Why does LLVM allocate a redundant variable?

Question

Here's a simple C file with an enum definition and a main function:

enum days {MON, TUE, WED, THU};

int main() {
    enum days d;
    d = WED;
    return 0;
}

It transpiles to the following LLVM IR:

define dso_local i32 @main() #0 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  store i32 2, i32* %2, align 4
  ret i32 0
}

%2 is evidently the d variable, which gets 2 assigned to it. What does %1 correspond to if zero is returned directly?

Answer 1

The %1 register was generated by clang to handle multiple return statements in a function. Imagine you were writing a function to compute an integer's factorial. Instead of this

int factorial(int n){
    int result;
    if(n < 2)
      result = 1;
    else{
      result = n * factorial(n-1);
    }
    return result;
}

You'd probably do this

int factorial(int n){
    if(n < 2)
      return 1;
    return n * factorial(n-1);
}

Why? Because Clang will insert that result variable that holds the return value for you. Yay. That's the reason for that %1 variable. Look at the IR for a slightly modified version of your code.

Modified code,

enum days {MON, TUE, WED, THU};

int main() {
    enum days d;
    d = WED;
    if(d) return 1;
    return 0;
}

IR,

define dso_local i32 @main() #0 !dbg !15 {
    %1 = alloca i32, align 4
    %2 = alloca i32, align 4
    store i32 0, i32* %1, align 4
    store i32 2, i32* %2, align 4, !dbg !22
    %3 = load i32, i32* %2, align 4, !dbg !23
    %4 = icmp ne i32 %3, 0, !dbg !23
    br i1 %4, label %5, label %6, !dbg !25

 5:                                                ; preds = %0
   store i32 1, i32* %1, align 4, !dbg !26
   br label %7, !dbg !26

 6:                                                ; preds = %0
  store i32 0, i32* %1, align 4, !dbg !27
  br label %7, !dbg !27

 7:                                                ; preds = %6, %5
  %8 = load i32, i32* %1, align 4, !dbg !28
  ret i32 %8, !dbg !28
}

Now you see %1 making itself useful huh? Most functions with a single return statement will have this variable stripped by one of LLVM's passes.