On Tue, 24 Jul 2007, Nick Piggin wrote: > Satyam Sharma wrote: > > From: Satyam Sharma <[EMAIL PROTECTED]> > > > > [6/8] i386: bitops: Don't mark memory as clobbered unnecessarily > > > > The goal is to let gcc generate good, beautiful, optimized code. > > > > But test_and_set_bit, test_and_clear_bit, __test_and_change_bit, > > and test_and_change_bit unnecessarily mark all of memory as clobbered, > > thereby preventing gcc from doing perfectly valid optimizations. > > > > The case of __test_and_change_bit() is particularly surprising, given > > that it's a variant where we don't make any guarantees at all. > > __test_and_change_bit is one that you could remove the memory clobber > from.
Yes, for the atomic versions we don't care if we're asking gcc to generate trashy code (even though I'd have wanted to only disallow problematic optimizations -- ones involving the passed bit-string address -- there, and allow other memory references to be optimized as and how the compiler feels like it) because the atomic variants are slow anyway and we probably want to be extra-safe there. But for the non-atomic variants, it does make sense to remove the memory clobber (and the unneeded __asm__ __volatile__ that another patch did -- for the non-atomic variants, again). OTOH, as per Linus' review it seems we can drop the "memory" clobber and specify the output operand for the extended asm as "+m". But I must admit I didn't quite understand that at all. [ I should probably start reading gcc sources, the docs are said to be insufficient/out-of-date, as per the reviews of the patches. ] Satyam - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
