[LLVMdev] use function address as global variable initializer

Tim Northover t.p.northover at gmail.com
Fri Sep 27 23:44:04 PDT 2013


> but how do i generate i8* bitcast (i32 ()* @main to i8*), align 8 in LLVM IR
> and use it as initializer to a GlobalVariable.

LLVM has a "C++" backend which is amazingly helpful in these
situations. You use llc to compile the bitcode you want to emit with
"-march=cpp" and get back some C++ code that would generate the
original IR. In this case:

PointerType* PointerTy_0 =
PointerType::get(IntegerType::get(mod->getContext(), 8), 0);
Constant* const_ptr_5 = ConstantExpr::getCast(Instruction::BitCast,
func_main, PointerTy_0);

It's not pretty (I'd suggest using ConstantExpr::getBitCast instead,
for example), but it often shows you where you should start looking.

> I know i will need to use the bitcast instruction. but as far i as i
> understand, every instruction in LLVM needs to be created with an insertion
> point.

A subclass of Instruction needs an insertion-point, but because @main
(like other globals) is actually a Constant, you can also create a
bitcast that's a Constant too (in this case a ConstantExpr), and they
don't need an insertion point.

> also, do i just give the LLVM main Function* to the bitcast instruction as its
> value ?

You could do. For a bitcast Instruction, you just need a subclass of
Value, which both other Instructions and Constants (including
Functions) are. For a Constant bitcast, you need another Constant (so
Function works there too).

The class diagrams at
http://llvm.org/docs/doxygen/html/classllvm_1_1Value.html can actually
be quite useful for ideas on what's possible.



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