On Mon, Aug 19, 2019 at 07:40:42AM +0200, Christophe Leroy wrote: > 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 ?
The top 32 bits of a 64-bit register holding an int are undefined. Take as example int x = 42; x = ~x; which may put ffff_ffff_ffff_ffd5 into the reg, not 0000_0000_ffff_ffd5 as you might expect or want. For tw instructions this makes no difference (they only look at the low 32 bits anyway); for td insns, it does. > 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. Sure, it isn't defined what types you can pass to that macro. Another thing that makes inline functions much saner to use. > >(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. But see my comment there about the compiler knowing all code after an unconditional trap is dead. Segher
