How to execute llvm code

Question

I have a c code that calculates the factorial of an int "factorial.c". I compile it to llvm readable code "factorial.ll" and I modify in the compiled llvm code. The objective is to execute the modified llvm code and to see its output, How can I do this?

Answer 1

It will depend on how your outputted LLVM is assembled and what libraries it links against, but for example executing the following factorial.ll with the shell command lli

$ lli factorial.ll
Factorial of 10 = 3628800

Will execute the main function with the JIT and use the standard printf to output the result to stdout.

@.str = private unnamed_addr constant [22 x i8] c"Factorial of %d = %d\0A\00", align 1
declare i32 @printf(i8*, ...)

define i32 @factorial(i32 %n) nounwind uwtable {
entry:
  %n.addr = alloca i32, align 4
  store i32 %n, i32* %n.addr, align 4
  %0 = load i32* %n.addr, align 4
  %cmp = icmp sle i32 %0, 1
  br i1 %cmp, label %cond.true, label %cond.false

cond.true:                                        ; preds = %entry
  br label %cond.end

cond.false:                                       ; preds = %entry
  %1 = load i32* %n.addr, align 4
  %2 = load i32* %n.addr, align 4
  %sub = sub nsw i32 %2, 1
  %call = call i32 @factorial(i32 %sub)
  %mul = mul nsw i32 %1, %call
  br label %cond.end

cond.end:                                         ; preds = %cond.false, %cond.true
  %cond = phi i32 [ 1, %cond.true ], [ %mul, %cond.false ]
  ret i32 %cond
}

define i32 @main(i32 %argc, i8** %argv) nounwind uwtable {
entry:
  %retval = alloca i32, align 4
  %argc.addr = alloca i32, align 4
  %argv.addr = alloca i8**, align 8
  store i32 0, i32* %retval
  store i32 %argc, i32* %argc.addr, align 4
  store i8** %argv, i8*** %argv.addr, align 8
  %call = call i32 @factorial(i32 10)
  %call1 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([22 x i8]* @.str, i32 0, i32 0), i32 10, i32 %call)
  ret i32 0
}