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

> 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>
> ---

>
> 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
>
>