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Re: question about memory constraint in atomic_add

To: Ralf Baechle <>
Subject: Re: question about memory constraint in atomic_add
From: Indigodfw <>
Date: Wed, 18 Feb 2004 16:11:32 -0800 (PST)
In-reply-to: <>
Original-recipient: rfc822;
Hi Ralf

Thanks for response, but I did not understand your
point. Please read inline.

--- Ralf Baechle <> wrote:
> On Sat, Feb 14, 2004 at 07:11:52AM -0800, Indigodfw
> wrote:
> > 2. Result of (C expression) should go into %xyz
> > register 
> > So v->counter goes into %1, IOW ll from an int!
> > 
> > Does not make sense to me.
> > Why does it work, What am I missing?
> > I mean in general what is the expression for a m
> > constraint ptr (because I want ptr to be in
> regiser)
> > or *ptr (because I wanna tell compiler that *ptr
> is
> > what gets changed) 
> "m" gives you *something* suitable to address a
> memory object; that isn't
> necessarily a memory address.  On MIPS it can't even
> be just an address
> in a register because "m" constraints are used with
> loads and stores and
> those only accept the offset(reg) addressing mode. 
> If you want an address
> use something like "r" (&v->counter), then lw
> reg,(%xxx).

Well, is not that what we want?
That is, we want to load (using ll) from &v->counter.

Should not the code have been
ll     %0, 0(%1) 
where %1 is "=m" (&v->counter)

The way I interpret it is:
a) %1 will contain address of v->counter
b) We would do ll (load with reservation/lock) from %1
which is &v->counter (and not from v->counter)
c)We want to tell the compiler that &v->counter is
output constraint and it may be modified. (Since
compiler does not look inside asm). 
But I fear that with the syntax "=m" (&v->counter) we
are informing the compiler that this ptr itself may be
modified instead of its contents. 


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