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From: Ian Lance Taylor <iant@google.com>
To: Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Forcing memory mode to some addresses
References: <CAJ-J9_XzXQZjp4RyHZpOoEAXyooRfOfytR1JmJO8bZc=Q4a8Zg@mail.gmail.com>
Date: Wed, 16 Nov 2011 18:29:00 -0000
In-Reply-To: <CAJ-J9_XzXQZjp4RyHZpOoEAXyooRfOfytR1JmJO8bZc=Q4a8Zg@mail.gmail.com>	(Aurelien Buhrig's message of "Wed, 16 Nov 2011 12:10:30 +0100")
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Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com> writes:

> I would like to force accesses to some memory variables in a specific mode.
> For example, I would like GCC to access a 16-bit wide hardware
> register always in HImode when using a 16bit wide bitfield struct...
> Currently, GCC can accesses part of this memory structure using
> QImode, which is not always permitted.
> Is there a way to do it using an attribute ? Or perhaps another way ?
>
> If a target specific attribute must be defined, what should it
> control, and how to recognized it in my backend ?

You should always use a volatile qualified pointer when accessing a
hardware register.  When you do that, gcc should always use the
appropriate mode.

Ian