[stdlib] Lemire’s Nearly Divisionless Random Integer Generation (#25286)

* [stdlib] Lemire’s nearly divisionless random int

Implementation of Daniel Lemire’s “Fast Random Integer Generation in Interval”
See https://arxiv.org/pdf/1805.10941.pdf

* [stdlib] Simpler, optimized expression

* [stdlib] O'Neill’s modulo optimization

See http://www.pcg-random.org/posts/bounded-rands.html#optimizing-modulo

* [stdlib] Remove modulo optimization

Swift, compared to C, seems unable to generate tightly fused instructions here for some reason (probably the division by zero check?)… removing.

* [stdlib] Keep OpenBSD debiasing method on 32-bit

systems until the https://bugs.swift.org/browse/SR-10910 is resolved.

* [stdlib] TODO FIXME SR-10912

Remove the old OpenBSD generation method, once 32-bit systems support multipliedFullWidth on UInt64.

Author: palimondo

stdlib/public/core/Random.swift

@@ -94,6 +94,7 @@ extension RandomNumberGenerator {
     upperBound: T
   ) -> T {
     _precondition(upperBound != 0, "upperBound cannot be zero.")
+#if arch(i386) || arch(arm) // TODO(FIXME) SR-10912
     let tmp = (T.max % upperBound) + 1
     let range = tmp == upperBound ? 0 : tmp
     var random: T = 0
@@ -103,6 +104,19 @@ extension RandomNumberGenerator {
     } while random < range
 
     return random % upperBound
+#endif
+#if arch(x86_64) || arch(arm64)
+    var random: T = next()
+    var m = random.multipliedFullWidth(by: upperBound)
+    if m.low < upperBound {
+      let t = (0 &- upperBound) % upperBound
+      while m.low < t {
+        random = next()
+        m = random.multipliedFullWidth(by: upperBound)
+      }
+    }
+    return m.high
+#endif
   }
 }