More efficient implementation of frandom() in stdlib

stdlib.ispc

@@ -2624,7 +2624,9 @@ static inline unsigned int random(reference RNGState state)
 
 static inline float frandom(reference RNGState state)
 {
-    return ((int)(random(state) & ((1<<24)-1))) / (float)(1 << 24);
+    unsigned int irand = random(state);
+    irand &= (1<<23)-1;
+    return floatbits(0x3F800000 | irand)-1.0f;
 }
 
 static inline uniform unsigned int __seed4(reference RNGState state, 

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;
+}

tests/rand-distrib.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RNGState state;
+    seed_rng(state, 1);
+    float sum = 0;
+    uniform int iters = 10000;
+    for (unsigned int i = 0; i < iters; ++i)
+        sum += frandom(state);
+    float avg = sum / iters;
+    RET[programIndex] = (avg > 0.495 && avg < 0.505) ? 1 : 0;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}