Add an LLVM implementation of 'random' to standard library (#1163)

Author: phischu

libraries/common/effekt.effekt

@@ -164,6 +164,10 @@ extern def unsafeSubstring(str: String, from: Int, to: Int) at {}: String =
 extern def random() at io: Double =
   js "Math.random()"
   chez "(random 1.0)"
+  llvm """
+    %r = call double @c_io_random()
+    ret double %r
+  """
   vm "effekt::random()"
 
 // References and state

libraries/llvm/forward-declare-c.ll

@@ -5,6 +5,7 @@ declare %Pos @c_get_arg(i64)
 
 declare void @c_io_println(%Pos)
 declare %Pos @c_io_readln()
+declare %Double @c_io_random()
 
 declare void @hole(i8*)
 declare void @duplicated_prompt()

libraries/llvm/io.c

@@ -4,8 +4,8 @@
 #include <uv.h>
 #include <string.h> // to compare flag names
 
-// Println and Readln
-// ------------------
+// Println and Readln and Random
+// -----------------------------
 
 void c_io_println(String text) {
     for (uint64_t j = 0; j < text.tag; ++j) {
@@ -34,6 +34,10 @@ String c_io_readln() {
     return result;
 }
 
+double c_io_random(void) {
+    return (double)rand() / (double)RAND_MAX;
+}
+
 // Lib UV Bindings
 // ---------------
 // Ideas behind the LLVM / libuv implementation.