How can I get variable name of GEP, and use it to create a if then instruction?

IR & Optimizations
1

I am a beginner in llvm ir, and now I am doing a llvm transform pass to add some code in C program, but I have some trouble in it.
The source C code is

void foo(int i) {
    a[i] = -1;
}

and its llvm ir is：

define void @foo(i32 noundef %0) #0 {
  %2 = alloca i32, align 4
  store i32 %0, ptr %2, align 4
  
  %3 = load i32, ptr %2, align 4
  %4 = sext i32 %3 to i64
  %5 = getelementptr inbounds [10 x i32], ptr @a, i64 0, i64 %4
  store i32 -1, ptr %5, align 4
  ret void
}

now i want to get the variable i, and build an if then instruction, which looks like:

void foo(int i) {
    a[i] = -1;
    if (i > 5)
        // do some things
}

My question is that how can I get the variable i and build an if statement in llvm ir? now I just know use llvm built-in function SplitBlockAndInsertIfThen to create an if…then… statement, but how can I get i first? Waiting for someone to answer my question, or give an idea, sincerely!

2

You can’t. Either write your transformation in such a way that it doesn’t need to know about variable names (because they’re meaningless, what matters are data and control flow) or transform the Clang AST.

3

Oh, thank you for replying me, I output all the operands of GEP and get i, which is sext i32 %3 to i64, is this meaningless?
Am I supposed to trace back to the instruction that allocates memory for the variable i，just like the first ir instruction %2 which uses allocate and give name to %2

4

Thank a lot, but could you explain more clearly, or provide some learning materials? :sob: :sob: :sob:

5

The variable i is the first argument of the function foo. In the IR you posted it would translate to %0. If you have access to the llvm function then you can use the getArg function on it.

e.g.

llvm::Function *FuncFoo = ...somehow get the function foo
llvm::Argument *FirstArg = FuncFoo ->getArg(0);

LLVM: llvm::Function Class Reference

On another note i is not sext i32 %3 to i64, i is %0 see this example for the difference: Compiler Explorer

1 Like
6

Oh, thank you! I understand what you said :+1: