Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

Trent Piepho
2008-05-20T22:16:26Z

On Wed, 21 May 2008, Andreas Schwab wrote:
> Trent Piepho <tpiepho@freescale.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"?