[LLVMdev] Should remove calling NULL pointer or not

[David Blaikie]

On Thu, Nov 7, 2013 at 11:02 AM, Yin Ma <yinma at codeaurora.org> wrote:

> Hi John,
>
>
>
> It seems the dereferencing a NULL pointer is undefined behavior but
>
> Calling a function through a null pointer seems o.k.
>

What is the well defined behavior of calling a null function pointer?


>
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> If so , for this place, we need comment out the check.
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> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#232
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>
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> look at Notes from the October 2003 meeting.
>

This doesn't seem to describe well-defined behavior of calling a null
function pointer. It talks about the narrow case of dereferencing a null
pointer but not performing an lvalue to rvalue conversion with the result.

- David


>
>
> Yin
>
>
>
> *From:* John Criswell [mailto:criswell at illinois.edu]
> *Sent:* Wednesday, November 06, 2013 6:28 PM
> *To:* Yin Ma; 'llvmdev Dev'
> *Subject:* Re: [LLVMdev] Should remove calling NULL pointer or not
>
>
>
> On 11/6/13 6:36 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?
>
>
> If calling a NULL function pointer yields undefined behavior (as defined
> by the C/C++ standards), then the optimization is correct: since the
> behavior is undefined, the compiler can change it as it sees fits.  In
> other words, the compiler is not required to maintain "incorrect" behavior.
>
> The remaining question, then, is whether the C/C++ standards consider
> calling a NULL function pointer undefined behavior.  I suspect that it is
> undefined behavior, but to be honest, I do not know for certain.
>
> -- John T.
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> Thanks,
>
>
>
> Yin
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