Optimise getentropy

[RReverser]

• Uses random filling directly into the destination when possible.
• Fills up the entire block at once to avoid overhead of asking the underlying implementation to generate each byte separately.
• Generates buffer chunks for / dev/{u,}random for the same reasons.

[RReverser]

Non-scientific benchmark compiled with -O3 showing that it still works correctly & is indeed way faster:

#include <emscripten.h>
#include <unistd.h>
#include <stdio.h>

// 256-bit entropy buffer
// volatile just to prevent it from getting optimized out
volatile unsigned char entropy[32] = {0};

void print_entropy() {
  printf("Entropy value: ");
  for (int i = 0; i < 32; ++i) {
    printf("%02x", entropy[i]);
  }
  printf("\n");
}

int main() {
  print_entropy();
  double start = emscripten_get_now();
  // preheat
  for (int i = 0; i < 100; ++i) {
    getentropy((void*) entropy, sizeof(entropy));
  }
  // actual benchmark
  int n = 100000;
  for (int i = 0; i < n; ++i) {
    getentropy((void*) entropy, sizeof(entropy));
  }
  double end = emscripten_get_now();
  printf("Time per call: %fns\n", (end - start) * 1000 / n);
  print_entropy();
}

---

Old (Node.js):

Entropy value: 0000000000000000000000000000000000000000000000000000000000000000
Time per call: 83.646909ns
Entropy value: eceacda315bc4aa0f4371b6b9b5f1266a999cd5fb3873bbdbbab5d5d514b7dd6

New (Node.js):

Entropy value: 0000000000000000000000000000000000000000000000000000000000000000
Time per call: 2.514285ns
Entropy value: ab5a6870feb9c14a8323e1666e210ced0704b2d7f8d2ef7ccdddc0d3e6594431

---

Old (browser):

Entropy value: 0000000000000000000000000000000000000000000000000000000000000000
Time per call: 24.255750ns
Entropy value: bf1902314e41ad063ecbdbe3235c3b81d4f8d45c74d10deee9684505ded5b4d4

New (browser):

Entropy value: 0000000000000000000000000000000000000000000000000000000000000000
Time per call: 1.656000ns
Entropy value: c47ca65b8ac13439ce5df870f5281b285e2072041fd39eb9ae57861b13e63935

---

Old (browser + pthread):

Entropy value: 0000000000000000000000000000000000000000000000000000000000000000
Time per call: 22.963799ns
Entropy value: 355a82b0790efe0c09c4f281bfdaf69bcad28a037733f2e2b994a2ae46c86a6f

New (browser + pthread):

Entropy value: 0000000000000000000000000000000000000000000000000000000000000000
Time per call: 2.961150ns
Entropy value: 26850aff38d2769714ede7ea0e0fddc834a10fb27acea21134236aa8a66e84ba

---

In fact, the new timing is pretty much constant-time whether you generate sizeof(int) or 256 bits like in my benchmark, but always faster than before.