[LLVMdev] use function address as global variable initializer

Xin Tong trent.tong at gmail.com
Fri Sep 27 18:47:15 PDT 2013


I would like to initialize a global variable to the address of a function.
Looking at how clang compiles the C code below, i see @main_ptr = global
i8* bitcast (i32 ()* @main to i8*), align 8

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

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. what should be the insertion point of this bitcast instruction.
also, do i just give the LLVM main Function* to the bitcast instruction as
its value ?

I am new to LLVM. please help.

Thank you,


* C code *

typedef int (*INVOKE_MAIN)(void);

int main();

void *main_ptr = main;

int main()
   return 0;

*generated LLVM code*

@main_ptr = global i8* bitcast (i32 ()* @main to i8*), align 8

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
  %retval = alloca i32, align 4
  store i32 0, i32* %retval
  %0 = load i8** @main_ptr, align 8
  %1 = bitcast i8* %0 to i32 ()*
  %call = call i32 %1()
  ret i32 0
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