| commit | 107fc11939c9c873ae890d583ad2153cd4b0eead | |
| author | Petr Ledvina <ledvinap@gmail.com> | |
| Thu, 29 Dec 2016 18:31:02 +0000 (29 19:31 +0100) | ||
| committer | Michael Keller <github@ike.ch> | |
| Wed, 25 Jan 2017 20:40:17 +0000 (26 09:40 +1300) | ||
| tree | 0195d8dae4d35498de32a6cdeae557dbdcd35024 | treesnapshot (tar.gz zip) |
| parent | 42256ca8b57feeb0feea4aa9b68f40802106b993 | commitdiff |
Fix atomic.h
`=m` output operand means that value is write only, gcc may discard previous value because it assumes that it will be overwritten. This bug was not triggered in gcc v4 because `asm volatile` triggered full memory barrier.
With old version, this code will increase `markme` only by 2, not 3:
```
static int markme = 0;
markme++;
ATOMIC_BLOCK_NB(0xff) {
ATOMIC_BARRIER(markme); // markme is marked as overwritten, previous increment can be discarded
markme++;
}
markme++;
```
`=m` output operand means that value is write only, gcc may discard previous value because it assumes that it will be overwritten. This bug was not triggered in gcc v4 because `asm volatile` triggered full memory barrier.
With old version, this code will increase `markme` only by 2, not 3:
```
static int markme = 0;
markme++;
ATOMIC_BLOCK_NB(0xff) {
ATOMIC_BARRIER(markme); // markme is marked as overwritten, previous increment can be discarded
markme++;
}
markme++;
```
| src/main/build/atomic.h | diffblobblamehistory |