Re: [PATCH] USB: ehci: use packed,aligned(4) instead of removingthe packed attribute

From: Alexander Holler
Date: Thu Jun 16 2011 - 14:16:34 EST

Am 16.06.2011 19:09, schrieb Alan Stern:
On Thu, 16 Jun 2011, Alexander Holler wrote:

In commit 139540170d9d9b7ead3caaf540f161756b356d56 the attribute
packed is removed from the structs which are used to access the EHCI-registers.

This is done to circumvent a problem with gcc 4.6, which might access members of
packed structs on a byte by byte basis. But using packed, aligned(4) fixes that
too and is imho the better solution. Otherwise (without packed) the compiler would be free
to choose whatever alignment he thinks fits best, which might be e.g. 8-byte on 64-bit machines.

Is that really true? I thought the compiler was not allowed to insert
padding if the natural alignment of the data types didn't require any.

Alan Stern

I wasn't sure and have searched c99 before posting the patch but I haven't found something which states what you are suggesting. Maybe I was too stupid to find it, I've searched for the words "alignment" and "padding".

The only statement I've found was

"There may be unnamed padding within a structure object, but not at its beginning."

in 13 and

"There may be unnamed padding at the end of a structure or union."

in 15 in my copy of c99.


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