2017-01-16 15:53 GMT+03:00 Peter Zijlstra <pet...@infradead.org>: > On Mon, Jan 16, 2017 at 10:42:29PM +1100, Anton Blanchard wrote: >> Hi Peter, >> >> > Last I checked I couldn't build a x86_64 kernel with llvm. So no, not >> > something I've ever ran into. >> > >> > Also, I would argue that this is broken in llvm, the kernel very much >> > relies on things like this all over the place. Sure, we're way outside >> > of what the C language spec says, but who bloody cares ;-) >> >> True, but is there anything preventing gcc from implementing this >> optimisation in the future? If we are relying on undefined behaviour we >> should have a -fno-strict-* option to cover it. >> >> > If llvm wants to compile the kernel, it needs to learn the C dialect >> > the kernel uses. >> >> LLVM has done that before (eg adding -fno-strict-overflow). I don't >> think that option covers this case however. > > Our comment there states: > > # disable invalid "can't wrap" optimizations for signed / pointers > KBUILD_CFLAGS += $(call cc-option,-fno-strict-overflow) > > So this option should apply to pointer arithmetic, therefore I would > expect -fno-strict-overflow to actually apply here, or am I missing > something?
That case is null pointer check optimization. '->member' has non-zero offset in struct, so LLVM assumes that pos->member != NULL and optimize away this check. LLVM/clang currently doesn't have -fno-delete-null-pointer-checks
