Re: [PATCH] cleanup: fix possible overflow errors in binary search, part 2

["René Scharfe" <l.s.r@web.de>]

Am 13.06.19 um 21:42 schrieb Martin Ågren:
> On Thu, 13 Jun 2019 at 19:54, René Scharfe <l.s.r@web.de> wrote:
>>
>> Calculating the sum of two array indexes to find the midpoint between
>> them can overflow, i.e. code like this is unsafe for big arrays:
>>
>>         mid = (first + last) >> 1;
>>
>> Make sure the intermediate value stays within the boundaries instead,
>> like this:
>>
>>         mid = first + ((last - first) >> 1);
>>
>> The loop condition of the binary search makes sure that 'last' is
>> always greater than 'first', so this is safe as long as 'first' is
>> not negative.  And that can be verified easily using the pre-context
>> of each change, except for name-hash.c, so add an assertion to that
>> effect there.
>
> Right, with "safe", one might mean something like "no undefined behavior
> due to shifting a signed value with the high bit set". Especially since
> we're worrying about overflows, we're obviously having large values in
> mind, so we're right to consider the sign bit. But, we're fine as you
> note.  Because we subtract, and `last` doesn't have its sign bit set,
> and `first` is non-negative and not greater than `last`, the sign bit of
> `(last - first)` is always zero.
>
> So all is well. But maybe we should write `(last - first) / 2` anyway.
> We could then drop the extra parenthesis, and we would keep future
> readers (and static analysis?) from wondering whether we might ever be
> shifting a signed value with the sign bit set. A few spots fewer to
> audit in the future...

Yes, thought about that.  When I saw Clang 8 emitting extra opcodes for
handling the sign for the version with division I decided to restrict
the patch to just do overflow prevention and leave the right shifts in
place..

René