| commit | 108d9d6b61b05d96163dfd4cdea2d4d86f413b8e | |
| author | Petr Ledvina <ledvinap@gmail.com> | |
| Thu, 29 Dec 2016 18:31:02 +0000 (29 19:31 +0100) | ||
| committer | borisbstyle <dreambb1982@gmail.com> | |
| Mon, 30 Jan 2017 23:50:26 +0000 (31 00:50 +0100) | ||
| tree | 2af901151c4fc5f93a929ec2f5e6e848d480bc6a | treesnapshot (tar.gz zip) |
| parent | 46fa01a4b82c1fa3adaab35ebf0716a234132c75 | 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 |