From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: "Martin Ågren" <martin.agren@gmail.com>
Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
Eric Sunshine <sunshine@sunshineco.com>
Subject: Re: [PATCH v2] sequencer: break out of loop explicitly
Date: Wed, 31 Oct 2018 15:54:03 +0100 (STD) [thread overview]
Message-ID: <nycvar.QRO.7.76.6.1810311553480.4546@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <20181030080937.11118-1-martin.agren@gmail.com>
[-- Attachment #1: Type: text/plain, Size: 2547 bytes --]
Hi Martin,
On Tue, 30 Oct 2018, Martin Ågren wrote:
> It came up in review [1, 2] that this non-idiomatic loop is a bit tricky.
> When we find a space, we set `len = i`, which gives us the answer we are
> looking for, but which also breaks out of the loop.
>
> It turns out that this loop can confuse compilers as well. My copy of
> gcc 7.3.0 realizes that we are essentially evaluating `(len + 1) < len`
> and warns that the behavior is undefined if `len` is `INT_MAX`. (Because
> the assignment `len = i` is guaranteed to decrease `len`, such undefined
> behavior is not actually possible.)
>
> Rewrite the loop to a more idiomatic variant which doesn't muck with
> `len` in the loop body. That should help compilers and human readers
> figure out what is going on here. But do note that we need to update
> `len` since it is not only used just after this loop (where we could
> have used `i` directly), but also later in this function.
>
> While at it, reduce the scope of `i`.
>
> [1] https://public-inbox.org/git/CAPig+cQbG2s-LrAo9+7C7=dXifbWFJ3SzuNa-QePHDk7egK=jg@mail.gmail.com/
>
> [2] https://public-inbox.org/git/CAPig+cRjU6niXpT2FrDWZ0x1HmGf1ojVZj3uk2qXEGe-S7i_HQ@mail.gmail.com/
>
> Helped-by: Eric Sunshine <sunshine@sunshineco.com>
> Signed-off-by: Martin Ågren <martin.agren@gmail.com>
> ---
ACK. Thanks for cleaning up after me,
Dscho
> Thanks for the comments on v1. Based on them, I decided to go for
> Eric's option 2, and to make the log message less technical in favor of
> "compilers and humans alike can get this wrong".
>
> sequencer.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/sequencer.c b/sequencer.c
> index 0c164d5f98..e7aa4d5020 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -2829,7 +2829,7 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
> struct tree_desc desc;
> struct tree *tree;
> struct unpack_trees_options unpack_tree_opts;
> - int ret = 0, i;
> + int ret = 0;
>
> if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
> return -1;
> @@ -2849,10 +2849,13 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
> }
> oidcpy(&oid, &opts->squash_onto);
> } else {
> + int i;
> +
> /* Determine the length of the label */
> for (i = 0; i < len; i++)
> if (isspace(name[i]))
> - len = i;
> + break;
> + len = i;
>
> strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
> if (get_oid(ref_name.buf, &oid) &&
> --
> 2.19.1.593.gc670b1f876.dirty
>
>
next prev parent reply other threads:[~2018-10-31 14:54 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-28 15:31 [PATCH] sequencer: clarify intention to break out of loop Martin Ågren
2018-10-28 19:01 ` Eric Sunshine
2018-10-28 20:37 ` Martin Ågren
2018-10-29 3:43 ` Junio C Hamano
2018-10-30 8:09 ` [PATCH v2] sequencer: break out of loop explicitly Martin Ågren
2018-10-31 14:54 ` Johannes Schindelin [this message]
2018-10-31 17:28 ` Eric Sunshine
2018-10-31 18:41 ` Martin Ågren
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