[LLVMdev] Telling the optimizer a value is always null at the start

[Stephan Tolksdorf]

On 2014-07-13 16:50, Carlo Kok wrote:
>   On Thu, Jul 10, 2014 at 11:29 AM, Philip Reames
>   <listmail at philipreames.com> wrote:
>> Hm, I don't know of an explicit way in the IR to do this.  If anyone else
>> does, feel free to chime in.
>>
>> One approach would be to add a branch at the beginning of the function
>> to an
>> unreachable block.  If you're testeh function started with:
>> if( *ex != null) llvm_unreachable();
>>
>> You might get lucky and have the GVN or EarlyCSE propagate the null
>> value.
>> You're going to run into pass ordering problems here though.  I
>> suspect we'd
>> drop the unreachable block before GVN runs.
>
> Looks like, at least I tried the code below and while it removes the
> unreachable, it doesn't remove the second checck. Philips code looks
> like it could do this though.

LLVM's can't exploit information introduced by unreachable() like GCC 
can (or like MSVC can for the equivalent __assume() expression). This 
has been a long-standing issue, see the later comments in
http://llvm.org/bugs/show_bug.cgi?id=810

- Stephan