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<p class="MsoNormal">I feel a bit silly asking this, but here goes.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The C spec does not require the null pointer to be implemented as an integer with the 0 bit-pattern. The implementation may arbitrarily choose any other (integer, but not necessarily) representation, as long as it’s distinct from any legal
pointer. The only requirement is that casting an integer 0 to a pointer type results in a null pointer. <o:p></o:p></p>
<p class="MsoNormal">LLVM IR appears to have similar the casting behavior. That is, if I try to create a pointer with integer value 0 (e.g. by constructing an inttoptr constant expression with a i32 0 argument), it gets immediately folded into a null pointer
constant. Furthermore, a PtrToInt from a null pointer results in 0 (which is even “stricter” than the C spec.)
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So, that raises a few question:<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="mso-list:Ignore">a)<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]><span dir="LTR"></span>Are those the desired semantics? I guess if clang generates this kind of IntToPtr instructions (as opposed to directly resolving the cast to a null pointer) this is unavoidable. But otherwise, why does LLVM treat
null pointers and pointers with the integer value 0 as equivalent? <br>
It’s completely natural for most platforms, but isn’t necessarily the right thing to do.<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="mso-list:Ignore">b)<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]><span dir="LTR"></span>Assuming this is really desirable, should the fact that casting an integer 0 to a pointer type results in a null pointer be documented in the langref (as part of the inttoptr/ptrtoint documentation)? Or is it too
“low-level”?<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="mso-list:Ignore">c)<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]><span dir="LTR"></span>Let’s say I want to create a pointer with the numeric value 0, distinct from the null pointer, because 0 really does represent a valid pointer on my platform. Should this possible at the IR level? If it should
be, then how?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Anyone has any inputs, except laughing me out of the (virtual) room? :-)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal"> Michael<o:p></o:p></p>
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