It is a JIT optimizer bug. It is unrolling the inner loop but not updating the oVec.y value properly:
for (oVec.x = 0; oVec.x < 2; oVec.x++) {
0000000a xor esi,esi ; oVec.x = 0
for (oVec.y = 0; oVec.y < 2; oVec.y++) {
0000000c mov edi,2 ; oVec.y = 2, WRONG!
oDoesSomething.Do(oVec);
00000011 push edi
00000012 push esi
00000013 mov ecx,ebx
00000015 call dword ptr ds:[00170210h] ; first unrolled call
0000001b push edi ; WRONG! does not increment oVec.y
0000001c push esi
0000001d mov ecx,ebx
0000001f call dword ptr ds:[00170210h] ; second unrolled call
for (oVec.x = 0; oVec.x < 2; oVec.x++) {
00000025 inc esi
00000026 cmp esi,2
00000029 jl 0000000C
The bug disappears when you let oVec.y increment to 4, that’s too many calls to unroll.
One workaround is this:
for (int x = 0; x < 2; x++) {
for (int y = 0; y < 2; y++) {
oDoesSomething.Do(new IntVec(x, y));
}
}
UPDATE: re-checked in August 2012, this bug was fixed in the version 4.0.30319 jitter. But is still present in the v2.0.50727 jitter. It seems unlikely they’ll fix this in the old version after this long.