[LLVMdev] global question

Andrew Haley aph at redhat.com
Sat Mar 7 12:09:57 PST 2009

Patrick Boettcher wrote:

> On Sat, 7 Mar 2009, John Regehr wrote:
>> Below is some C code and its LLVM translation (using a recent rev).  The
>> compiler goes ahead and loads from x in main, but it seems clear that a
>> constant propagation pass could have inferred that x is 0. 

I'm not sure I believe that, unless you're suggesting we treat main()
as special.

>> In fact that
>> is what happens if x is static, but in this case I cannot see how
>> external linkage of x would invalidate that optimization.
>> int x;
>> int main (void)
>> {
>>   return x;
>> }
> Please correct me if I'm wrong, but how can the compiler know, that x is not 
> initialized in another file which defines x as extern? It can only be sure, 
> when x is declared static.
> So, for me the LLVM-code is right.
> (I even remember someone, that it is platform specific whether x is initialized 
> to 0 or not... or was it a question of C89 or C99?)

It's initialized to zero.  Always, for all versions of C.


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