On Mon, 2013-09-16 at 11:44 +0300, Michael S. Tsirkin wrote: > On Mon, Sep 16, 2013 at 10:40:00AM +0200, Oliver Neukum wrote: > > On Mon, 2013-09-16 at 13:38 +0930, Rusty Russell wrote: > > > Predates git, does anyone remember the rationale? > > > > > > ie: > > > int test_bit(int nr, const volatile unsigned long *addr) > > > > > > I noticed because gcc failed to elimiate some code in a patch I was > > > playing with. > > > > > > I'm nervous about subtle bugs involved in ripping it out, even if noone > > > knows why. Should I add __test_bit()? > > > > It seems to me that if you do > > > > b = *ptr & 0xf; > > c = b << 2; > > if (test_bit(1, ptr)) > > > > the compiler could optimize away the memory access on ptr without > > the volatile. We'd have to add a lot of mb(). > > > > Regards > > Oliver > > What is this code supposed to do? > Any specific examples? >
Often you see while (test_bit(...) && condition) ... ; If the compiler can show that you don't change the memory you do the test_bit on, it can change this to: if (test_bit(...)) while (condition) ...; That must not happen. Regards Oliver -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/