipoakley@iee.org

If there are concens that future upgrades may make the (i < 31) test a 
necessity that it should be added before the &&. If the compiler is able 
to determine it's dead code in that case then that's OK, at least it 
would be future proof. Then again it's probably not likely in the near 
future.

>
> Philip,
> thanks for the wording of your mail. I get quickly derailed from the
> warnings of the STACK tool and write the other commit messages than
> I ought to write. I think the wording of your mail nails it pretty
> good, so I put it in the commit message as well.
>
> Stefan
>
>
> diff-delta.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/diff-delta.c b/diff-delta.c
> index 93385e1..3797ce6 100644
> --- a/diff-delta.c
> +++ b/diff-delta.c
> @@ -155,7 +155,7 @@ struct delta_index * create_delta_index(const void 
> *buf, unsigned long bufsize)
>  entries = 0xfffffffeU / RABIN_WINDOW;
>  }
>  hsize = entries / 4;
> - for (i = 4; (1u << i) < hsize && i < 31; i++);
> + for (i = 4; (1u << i) < hsize; i++);
>  hsize = 1 << i;
>  hmask = hsize - 1;
>
> -- 
> 1.8.4.rc3.2.g2c2b664
>
>
Philip