Re: [PATCH] powerpc: optimise WARN_ON()

Le 18/08/2019 à 14:01, Segher Boessenkool a écrit :

On Sat, Aug 17, 2019 at 09:04:42AM +0000, Christophe Leroy wrote:

quoted

Unlike BUG_ON(x), WARN_ON(x) uses !!(x) as the trigger
of the t(d/w)nei instruction instead of using directly the
value of x.

This leads to GCC adding unnecessary pair of addic/subfe.

And it has to, it is passed as an "r" to an asm, GCC has to put the "!!"
value into a register.

quoted

By using (x) instead of !!(x) like BUG_ON() does, the additional
instructions go away:

But is it correct?  What happens if you pass an int to WARN_ON, on a
64-bit kernel?

On a 64-bit kernel, an int is still in a 64-bit register, so there would 
be no problem with tdnei, would it ? an int 0 is the same as an long 0, 
right ?

It is on 32-bit kernel that I see a problem, if one passes a long long 
to WARN_ON(), the forced cast to long will just drop the upper size of 
it. So as of today, BUG_ON() is buggy for that.

(You might want to have 64-bit generate either tw or td.  But, with
your __builtin_trap patch, all that will be automatic).

Yes I'll discard this patch and focus on the __builtin_trap() one which 
should solve most issues.

Christophe