How to make custom llvm pass for changing control flow?

Question

I want to make a pass that changing control flow.

The pass should change the if condition.

Let's assume the original code is like below.

int main(int argc, char *argv[])
{
  if (atoi(argv[1]) % 2 == 0)
    printf("even\n");
  else
    printf("odd\n");

  return 0;
}

After apply my pass the code should be changed to below. (Not mean that changing the source code, but IR code in real.)

int main(int argc, char *argv[])
{
  if (atoi(argv[1]) % 2 == 1)    //the condition of if statement is changed to 1
    printf("even\n");
  else
    printf("odd\n");

  return 0;
}

this is just a toy example of what I really want to do but I have difficulties about

finding appropriate instructions want to change
and changing the control flows.

To change instructions, you should manipulate the module on bitcode level, delete the old instruction and use IRBuilder to add new instructions. You could check here llvm.org/docs/ProgrammersManual.html#making-simple-changes — Yushan
– Yushan, Commented Jan 21, 2019 at 5:40

arrowd · Accepted Answer · 2019-01-17 07:04:50Z

2

Start with compiling this code to LLVM IR to get an idea what you are going to work on:

 # clang -S -emit-llvm -o - main.c

Then you will see that you are interested in icmp instruction and its operands. In your pass, iterate over all instructions in a Function, search for ICmpInst using isa<> or dyn_cast<>, then analyze its operands with getOperand() method and replace ConstantInt operand with 0 value by the same ConstantInt with 1 value.

answered Jan 17, 2019 at 7:04

arrowd

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1 Comment

MangoTatsy Over a year ago

really appreciate about your answer! much better understanding about pass.

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