Implement string concatenation

compiler.go

@@ -1835,7 +1835,13 @@ func (c *Compiler) parseBinOp(frame *Frame, binop *ssa.BinOp) (llvm.Value, error
 	}
 	switch binop.Op {
 	case token.ADD: // +
-		return c.builder.CreateAdd(x, y, ""), nil
+		if typ, ok := binop.X.Type().(*types.Basic); ok && typ.Kind() == types.String {
+			// string concatenation
+			fn := c.mod.NamedFunction("runtime.stringConcat")
+			return c.builder.CreateCall(fn, []llvm.Value{x, y}, ""), nil
+		} else {
+			return c.builder.CreateAdd(x, y, ""), nil
+		}
 	case token.SUB: // -
 		return c.builder.CreateSub(x, y, ""), nil
 	case token.MUL: // *

src/runtime/string.go

@@ -2,11 +2,17 @@ package runtime
 
 // This file implements functions related to Go strings.
 
+import (
+	"unsafe"
+)
+
+// The underlying struct for the Go string type.
 type _string struct {
 	length lenType
 	ptr    *uint8
 }
 
+// Return true iff the strings match.
 func stringEqual(x, y string) bool {
 	if len(x) != len(y) {
 		return false
@@ -18,3 +24,18 @@ func stringEqual(x, y string) bool {
 	}
 	return true
 }
+
+// Add two strings together.
+func stringConcat(x, y _string) _string {
+	if x.length == 0 {
+		return y
+	} else if y.length == 0 {
+		return x
+	} else {
+		length := uintptr(x.length + y.length)
+		buf := alloc(length)
+		memcpy(buf, unsafe.Pointer(x.ptr), uintptr(x.length))
+		memcpy(unsafe.Pointer(uintptr(buf)+uintptr(x.length)), unsafe.Pointer(y.ptr), uintptr(y.length))
+		return _string{lenType(length), (*uint8)(buf)}
+	}
+}