Re: [PATCH 3/3] powerpc: Fix pre-update addressing in inline assembly

[Christophe Leroy]

Le 19/10/2020 à 22:24, Segher Boessenkool a écrit :
> On Mon, Oct 19, 2020 at 12:12:48PM +0000, Christophe Leroy wrote:
> > In several places, inline assembly uses the "%Un" modifier
> > to enable the use of instruction with pre-update addressing,
>
> Calling this "pre-update" is misleading: the register is not updated
> before the address is generated (or the memory access done!), and the
> addressing is exactly the same as the "non-u" insn would use.  It is
> called an "update form" instruction, because (at the same time as doing
> the memory access, logically anyway) it writes back the address used to
> the base register.
>
> > but the associated "<>" constraint is missing.
>
> But that is just fine.  Pointless, sure, but not a bug.

Most of those are from prehistoric code. So at some point in time it was effective. Then one day GCC 
changed it's way and they became pointless. So, not a software bug, but still a regression at some 
point.

> > Use UPD_CONSTR macro everywhere %Un modifier is used.
>
> Eww.  My poor stomach.

There are not that many :)

> Have you verified that update form is *correct* in all these, and that
> we even *want* this there?

I can't see anything that would militate against it, do you ?

I guess if the elders have put %Us there, it was wanted.

Christophe