[LLVMdev] Storing values in global variables

Jan Teske jan.teske at gmail.com
Tue Oct 28 05:16:02 PDT 2014


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!

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