This reduces memory usage because we only need one allocation instead of two
for the dynamic atom and its chars, and because we don't need to store a
refcount and a size. It precludes sharing of chars between dynamic atoms, but
we weren't benefiting much from that anyway.
This reduces per-process memory usage by up to several hundred KiB on my
Linux64 box.
One consequence of this change is that we need to allocate + copy in
DOMString::SetKnownLiveAtom(), which could make some things slower.
--HG--
extra : rebase_source : ba4065ea31e509dd985c003614199f73def0596c
Currently static atoms are stored on the heap, but their char buffers are
stored in read-only static memory.
This patch changes the representation of nsStaticAtom (thus making it a
non-trivial subclass of nsAtom). Instead of a pointer to the string, it now has
an mStringOffset field which is a 32-bit offset to the string. (This requires
placement of the string and the atom within the same object so that the offset
is known to be small. The docs and macros in nsStaticAtom.h handle that.)
Static and dynamic atoms now store their chars in different ways: nsStaticAtom
stores them inline, nsDynamicAtom has a pointer to separate storage. So
`mString` and GetStringBuffer() move from nsAtom to nsDynamicAtom.
The change to static atoms means they can be made constexpr and stored in
read-only memory instead of on the heap. On 64-bit this reduces the per-process
overhead by 16 bytes; on 32-bit the saving is 12 bytes. (Further reductions
will be possible in follow-up patches.)
The increased use of constexpr required multiple workarounds for MSVC.
- Multiple uses of MOZ_{PUSH,POP}_DISABLE_INTEGRAL_CONSTANT_OVERFLOW_WARNING to
disable warnings about (well-defined!) overflow of unsigned integer
arithmetic.
- The use of -Zc:externConstexpr on all files defining static atoms, to make
MSVC follow the C++ standard(!) and let constexpr variables have external
linkage.
- The use of -constexpr:steps300000 to increase the number of operations
allowed in a constexpr value, in order to handle gGkAtoms, which requires
hashing ~2,500 atom strings.
The patch also changes how HTML5 atoms are handled. They are now treated as
dynamic atoms, i.e. we have "dynamic normal" atoms and "dynamic HTML5 atoms",
and "dynamic atoms" covers both cases, and both are represented via
nsDynamicAtom. The main difference between the two kinds is that dynamic HTML5
atoms still aren't allowed to be used in various operations, most notably
AddRef()/Release(). All this also required moving nsDynamicAtom into the header
file.
There is a slight performance cost to all these changes: now that nsStaticAtom
and nsDynamicAtom store their chars in different ways, a conditional branch is
required in the following functions: Equals(), GetUTF16String(),
WeakAtom::as_slice().
Finally, in about:memory the "explicit/atoms/static/atom-objects" value is no
longer needed, because that memory is static instead of heap-allocated.
MozReview-Commit-ID: 4AxPv05ngZy
By removing the "Atom" suffix, which is redundant.
MozReview-Commit-ID: 4MCX9Icfjrw
--HG--
extra : rebase_source : c3c759a508a8938b59d36dbb20448d2964b98c91
The renaming here is like this:
SetStringBuffer -> SetKnownLiveStringBuffer
SetEphemeralStringBuffer -> SetStringBuffer
SetOwnedString -> SetKnownLiveString
SetOwnedAtom -> SetKnownLiveAtom
This should make it clearer what the lifetime expectations are on the caller side.
MozReview-Commit-ID: ERHbB3r6paN
This removes an unnecessary level of indirection by replacing all
nsStringGlue.h instances with just nsString.h.
--HG--
extra : rebase_source : 340989240af4018f3ebfd92826ae11b0cb46d019
Currently nsAtom::mString points to the interior of an nsStringBuffer. For
static atoms this requires the use of nsFakeStringBuffer, which is pretty
gross.
This patch changes things so that nsAtom::mString points to a static char
buffer for static atoms. This simplifies a number of things:
- nsFakeStringBuffer and CheckStaticAtomSizes are no longer needed.
- FakeBufferRefCountHelper is no longer needed.
- nsAtom's constructor for static atoms is simpler.
- RegisterStaticAtoms() is simpler.
On the flip-side, a couple of things get more complicated.
- nsAtom::ToString() treats static and dynamic atoms differently.
- nsAtom::GetStringBuffer() is now only valid for dynamic atoms. This
function is only used in two places, both involving DOMString, so those
locations are updated appropriately. This also requires updating some other
code assigning nsStrings to DOMStrings, because we can't assume that
nsStrings are shared.
On Linux64 this change reduces the size of the binary by 8752 B, and moves
81968 B from the .data to the .rodata section, where it can be shared between
processes.
--HG--
extra : rebase_source : 0f6fcdec1c525aa66222e208b66a9f9026f69bcb
(Path is actually r=froydnj.)
Bug 1400459 devirtualized nsIAtom so that it is no longer a subclass of
nsISupports. This means that nsAtom is now a better name for it than nsIAtom.
MozReview-Commit-ID: 91U22X2NydP
--HG--
rename : xpcom/ds/nsIAtom.h => xpcom/ds/nsAtom.h
extra : rebase_source : ac3e904a21b8b48e74534fff964f1623ee937c67
Boris Zbarsky
Nicholas Nethercote
Eric Rahm
Andrew McCreight
Ehsan Akhgari
Seth Fowler
Olli Pettay
Ryan VanderMeulen