From: Denton Liu <liu.denton@gmail.com>
To: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH 02/13] msvc: avoid using minus operator on unsigned types
Date: Thu, 26 Sep 2019 16:57:53 -0700 [thread overview]
Message-ID: <20190926235753.GA94990@generichostname> (raw)
In-Reply-To: <nycvar.QRO.7.76.6.1909262236550.15067@tvgsbejvaqbjf.bet>
Hi Dscho,
On Thu, Sep 26, 2019 at 11:01:32PM +0200, Johannes Schindelin wrote:
> Hi Denton,
>
> On Thu, 26 Sep 2019, Denton Liu wrote:
>
> > Hi Dscho,
> >
> > On Thu, Sep 26, 2019 at 01:30:10AM -0700, Johannes Schindelin via GitGitGadget wrote:
> > > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> > >
> > > 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?
>
> I guess we should use `signed_add_overflows()` to make extra certain
> that it does what we want it to do, kind of like `st_add()`. Or just do
> the check explicitly, like so:
>
> if (istate->cache_nr > INT_MAX)
> die("overflow: -1 - %u", istate->cache_nr);
> pos = -1 - istate->cache_nr;
Could we change this to
pos = -1 - (int) istate->cache_nr;
so that we alleviate the problem I was talking about above?
Other than that, it looks good. Well, it might break on one's complement
systems but I don't think we support UNIVACs anyway. ;)
> }
> >
> > > 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.
>
> This is even more critical, as `lo` has the type `size_t`:
>
> if (lo > INT_MAX)
> die("overflow: -1 - %"PRIuMAX, (uintmax_t)lo);
> return -1 - lo;
Also, could we change this to
return -1 - (int) lo;
Thanks,
Denton
> >
> What do you think?
> Dscho
>
> > [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
> > >
> >
next prev parent reply other threads:[~2019-09-26 23:57 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-26 8:30 [PATCH 00/13] ci: include a Visual Studio build & test in our Azure Pipeline Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 01/13] push: do not pretend to return `int` from `die_push_simple()` Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 02/13] msvc: avoid using minus operator on unsigned types Johannes Schindelin via GitGitGadget
2019-09-26 17:20 ` Denton Liu
2019-09-26 21:01 ` Johannes Schindelin
2019-09-26 23:57 ` Denton Liu [this message]
2019-09-30 9:50 ` Johannes Schindelin
2019-09-26 8:30 ` [PATCH 03/13] winansi: use FLEX_ARRAY to avoid compiler warning Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 04/13] compat/win32/path-utils.h: add #include guards Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 05/13] msvc: ignore some libraries when linking Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 06/13] msvc: handle DEVELOPER=1 Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 07/13] msvc: work around a bug in GetEnvironmentVariable() Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 08/13] vcxproj: only copy `git-remote-http.exe` once it was built Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 09/13] vcxproj: include more generated files Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 10/13] test-tool run-command: learn to run (parts of) the testsuite Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 11/13] tests: let --immediate and --write-junit-xml play well together Johannes Schindelin via GitGitGadget
2019-09-28 22:22 ` Junio C Hamano
2019-09-30 9:52 ` Johannes Schindelin
2019-09-26 8:30 ` [PATCH 12/13] ci: really use shallow clones on Azure Pipelines Johannes Schindelin via GitGitGadget
2019-09-26 8:30 ` [PATCH 13/13] ci: also build and test with MS Visual Studio " Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 00/13] ci: include a Visual Studio build & test in our Azure Pipeline Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 01/13] push: do not pretend to return `int` from `die_push_simple()` Johannes Schindelin via GitGitGadget
2019-10-03 22:37 ` Junio C Hamano
2019-10-04 9:36 ` Johannes Schindelin
2019-09-30 9:55 ` [PATCH v2 02/13] msvc: avoid using minus operator on unsigned types Johannes Schindelin via GitGitGadget
2019-10-03 22:44 ` Junio C Hamano
2019-10-04 9:55 ` Johannes Schindelin
2019-10-04 17:09 ` Johannes Sixt
2019-10-04 21:24 ` Johannes Schindelin
2019-10-06 0:02 ` Junio C Hamano
2019-10-06 10:53 ` Johannes Sixt
2019-10-08 12:04 ` Johannes Schindelin
2019-10-08 21:13 ` Johannes Sixt
2019-09-30 9:55 ` [PATCH v2 03/13] winansi: use FLEX_ARRAY to avoid compiler warning Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 04/13] compat/win32/path-utils.h: add #include guards Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 05/13] msvc: ignore some libraries when linking Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 06/13] msvc: handle DEVELOPER=1 Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 07/13] msvc: work around a bug in GetEnvironmentVariable() Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 09/13] vcxproj: include more generated files Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 08/13] vcxproj: only copy `git-remote-http.exe` once it was built Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 10/13] test-tool run-command: learn to run (parts of) the testsuite Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 11/13] tests: let --immediate and --write-junit-xml play well together Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 13/13] ci: also build and test with MS Visual Studio on Azure Pipelines Johannes Schindelin via GitGitGadget
2019-09-30 9:55 ` [PATCH v2 12/13] ci: really use shallow clones " Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 00/13] ci: include a Visual Studio build & test in our Azure Pipeline Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 01/13] push: do not pretend to return `int` from `die_push_simple()` Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 02/13] msvc: avoid using minus operator on unsigned types Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 03/13] winansi: use FLEX_ARRAY to avoid compiler warning Johannes Schindelin via GitGitGadget
2019-10-07 19:16 ` Alban Gruin
2019-10-04 15:09 ` [PATCH v3 04/13] compat/win32/path-utils.h: add #include guards Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 05/13] msvc: ignore some libraries when linking Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 06/13] msvc: handle DEVELOPER=1 Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 07/13] msvc: work around a bug in GetEnvironmentVariable() Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 08/13] vcxproj: only copy `git-remote-http.exe` once it was built Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 09/13] vcxproj: include more generated files Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 10/13] test-tool run-command: learn to run (parts of) the testsuite Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 11/13] tests: let --immediate and --write-junit-xml play well together Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 12/13] ci: really use shallow clones on Azure Pipelines Johannes Schindelin via GitGitGadget
2019-10-04 15:09 ` [PATCH v3 13/13] ci: also build and test with MS Visual Studio " Johannes Schindelin via GitGitGadget
2019-10-06 0:19 ` [PATCH v3 00/13] ci: include a Visual Studio build & test in our Azure Pipeline Junio C Hamano
2019-10-06 10:45 ` Johannes Schindelin
2019-10-06 20:38 ` Johannes Schindelin
2019-10-07 1:14 ` Junio C Hamano
2019-10-07 21:51 ` Johannes Schindelin
2019-10-08 2:19 ` Junio C Hamano
2019-10-08 12:46 ` Johannes Schindelin
2019-10-09 13:57 ` Philip Oakley
2019-10-10 9:03 ` Johannes Schindelin
2019-10-10 10:12 ` Philip Oakley
2019-10-07 0:59 ` Junio C Hamano
2019-10-07 16:08 ` Thomas Gummerer
2019-10-11 22:06 ` Johannes Schindelin
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