PolicyArray splits the ArrayData implementation in two parts. ArrayShell implements the baroque ArrayData implementation in terms of a much smaller statically-bound core that concerns itself exclusively with the storage strategy for the array. This way the two aspects can be worked on separately, and different stores can be easily plugged into the given ArrayShell.
To give a better perspective, the featured SimpleArrayStore has about 340 lines all told (including solid documentation), whereas ArrayShell has some 1100 lines. The shell can be reused with other stores of unbounded sophistication. The store needs to implement only 22 primitives, some of which are trivial. Basically a new store implementation saves 1100 of difficult-to-get-right lines of code right off the bat, and only needs to focus on implementing 22 well-defined primitives.
Things to watch for when reviewing:
- Is the store API small/expressive enough? What should be added/removed?
- Are various forms of duplication present? If so, how could code be factored better?
- Any subtle change in semantics from HphpArray and friends that should be minded?
Known issues:
* Currently ArrayShell is defined as:
class ArrayShell : public ArrayData, private SimpleArrayStore { ... };
when in fact it should be:
template <class StorePolicy>
class ArrayShell : public ArrayData, private StorePolicy { ... };
Extenuating circumstances related to allocator design prevent use of templates at this point. I'll work on these in parallel with the review.
* growNoResize is almost always prefaced by a test-and-grow. This sequence should be encoded in a function.
* The growth policy is spread all over the place in a subtle form of duplication.
* The current store design makes almost no effort to be particularly efficient.
Streamline the array access methods by always returning an array
pointer instead of a new pointer or null. Callsites compare
(new != old) to detect escalation, rather than (new != null).
Replaces the awkward getFullPos() and setFullPos() methods with a more
intuitive advanceFullPos() method. This refactoring also reduces the number
of virtual calls made when doing mutable iteration on an array.
Of the four horsemen of the SmartAllocator, ArrayData was the only virtual
call. This meant an extra layer of indirection when coming from the TC
to allow the c++ compiler to emit its virtual call, and slightly larger
callsites when using non-generic paths.
While we're moving in this direction, consolidate ArrayData introspection
on its type enum. isSharedMap() was previously implemented with a vtable
slot, and we had no way of asking if an ArrayData was a NameValueTable.
A few simple refactorings and optimizations for array_data. Perflab results (CPU time %) are on the desired side of noise: 22/23 show reduced times, 12/23 green, no red, best -2.2%, worst +0.3%.
SharedMap was the last dependency on ZendArray. For its localCache,
use a TypedValue[] array indexed by SharedVariant.getIndex(), and
for escalate(mutableIteration), escalate to an HphpArray instead of
a ZendArray.
Some StringData* helpers were forwarding to CStrRef
methods - which now forward to StringData* methods. Just
call the StringData* methods directly
Other methods were using stack allocated StringData's.
Thats pointlessly risky for slow-path code. Use a String
instead.
g++-4.7.1 treats "FOO"bar as a c++-11 literal operator, even
if bar is a macro with an expansion such as "BAR" - so add a space
after the quote (this seems like a bug, and I fixed a bunch of these
a while ago, but we just added a slew of PRI*64 macros which break
under 4.7.1).
Also, it warned that "explicit by-copy capture of 'this' redundant"
for a lambda declared [=, this] - so I removed the this.
We also needed more than the 60 levels of template expansion that was
allowed by the makefile.
This diff refactors some of the VM's logic for iterators (with a focus on
mutable iteration), delivering several improvements:
1) MIterCtx was renamed to MArrayIter, and the m_key and m_val fields
were eliminated.
2) Eliminated the need for MArrayIter to dynamically allocate a
MutableArrayIter object, and removed other layers of indirection as
well.
3) Reduced the size of HPHP::VM::Iter from 64 bytes down to 32 bytes.
4) Removed the "if (siPastEnd())" check when adding a new element to an
HphpArray or a ZendArray.
5) Moved all of the iterator logic into a single .cpp file.
This diff reworks FullPos's to point to current element instead of pointing
to the next element. It also splits up the IterFree instruction into two
instructions (IterFree and MIterFree). These changes allowed various logic
to be simplified and data structures to be reduced in size. There is
definitely more opportunity for refactoring, but I know the JIT helpers for
iteration have been carefully tuned and so I'll leave further refactoring
for future diffs.
Finally, I spent a little time cleaning up the bytecode spec a bit, mostly
with respect to iteration.
Per @mwilliams' suggestion, this is the first stage in a staggered approach to replacing int64 with int64_t. More precisely I inserted "typedef ::int64_t int64;" in util/base.h and dealt with the consequences.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.
Paul Tarjan