More efficient implementation of frandom() in stdlib

tests/rand-distrib-1.ispc

@@ -0,0 +1,27 @@
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    RNGState state;
+    seed_rng(state, 1);
+    int count[32];
+    for (uniform int i = 0; i < 32; ++i)
+        count[i] = (b == 5.) ? 0 : 1;
+    uniform int iters = 10000;
+    for (uniform int i = 0; i < iters; ++i) {
+        unsigned int val = random(state);
+        for (uniform int j = 0; j < 32; ++j) {
+            if (val & (1<<j))
+                ++count[j];
+        }
+    }
+
+    bool ok = true;
+    for (uniform int i = 0; i < 32; ++i) 
+        ok |= (count[i] > 0.495 * iters && count[i] < 0.505 * iters);
+    RET[programIndex] = ok ? 1 : 0;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}