llvm replace a function with another one

Question

I am trying to replace a function call with another one. e.g. here is the code with 3 functions - print1, print2 and main:

#include <stdio.h>
extern "C" { 
int print1()
{
   printf("Inside print1\n");
   return 0xdeadbeef;
}
int print2()
{
   printf("Inside print2\n");
   return 0xbeefdead;
}
int main(void)
{
   return print1();
}
}"

My goal is to replace use of print1 (in main) with print2. I compile the above code into an llvm::Module* (called main in the code below) and then create an execution engine out of it.

std::string errMsg;
llvm::ExecutionEngine *ee =
   llvm::EngineBuilder( main ).setErrorStr( &errMsg ).create();    
ASSERT_NE( ee, nullptr )<<"Execution engine is nullptr:"<<errMsg;

At this point, I am able to get all the 3 functions (print1, print2 and main) from the execution engine and am able to execute them fine. However, problem occurs when I try to replace function "print1" with "print2", as follows:

llvm::Function *print1f = main->getFunction( "print1" );
llvm::Function *print2f = main->getFunction( "print2" );
llvm::Function *mainf = main->getFunction( "main" );

//carry out the replacement
print2f->takeName( print1f );
ee->freeMachineCodeForFunction( mainf );
ee->freeMachineCodeForFunction( print1f );
print1f->replaceAllUsesWith( print2f );
print1f->deleteBody();
print1f->dropAllReferences();
print1f->eraseFromParent();

//run main
void *mainfPtr = ee->getPointerToFunction( mainf );
mainfPtr = ee->recompileAndRelinkFunction( mainf );
ASSERT_NE( mainfPtr, nullptr );
ret = ((int(*)(void))(mainfPtr))();
*EXPECT_EQ(0xbeefdead, ret);*

However, ret is returned as 0xdeadbeef, as if print1 is being called and not print2. Can someone please let me know if I am following the right steps to replace the function call. If there is other method, please let me know.

thx Vikas.

==========

Answer 1

If the compiler were to inline print1 in main, the function would never actually be called; Instead, main would have its own private version of print1's code pasted in. Since it doesn't actually have to refer to the shared print1 anymore, swapping in print2 might not affect main's behavior.

If you want to verify that this is the problem (and/or keep it from happening, if it is), try telling the compiler not to inline.