Add snapshot mode for heap profiling.

Add an ability to initiate GC through V8 API.

I'm planning to use it in DevTools heap profiler. It is a common scenario in debugging memory leaks to enforce GC, then perform an operation, then enforce GC again to check for non-collected (that is, leaked) objects. Using the existing GC extension isn't possible because it doesn't exposed in the normal operation mode of Chromium.

Committed: http://code.google.com/p/v8/source/detail?r=2619

[Søren Thygesen Gjesse, 2009-08-03 14:06:48]

Exposing the (full) GC directly in the API is something we are trying very hard
to avoid, as it will be seen as as something generally useful for the embedding
application. However in most cases it just leads to performance degradations as
running the full GC's too often is expensive.

Having the ability to run GC's to identify actual memory used when performing
heap profiling makes perfect sense, but it should be hidden somewhere in the
Profiling API, and not exposed directly as a general API.

We do full GC's before scanning the heap in a number of places related to
debugging which have lead to strange bugs as the full heap scan is somewhat
dangerous.

In the long term we should replace the full GC followed by a heap scan with a
heap scanner that will visit only live objects using the actual mark-sweep
algorithm.

[mnaganov (inactive), 2009-08-04 13:29:16]

OK, now explicit GC is hidden inside Profiling API. I also moved flag
manipulation logic into Log class.

[Søren Thygesen Gjesse, 2009-08-04 13:44:55]

LGTM

http://codereview.chromium.org/159787/diff/1005/4
File include/v8.h (right):

http://codereview.chromium.org/159787/diff/1005/4#newcode1973
Line 1973: * kinds of heap profiling. Each can be turned on / off independently.
Please add a comment on PROFILER_MODULE_HEAP_SNAPSHOT.

http://codereview.chromium.org/159787/diff/1005/5
File src/api.cc (right):

http://codereview.chromium.org/159787/diff/1005/5#newcode3246
Line 3246: flags &= ~PROFILER_MODULE_HEAP_SNAPSHOT;
Should PROFILER_MODULE_CPU be masked out here as well? What is the exact
semantics of combining PROFILER_MODULE_CPU and PROFILER_MODULE_HEAP_SNAPSHOT?

[mnaganov (inactive), 2009-08-04 14:12:38]

http://codereview.chromium.org/159787/diff/1005/4
File include/v8.h (right):

http://codereview.chromium.org/159787/diff/1005/4#newcode1973
Line 1973: * kinds of heap profiling. Each can be turned on / off independently.
On 2009/08/04 13:44:55, Søren Gjesse wrote:
> Please add a comment on PROFILER_MODULE_HEAP_SNAPSHOT.

Done.

http://codereview.chromium.org/159787/diff/1005/5
File src/api.cc (right):

http://codereview.chromium.org/159787/diff/1005/5#newcode3246
Line 3246: flags &= ~PROFILER_MODULE_HEAP_SNAPSHOT;
On 2009/08/04 13:44:55, Søren Gjesse wrote:
> Should PROFILER_MODULE_CPU be masked out here as well? What is the exact
> semantics of combining PROFILER_MODULE_CPU and PROFILER_MODULE_HEAP_SNAPSHOT?

Combining CPU profiling with snapshot makes no sense to me. Added masking for
it.