linux-kernel - Re: [PATCH 2/2] radix-tree: Fix optimisation problem

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Message-ID: <CALXu0Ucx-6PeEk9nTD-4nZvwyVr9LLXcFGFzhctX-ucKfCygGA@mail.gmail.com>
Date:   Sun, 25 Sep 2016 01:35:27 +0200
From:   Cedric Blancher <cedric.blancher@...il.com>
To:     Matthew Wilcox <mawilcox@...uxonhyperv.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 2/2] radix-tree: Fix optimisation problem

On 22 September 2016 at 20:53, Matthew Wilcox
<mawilcox@...uxonhyperv.com> wrote:
> From: Matthew Wilcox <mawilcox@...rosoft.com>
>
> When compiling the radix tree with -O2, GCC thinks it can optimise:
>
>         void *entry = parent->slots[offset];
>         int siboff = entry - parent->slots;

If entry is a pointer to void, how can you do pointer arithmetic with it?
Also, if you use pointer distances, the use of int is not valid, it
should then be ptrdiff_t siboff.

lint(1) would bite your arse off in both cases.
Sadly only UNIX (Solaris, AIX, ...) use lint(1) as mandatory part of
the build process and make warnings and errors of lint(1) fatal...

Ced
-- 
Cedric Blancher <cedric.blancher@...il.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur