Messages in this thread | | | Subject | Re: [PATCH] powerpc: optimise WARN_ON() | From | Christophe Leroy <> | Date | Mon, 19 Aug 2019 07:40:42 +0200 |
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Le 18/08/2019 à 14:01, Segher Boessenkool a écrit : > On Sat, Aug 17, 2019 at 09:04:42AM +0000, Christophe Leroy wrote: >> 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. > >> 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
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