Hi Dscho,
On Thu, Sep 26, 2019 at 01:30:10AM -0700, Johannes Schindelin via GitGitGadget
wrote:
> From: Johannes Schindelin <[email protected]>
>
> MSVC complains about this with `-Wall`, which can be taken as a sign
> that this is indeed a real bug. The symptom is:
>
> C4146: unary minus operator applied to unsigned type, result
> still unsigned
>
> Let's avoid this warning in the minimal way, e.g. writing `-1 -
> <unsigned value>` instead of `-<unsigned value> - 1`.
[...]
> ---
> read-cache.c | 4 ++--
> sha1-lookup.c | 2 +-
> 2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/read-cache.c b/read-cache.c
> index c701f7f8b8..11f3357216 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -1276,7 +1276,7 @@ static int add_index_entry_with_check(struct
> index_state *istate, struct cache_e
> */
> if (istate->cache_nr > 0 &&
> strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
> - pos = -istate->cache_nr - 1;
> + pos = -1 - istate->cache_nr;
I've been thinking about this and I'm still not certain that this 100%
correct from a language-lawyer perspective.
If we do `-1 - istate->cache_nr`, then the unsignedness of
istate->cache_nr takes over and the whole expression is a very large
unsigned number.
Then, when we assign to `int pos`, we are converting an unsigned number
which is out of the range of the signed number. According to a
StackOverflow post citing the C99 standard[1]:
Otherwise, the new type is signed and the value cannot be
represented in it; either the result is implementation-defined
or an implementation-defined signal is raised.
I'm sure that most platforms that we support will handle it sanely but
could we write this as
pos = -1 - (int) istate->cache_nr;
to be doubly sure that no funny business will happen?
> else
> pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce),
> ce_stage(ce));
>
> @@ -1894,7 +1894,7 @@ static size_t estimate_cache_size(size_t ondisk_size,
> unsigned int entries)
> /*
> * Account for potential alignment differences.
> */
> - per_entry += align_padding_size(sizeof(struct cache_entry),
> -sizeof(struct ondisk_cache_entry));
> + per_entry += align_padding_size(per_entry, 0);
> return ondisk_size + entries * per_entry;
> }
>
> diff --git a/sha1-lookup.c b/sha1-lookup.c
> index 796ab68da8..c819687730 100644
> --- a/sha1-lookup.c
> +++ b/sha1-lookup.c
> @@ -97,7 +97,7 @@ int sha1_pos(const unsigned char *sha1, void *table, size_t
> nr,
> lo = mi + 1;
> mi = lo + (hi - lo) / 2;
> } while (lo < hi);
> - return -lo-1;
> + return -1 - lo;
Same thing here.
[1]:
https://stackoverflow.com/questions/50605/signed-to-unsigned-conversion-in-c-is-it-always-safe
> }
>
> int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo,
> --
> gitgitgadget
>
