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From: Denton Liu <liu.denton@gmail.com>
To: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: Re: [PATCH 02/13] msvc: avoid using minus operator on unsigned types
Date: Thu, 26 Sep 2019 10:20:22 -0700
Message-ID: <20190926172022.GA41037@generichostname> (raw)
In-Reply-To: <2abe1e1fb0bf3025489c2e543b9a9c648a164827.1569486607.git.gitgitgadget@gmail.com>

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?

>  	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
> 

  reply index

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 [this message]
2019-09-26 21:01     ` Johannes Schindelin
2019-09-26 23:57       ` Denton Liu
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 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 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 03/13] winansi: use FLEX_ARRAY to avoid compiler warning 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 04/13] compat/win32/path-utils.h: add #include guards 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 08/13] vcxproj: only copy `git-remote-http.exe` once it was built 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 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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