Date: Wed, 18 Jan 2012 10:26:17 +0100
From: Richard Guenther <rguenther@suse.de>
To: =?utf-8?B?UGF3ZcWC?= Sikora <pluto@agmk.net>
Cc: arekm@pld-linux.org, neilb@suse.de
Subject: Re: ugly way to avoiding gcc strict-aliasing warning?
In-Reply-To: <18186905.qOT7x4uFqU@vmx>
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References: <18186905.qOT7x4uFqU@vmx>
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On Tue, 17 Jan 2012, Pawe=C5=82 Sikora wrote:

> Hi,
>=20
> recently i've seen a tricky commit in the mdadm git repository:
> http://neil.brown.name/git?p=3Dmdadm;a=3Dcommit;h=3D90fa1a292929ff8a6c7=
357254b6f616608ec01b5
>=20
> imho, this is a tricky way to avoid gcc-4.7 diagnostics machinery
> but it still violates aliasing rules. am i right?
>=20
> thanks in advance for any comments.

#ifndef AVOID_WARNING
  *(md5_uint32 *) &ctx->buffer[bytes + pad + 4] =3D SWAP (ctx->total[0] <=
<=20
3);
#else
  ptr =3D (md5_uint32 *) &ctx->buffer[bytes + pad + 4];
  *ptr =3D SWAP (ctx->total[0] << 3);
#endif

Both code-snippets are indeed exactly the same.  In recent GCC
(>=3D 4.5), if you ever read from ctx->buffer[] with an effective
type other than md5_uint32 after writing to it using that type
you can get miscompilations (reading via an effective type
of char is ok, which happens to be the type of ctx->buffer).

In older GCC the above can result in miscompiles even if you
happen to follow that restriction (thus, later GCC got more
permissive in this area).  A portable and safe way of doing the above is

 md5_uint32 tem =3D SWAP (ctx->total[0] << 3);
 memcpy (&ctx->buffer[bytes + pad + 4], &tem, sizeof (md5_uint32));

GCC will inline that memcpy.

Richard.

--=20
Richard Guenther <rguenther@suse.de>
SUSE / SUSE Labs
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
GF: Jeff Hawn, Jennifer Guild, Felix Imend=C3=B6rffer
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