[LLVMdev] Should remove calling NULL pointer or not

[Sean Silva]

On Mon, Nov 11, 2013 at 3:01 PM, Philip Reames <listmail at philipreames.com>wrote:

>  I've definitely seen code like the following in various production code
> bases:
>
> namespace {
>   bool global = false;
> }
> class Foo {
>    void modify_global() { global = true; }
> };
>
> // usually, there's a layer of indirection (or two, or three) here
> ((Foo*)NULL)->modify_global();
>
>
> Discussions on StackOverflow seem to clearly indicate that the above code
> is undefined.  Having said that, I have not tried to parse apart the actual
> spec on this one, so I'll leave that discussion to others.
>

IIRC the rule that makes this undefined is that a method must be called on
an object of that type, and by definition null doesn't point at any object.
(i.e. `this` can never be null).

-- Sean Silva


>
> If we do decide this is undefined, do we have a general policy on how to
> warn about, or under which optimization levels to enable, such
> optimizations?  If not, it would probably worth some discussion on what a
> general policy might look like.
>
> Philip
>
>
> On 11/6/13 4:36 PM, Yin Ma wrote:
>
>  Hi,
>
>
>
> For a small case, that calls NULL pointer function. LLVM explicitly
> converts
>
> It to a store because it thinks it is not reachable like calling
> undefvalue.
>
> In InstCombineCalls.cpp:930
>
>
>
> I think it is not a right approach because calling null pointer function
>
> Will segfault the program. Converting to a store will make program pass
>
> Silently. This changes the behavior of a program.
>
>
>
> So we need remove the case if (isa<ConstantPointerNull>(Callee) at
>
> InstCombineCalls.cpp:918 and treat calling Null pointer reachable.
>
>
>
> How do you think? Is there any reason that we should convert
>
> a calling null pointer to a store?
>
>
>
> Thanks,
>
>
>
> Yin
>
>
>
>
>
>
>
>
>
>
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