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On Tue, 17 Jan 2012, Paweł Sikora wrote:
> Hi,
>
> recently i've seen a tricky commit in the mdadm git repository:
> http://neil.brown.name/git?p=mdadm;a=commit;h=90fa1a292929ff8a6c7357254b6f616608ec01b5
>
> imho, this is a tricky way to avoid gcc-4.7 diagnostics machinery
> but it still violates aliasing rules. am i right?
>
> thanks in advance for any comments.
#ifndef AVOID_WARNING
*(md5_uint32 *) &ctx->buffer[bytes + pad + 4] = SWAP (ctx->total[0] <<
3);
#else
ptr = (md5_uint32 *) &ctx->buffer[bytes + pad + 4];
*ptr = SWAP (ctx->total[0] << 3);
#endif
Both code-snippets are indeed exactly the same. In recent GCC
(>= 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 = SWAP (ctx->total[0] << 3);
memcpy (&ctx->buffer[bytes + pad + 4], &tem, sizeof (md5_uint32));
GCC will inline that memcpy.
Richard.
--
Richard Guenther <rguenther@xxxxxxx>
SUSE / SUSE Labs
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer
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