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From: Junio C Hamano <gitster@pobox•com>
To: "René Scharfe" <l.s.r@web•de>
Cc: Git List <git@vger•kernel.org>
Subject: Re: [PATCH 1/3] add QSORT
Date: Thu, 29 Sep 2016 15:36:24 -0700	[thread overview]
Message-ID: <xmqqponmcp07.fsf@gitster.mtv.corp.google.com> (raw)
In-Reply-To: <67bddc37-4ee2-fef0-c852-e32645421e4c@web.de> ("René Scharfe"'s message of "Thu, 29 Sep 2016 17:23:43 +0200")

René Scharfe <l.s.r@web•de> writes:

> Add the macro QSORT, a convenient wrapper for qsort(3) that infers the
> size of the array elements and supports the convention of initializing
> empty arrays with a NULL pointer, which we use in some places.
>
> Calling qsort(3) directly with a NULL pointer is undefined -- even with
> an element count of zero -- and allows the compiler to optimize away any
> following NULL checks.  Using the macro avoids such surprises.
>
> Add a semantic patch as well to demonstrate the macro's usage and to
> automate the transformation of trivial cases.
>
> Signed-off-by: Rene Scharfe <l.s.r@web•de>
> ---
>  contrib/coccinelle/qsort.cocci | 19 +++++++++++++++++++
>  git-compat-util.h              |  8 ++++++++
>  2 files changed, 27 insertions(+)
>  create mode 100644 contrib/coccinelle/qsort.cocci

The direct calls to qsort(3) that this series leaves behind are
interesting.

1. builtin/index-pack.c has this:

	if (1 < opts->anomaly_nr)
		qsort(opts->anomaly, opts->anomaly_nr, sizeof(uint32_t), cmp_uint32);

where opts->anomaly is coming from pack.h:

    struct pack_idx_option {
            unsigned flags;
            ...
            int anomaly_alloc, anomaly_nr;
            uint32_t *anomaly;
    };

I cannot quite see how the automated conversion misses it?  It's not
like base and nmemb are type-restricted in the rule (they are both
just "expression"s).

2. builtin/shortlog.c has this:

	qsort(log->list.items, log->list.nr, sizeof(struct string_list_item),
	      log->summary ? compare_by_counter : compare_by_list);

where log->list is coming from shortlog.h:

    struct shortlog {
            struct string_list list;
    };

and string-list.h says:

    struct string_list {
            struct string_list_item *items;
            unsigned int nr, alloc;
            ...
    };

which seems to be a good candidate for this rule:

    type T;
    T *base;
    expression nmemb, compar;
    @@
    - qsort(base, nmemb, sizeof(T), compar);
    + QSORT(base, nmemb, compar);

if we take "T == struct string_list_item".

3. builtin/show-branch.c does this:

    qsort(ref_name + bottom, top - bottom, sizeof(ref_name[0]),
          compare_ref_name);

where ref_name[] is a file-scope global:

    static char *ref_name[MAX_REVS + 1];

and top and bottom are plain integers.  The sizeof() does not take
the size of *base, so it is understandable that this does not get
automatically converted.

It seems that some calls to this function _could_ send the same top
and bottom, asking for 0 element array to be sorted, by the way.

Thanks for an amusing read.

next prev parent reply	other threads:[~2016-09-29 22:36 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-29 15:23 [PATCH 1/3] add QSORT René Scharfe
2016-09-29 15:27 ` [PATCH 2/3] use QSORT René Scharfe
2016-09-29 15:29 ` [PATCH 3/3] remove unnecessary check before QSORT René Scharfe
2016-09-29 22:36 ` Junio C Hamano [this message]
2016-09-29 23:21   ` [PATCH 1/3] add QSORT René Scharfe
2016-09-29 23:40     ` René Scharfe
2016-10-01 16:19   ` René Scharfe
2016-10-03 16:46     ` Kevin Bracey
2016-10-03 17:09     ` Kevin Bracey
2016-10-03 22:00       ` René Scharfe
2016-10-04  5:28         ` Kevin Bracey
2016-10-04 20:31           ` René Scharfe
2016-10-05 15:00             ` Kevin Bracey

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