From: Andi Kleen <andi@firstfloor.org>
To: Mikael Pettersson <mikpe@it.uu.se>
Cc: Andi Kleen <andi@firstfloor.org>,
roel kluin <roel.kluin@gmail.com>,
David Miller <davem@davemloft.net>,
herbert@gondor.apana.org.au, linux-crypto@vger.kernel.org,
akpm@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sha: prevent removal of memset as dead store in sha1_update()
Date: Fri, 26 Feb 2010 16:55:53 +0100 [thread overview]
Message-ID: <20100226155553.GD16335@basil.fritz.box> (raw)
In-Reply-To: <19335.60634.75667.480609@pilspetsen.it.uu.se>
> Instead of barrier(), this works with gcc-3.2.3 up to gcc-4.4.3
> for the purpose of making the memset() not disappear:
>
> {
> struct s { char c[n]; };
> asm("" : : "m"(*(struct s *)p));
> }
>
> Every byte in the [p,p+n[ range must be used. If you only use the
> first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
> _will_ skip scrubbing bytes beyond the first.
>
> An explicit loop that uses each byte individually also works, but
> results in awful code with older compilers.
I suspect it would be worth asking for a __builtin for this.
iirc Visual C already has one.
BTW I remember finding a few more such cases in random user space
code in past code review.
-Andi
--
ak@linux.intel.com -- Speaking for myself only.
next prev parent reply other threads:[~2010-02-26 15:55 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-02-25 15:10 [PATCH] sha: prevent removal of memset as dead store in sha1_update() Roel Kluin
2010-02-25 15:17 ` David Miller
2010-02-25 15:31 ` roel kluin
2010-02-25 15:37 ` David Miller
2010-02-26 11:55 ` Andi Kleen
2010-02-26 14:20 ` Mikael Pettersson
2010-02-26 15:46 ` Mikael Pettersson
2010-02-26 15:55 ` Andi Kleen [this message]
2010-02-25 15:56 ` Mikael Pettersson
2010-02-25 16:16 ` Pekka Enberg
2010-02-25 16:29 ` Mikael Pettersson
2010-02-25 16:33 ` roel kluin
2010-02-25 17:06 ` roel kluin
2010-02-25 16:33 ` Brian Gerst
2010-02-25 17:09 ` Mikael Pettersson
2010-02-25 17:32 ` Brian Gerst
2010-02-25 19:47 ` Roel Kluin
2010-02-25 20:43 ` Roel Kluin
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