linux-kernel - Re: [PATCH] [POWERPC] Improve (in|out)

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Date:	Tue, 20 May 2008 15:11:02 -0700 (PDT)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Andreas Schwab <schwab@...e.de>
cc:	linuxppc-dev@...abs.org, Scott Wood <scottwood@...escale.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

On Wed, 21 May 2008, Andreas Schwab wrote:
> Trent Piepho <tpiepho@...escale.com> writes:
>
>> For the LE versions, eventually they boil down to an asm that will look
>> something like this:
>> asm("sync; stwbrx %1,0,%2" : "=m" (*addr) : "r" (val), "r" (addr));
>>
>> While not perfect, this appears to be the best one can do.  The issue is
>> that the "stwbrx" instruction only comes in an indexed, or 'x', version, in
>> which the address is represented by the sum of two registers (the "0,%2").
>> Unfortunately, gcc doesn't have a constraint for an indexed memory
>> reference.
>
> There is the "Z" constraint, which matches either an indirect or an
> indexed memory address.  That should fit here.

This came up on the Freescale list.  I should have put what I wrote there into
my patch descrition:

It's the _le versions that have a problem, since we can't get gcc to just use
the register indexed mode.  It seems like an obvious thing to have a
constraint for, but I guess there weren't enough instructions that only come
in 'x' versions to bother with it.  There is a 'Z' constraint, "Memory operand
that is an indexed or indirect from a register", but I tried it and it can use
both "rb,ri" and "disp(rb)" forms.  Actually, I'm not sure how 'Z' is any
different than "m"?
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