[LLVMdev] loads from a null address and optimizations

Bill Wendling isanbard at gmail.com
Sat Sep 5 14:39:17 PDT 2009

Hi Zoltan,

We've come across this before where people meant to induce a trap by  
dereferencing a null. It doesn't work for LLVM (as you found out).  
Essentially, it's a valid transformation to turn this into  
unreachable. The better solution is to use something like  


On Sep 5, 2009, at 2:19 PM, Zoltan Varga <vargaz at gmail.com> wrote:

>                 Hi,
>   Currently, llvm treats the loads from a null address as  
> unreachable code, i.e.:
>      load i32* null
> is transformed by some optimization pass into
>     unreachable
> This presents problems in JIT compilers like mono which implement  
> null pointer checks by trapping SIGSEGV signals. It also
> looks incorrect since it changes program behavior, which might be  
> undefined in general, but it is quite well defined on unix.
> Is there a way to prevent llvm from doing this besides marking all  
> loads as volatile ?
>                           thanks
>                               Zoltan
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