Re: llist code relies on undefined behaviour, upsets llvm/clang

2017-01-16 15:53 GMT+03:00 Peter Zijlstra [off-list ref]:

On Mon, Jan 16, 2017 at 10:42:29PM +1100, Anton Blanchard wrote:

quoted

Hi Peter,

quoted

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.

quoted

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