The following code (reduced varint encoding) works as expected on Linux x86_64 if built without flags or just one of -O and -release, but fails if both -O -release are specified:
---
import core.stdc.stdio;

ubyte foo(uint n) {
  ubyte[5] buf = void;
  ubyte wsize;

  while (true) {
    if ((n & ~0x7F) == 0) {
      buf[wsize++] = cast(ubyte)n;
      break;
    } else {
      buf[wsize++] = cast(ubyte)((n & 0x7F) | 0x80);
      n >>= 7;
    }
  }

  printf("%hhu\n", wsize);
  return buf[0];
}

void main() {
  printf("%hhx\n", foo(3));
}
---

More specifically, the output (printf()s for shorter assembly) is »1 e0« for the optimized build instead of »1 3«, and the program crashes most of the time (different errors: segfaults, illegal instruction, glibc free() assert triggers, …) – stack corruption?