[LLVMdev] Storing values in global variables

Jan Teske jan.teske at gmail.com
Thu Oct 30 03:10:33 PDT 2014

Hello John,

thank you very much for the hint! I replaced all calls to
getGlobalContext() by calls that retrieve the module context and now
it works.


On 10/29/2014 05:06 PM, John Criswell wrote:
> Dear Jan,
> I can't see it in your code, but my guess is that, somewhere in
> your pass, you're grabbing the global context using
> getGlobalContext() and then using that to create some LLVM value
> (e.g., a global variable, a GEP, etc.).
> Passes should avoid using getGlobalContext() to get a context.
> Instead, they should get a pointer to the LLVMContext in which the
> Module or Function they are transforming belongs, and then create
> all new values (including global variables) within that context.
> Regards,
> John Criswell
> On 10/28/14, 5:16 AM, Jan Teske wrote:
>> Hello,
>> I have written a pass that works nicely if I use it with the opt
>> tool, as proposed by the "Writing an LLVM Pass" tutorial. Now I
>> want to execute this pass directly when clang is executed. I use
>> the technique described here: 
>> http://homes.cs.washington.edu/~asampson/blog/clangpass.html. The
>> pass is loaded and executed, when I run clang with the right
>> command line. But clang proceeds to crash.
>> It always crashes because of assertions that test types (e.g.
>> the types of arguments of a call). So far i figured out that this
>> is because while the types are the same, their contexts differ
>> and thus they do not have the same address in memory. I guess,
>> this is a problem now but not when using the opt way, because
>> when I use opt the global context and the module context are the
>> same.
>> Anyway, it seems I have to fix my (until now quite careless) use
>> of the different contexts. And this is where I could need help.
>> The special case I am stuck at is about storing a value in a
>> global variable. Here is some code:
>> IRBuilder<> builder(pi); Value *elem = builder.CreateCall(...); 
>> Value *elem_ptr = builder.CreateGEP(global_array, idx); 
>> builder.CreateStore(get_crypted_funcptr(func, builder),
>> elem_ptr);
>> I get a IRBuilder before some instruction (pi) in main. The
>> element I want to store (elem) is created by some call
>> instruction which I create via the builder. To store elem in
>> global_array, I need a pointer, which I get via a GEP instruction
>> which is also created by the bulder. Now the store fails.
>> Both elem and the element pointed to by elem_ptr have the same
>> type. But their contexts are different. elem has module context
>> (I guess this is because builder has module context) and elem_ptr
>> points to an element with global context (I guess this is because
>> it's a global variable with external linkage). Since both
>> contexts are different, the CreateStore fails at a type-check
>> assertion.
>> Does that mean I cannot store any values in a global variable? I
>> guess not. So what would be the correct way to go in this case?
>> The only one I can think of is trying to give every value the
>> global context. But I don't think that would be right.
>> Thanks in advance! Jan 
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