Added JIT compiler abstraction (#19)

* Added JIT compiler abstraction

* Added missing access control and fixed comment nits.

Author: harlanhaskins

Sources/LLVM/JIT.swift

@@ -0,0 +1,114 @@
+import cllvm
+
+/// JITError represents the different kinds of errors the JIT compiler can
+/// throw.
+public enum JITError: Error, CustomStringConvertible {
+    /// The JIT was unable to be initialized. A message is provided explaining
+    /// the failure.
+    case couldNotInitialize(String)
+
+    /// A human-readable description of the error.
+    public var description: String {
+        switch self {
+        case .couldNotInitialize(let message):
+            return "could not initialize JIT: \(message)"
+        }
+    }
+}
+
+/// A `JIT` is a Just-In-Time compiler that will compile and execute LLVM IR
+/// that has been generated in a `Module`. It can execute arbitrary functions
+/// and return the value the function generated, allowing you to write
+/// interactive programs that will run as soon as they are compiled.
+public final class JIT {
+    /// The underlying LLVMExecutionEngineRef backing this JIT.
+    internal let llvm: LLVMExecutionEngineRef
+
+    /// Creates a Just In Time compiler that will compile the code in the
+    /// provided `Module` to the architecture of the provided `TargetMachine`,
+    /// and execute it.
+    ///
+    /// - parameters:
+    ///   - module: The module containing code you wish to execute
+    ///   - machine: The target machine which you're compiling for
+    /// - throws: JITError
+    public init(module: Module, machine: TargetMachine) throws {
+        var jit: LLVMExecutionEngineRef?
+        var error: UnsafeMutablePointer<Int8>?
+        if LLVMCreateExecutionEngineForModule(&jit, module.llvm, &error) != 0 {
+            let str = String(cString: error!)
+            throw JITError.couldNotInitialize(str)
+        }
+        guard let _jit = jit else {
+            throw JITError.couldNotInitialize("JIT was NULL")
+        }
+        self.llvm = _jit
+    }
+
+
+    /// Runs the specified function with the provided arguments by compiling
+    /// it to machine code for the target architecture used to initialize this
+    /// JIT.
+    ///
+    /// - parameters:
+    ///   - function: The function you wish to execute
+    ///   - args: The arguments you wish to pass to the function
+    /// - returns: The LLVM value that the function returned
+    public func runFunction(_ function: Function, args: [IRValue]) -> IRValue {
+        var irArgs = args.map { $0.asLLVM() as Optional }
+        return irArgs.withUnsafeMutableBufferPointer { buf in
+            return LLVMRunFunction(llvm, function.asLLVM(),
+                                   UInt32(buf.count), buf.baseAddress)
+        }
+    }
+
+
+    /// Runs the specified function as if it were the `main` function in an
+    /// executable. It takes an array of argument strings and passes them
+    /// into the function as `argc` and `argv`.
+    ///
+    /// - parameters:
+    ///   - function: The `main` function you wish to execute
+    ///   - args: The string arguments you wish to pass to the function
+    /// - returns: The numerical exit code returned by the function
+    public func runFunctionAsMain(_ function: Function, args: [String]) -> Int {
+        // FIXME: Also add in envp.
+        return withCArrayOfCStrings(args) { buf in
+            return Int(LLVMRunFunctionAsMain(llvm, function.asLLVM(),
+                                             UInt32(buf.count),
+                                             buf.baseAddress, nil))
+        }
+    }
+}
+
+
+/// Runs the provided block with the equivalent C strings copied from the
+/// passed-in array. The C strings will only be alive for the duration
+/// of the block, and they will be freed when the block exits.
+///
+/// - parameters:
+///   - strings: The strings you intend to convert to C strings
+///   - block: A block that uses the C strings
+/// - returns: The result of the passed-in block.
+/// - throws: Will only throw if the passed-in block throws.
+internal func withCArrayOfCStrings<T>(_ strings: [String], _ block:
+    (UnsafeBufferPointer<UnsafePointer<Int8>?>) throws -> T) rethrows -> T {
+    var cStrings = [UnsafeMutablePointer<Int8>?]()
+    for string in strings {
+        string.withCString {
+            cStrings.append(strdup($0))
+        }
+    }
+    defer {
+        for cStr in cStrings {
+            free(cStr)
+        }
+    }
+    return try cStrings.withUnsafeBufferPointer { buf in
+        // We need to make this "immutable" but that doesn't change
+        // their size or contents.
+        let constPtr = unsafeBitCast(buf.baseAddress,
+                            to: UnsafePointer<UnsafePointer<Int8>?>.self)
+        return try block(UnsafeBufferPointer(start: constPtr, count: buf.count))
+    }
+}