The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in this code slice is:

The loop condition and increment in the for loop allow 'i' to potentially exceed half the size of the destination buffer 'r', leading to an out-of-bounds write when 'memcpy' is called.

Detailed explanation:

1. The loop starts with i = 3 and doubles i in each iteration (i *= 2).
2. The loop continues as long as i < size / 2.
3. However, the last valid iteration could result in i being just below size / 2.
4. In the memcpy call, 'i' is used both as the destination offset (r + i) and the number of bytes to copy.
5. This can lead to writing beyond the bounds of 'r' when i approaches or exceeds size / 2.

For example, if size = 100:
- The last valid iteration might have i = 48 (next iteration would be 96, which exceeds size/2 = 50).
- The memcpy call would then attempt to copy 48 bytes starting at index 48.
- This would write up to index 95, which is beyond the allocated size of 'r'.

To fix this, the code should ensure that the memcpy operation never writes beyond the bounds of 'r', possibly by adjusting the loop condition or adding a bounds check before the memcpy call.