Compatible atomics: volatile for LoadRelaxed, replace FetchAndAdd with FetchAndDecrement.

Compatible atomics: volatile for LoadRelaxed, replace FetchAndAdd with FetchAndDecrement.

Using volatile on a word size location will generate a single move instruction without order guarantees and is supported on all compilers.

Replace FetchAndAdd with FetchAndDecrement, since the latter is sufficient and eaiser to implement portably (in particular on some Windows setups).

BUG=https://github.com/dart-lang/sdk/issues/24049
R=iposva@google.com

Committed: https://github.com/dart-lang/sdk/commit/e7fb1c8516c99279551471986f044655285e7bbb

[koda, 2015-08-12 20:16:27]

koda@google.com changed reviewers:
+ iposva@google.com

[Ivan Posva, 2015-08-13 01:43:22]

LGTM with comments added.

-Ivan

https://codereview.chromium.org/1287853005/diff/20001/runtime/vm/atomic.h
File runtime/vm/atomic.h (right):

https://codereview.chromium.org/1287853005/diff/20001/runtime/vm/atomic.h#new...
runtime/vm/atomic.h:24: static uintptr_t FetchAndDecrement(uintptr_t* p);
This also needs the comment from above. Especially the NOTE about being accessed
by generated code.

https://codereview.chromium.org/1287853005/diff/20001/runtime/vm/atomic.h#new...
runtime/vm/atomic.h:26: static uword CompareAndSwapWord(uword* ptr, uword
old_value, uword new_value);
Please add a comment here as well plus the NOTE that CompareAndSwapWord is safe
for use with generated code.

[koda, 2015-08-13 16:21:42]

Committed patchset #3 (id:40001) manually as
e7fb1c8516c99279551471986f044655285e7bbb (presubmit successful).

[koda, 2015-08-13 16:22:38]

https://codereview.chromium.org/1287853005/diff/20001/runtime/vm/atomic.h
File runtime/vm/atomic.h (right):

https://codereview.chromium.org/1287853005/diff/20001/runtime/vm/atomic.h#new...
runtime/vm/atomic.h:24: static uintptr_t FetchAndDecrement(uintptr_t* p);
On 2015/08/13 01:43:21, Ivan Posva wrote:
> This also needs the comment from above. Especially the NOTE about being
accessed
> by generated code.

Done.

https://codereview.chromium.org/1287853005/diff/20001/runtime/vm/atomic.h#new...
runtime/vm/atomic.h:26: static uword CompareAndSwapWord(uword* ptr, uword
old_value, uword new_value);
On 2015/08/13 01:43:21, Ivan Posva wrote:
> Please add a comment here as well plus the NOTE that CompareAndSwapWord is
safe
> for use with generated code.

Done.