We currently turn off the C++14 sized-deallocation facility on MSVC, and
we'd like to ensure we do the same thing for clang and gcc. To do so,
we add new functionality to moz.configure for checking and adding
compilation flags, similar to the facility for checking and adding
warning flags. The newly added facility is then used to add
-fno-sized-deallocation to the compilation flags, when the option is
supported.
Once we do this, we can't define the sized deallocation functions in
mozalloc.h; the compiler will complain that we are using
-fno-sized-deallocation, yet defining these special functions that we'll
never use. These functions were added for MinGW, where we needed to
compile with C++14 ahead of other platforms to be compatible with MSVC
headers. But they're no longer necessary, though they would be if we
removed -fno-sized-deallocation; the compiler will complain if we do
that and we'll add them back at that point.
SRWLock is more lightweight than CriticalSection, but is only available
on Windows Vista and more. So until we actually dropped support Windows
XP, we had to use CriticalSection.
Now that all supported Windows versions do have SRWLock, this is a
switch we can make, and not only because SRWLock is more lightweight,
but because it can be statically initialized like on other platforms,
allowing to use the same initialization code as on other platforms,
and removing the requirement for a DllMain, which in turn can allow
to statically link mozjemalloc in some cases, instead of requiring a
shared library (DllMain only works on shared libraries), or manually
call the initialization function soon enough.
There is a downside, though: SRWLock, as opposed to CriticalSection, is
not fair, meaning it can have thread scheduling implications, and can
theoretically increase latency on some threads. However, it is the
default used by Rust Mutex, meaning it's at least good enough there.
Let's see how things go with this.
--HG--
extra : rebase_source : 337dc4e245e461fd0ea23a2b6b53981346a545c6
Currently, huge allocations are completely independent from arenas. But
in order to ensure that e.g. moz_arena_realloc can't reallocate huge
allocations from another arena, we need to track which arena was
responsible for the huge allocation. We do that in the corresponding
extent_node_t.
Both functions do essentially the same thing, one having more validation
than the other. We can use a template with a boolean parameter to avoid
the duplication.
Furthermore, we're soon going to require, in some cases, more
information than just the size of the allocation, so we wrap their
result in a helper class that gives information about an active
allocation.
FloorLog2 expands to, essentially, a compiler builtin/intrinsic, that,
in turn, expands to a single machine instruction on tier 1 and other
platforms. On platforms where that's not the case, we can expect the
compiler to generate fast code anyways. So overall, this is all better
than manually using a log2 lookup table.
Also replace a manual power-of-two check with mozilla::IsPowerOfTwo,
which does the same test.
--HG--
extra : rebase_source : e8164c254723c74ef83e798073327ec6afa6f1fb
They are both only used once, are trivial wrappers, and even repeat the
same assertions.
--HG--
extra : rebase_source : b40b26e303cb69a451e63937efd8a666053954e5
There are multiple flaws to the current code:
- The loop calculating the right parameters for a given run size is
repeated.
- The loop trying different run sizes doesn't actually work to fulfil
the overhead constraint: while it stops when the constraint is
fulfilled, the values that are kept are those from the previous
iteration, which may well be well over the constraint.
In practice, the latter resulted in a few surprising results:
- most size classes had an overhead slightly over the constraint
(1.562%), which, while not terribly bad, doesn't match the set
expectations.
- some size classes ended up with relatively good overheads only because
of the additional constraint that run sizes had to be larger than the
run size of smaller size classes. Without this constraint, some size
classes would end up with overheads well over 2% just because that
happens to be the last overhead value before reaching below the 1.5%
constraint.
Furthermore, for higher-level fragmentation concerns, smaller run sizes
are better than larger run sizes, and in many cases, smaller run sizes
can yield the same (or even sometimes, better) overhead as larger run
sizes. For example, the current code choses 8KiB for runs of size 112,
but using 4KiB runs would actually yield the same number of regions, and
the same overhead.
We thus change the calculation to:
- not force runs to be smaller than those of smaller classes.
- avoid the code repetition.
- actually enforce its overhead constraint, but make it 1.6%.
- for especially small size classes, relax the overhead constraint to
2.4%.
This leads to an uneven set of run sizes:
size class before after
4 4 KiB 4 KiB
8 4 KiB 4 KiB
16 4 KiB 4 KiB
32 4 KiB 4 KiB
48 4 KiB 4 KiB
64 4 KiB 4 KiB
80 4 KiB 4 KiB
96 4 KiB 4 KiB
112 8 KiB 4 KiB
128 8 KiB 8 KiB
144 8 KiB 4 KiB
160 8 KiB 8 KiB
176 8 KiB 4 KiB
192 12 KiB 4 KiB
208 12 KiB 8 KiB
224 12 KiB 4 KiB
240 12 KiB 4 KiB
256 16 KiB 16 KiB
272 16 KiB 4 KiB
288 16 KiB 4 KiB
304 16 KiB 12 KiB
320 20 KiB 12 KiB
336 20 KiB 4 KiB
352 20 KiB 8 KiB
368 20 KiB 4 KiB
384 24 KiB 8 KiB
400 24 KiB 20 KiB
416 24 KiB 16 KiB
432 24 KiB 12 KiB
448 28 KiB 4 KiB
464 28 KiB 16 KiB
480 28 KiB 8 KiB
496 28 KiB 20 KiB
512 32 KiB 32 KiB
1024 64 KiB 64 KiB
2048 132 KiB 128 KiB
* Note: before is before this change only, not before the set of changes
from this bug; before that, the run size for 96 could be 8 KiB in some
configurations.
In most cases, the overhead hasn't changed, with a few exceptions:
- Improvements:
size class before after
208 1.823% 0.977%
304 1.660% 1.042%
320 1.562% 1.042%
400 0.716% 0.391%
464 1.283% 0.879%
480 1.228% 0.391%
496 1.395% 0.703%
- Regressions:
352 0.312% 1.172%
416 0.130% 0.977%
2048 1.515% 1.562%
For the regressions, the values are either still well within the
constraint or very close to the previous value, that I don't feel like
it's worth trying to avoid them, with the risk of making things worse
for other size classes.
--HG--
extra : rebase_source : fdff18df8a0a35c24162313d4adb1a1c24fb6e82
On 64-bit platforms, sizeof(arena_run_t) includes a padding at the end
of the struct to align to 64-bit, since the last field, regs_mask, is
32-bit, and its offset can be a multiple of 64-bit depending on the
configuration. But we're doing size calculations for a dynamically-sized
regs_mask based on sizeof(arena_run_t), completely ignoring that
padding.
Instead, we use the offset of regs_mask as a base for the calculation.
Practically speaking, this doesn't change much with the current set of
values, but could affect the overheads when we squeeze run sizes more.
--HG--
extra : rebase_source : a3bdf10a507b81aa0b2b437031b884e18499dc8f
This makes the run header larger than necessary, which happens to make
the current arena_bin_run_calc_size pick 8KiB runs for size class 96
when MOZ_DIAGNOSTIC_ASSERT_ENABLED is set. This change makes it pick
4KiB runs, making MOZ_DIAGNOSTIC_ASSERT_ENABLED builds use the same set
of run sizes as non-MOZ_DIAGNOSTIC_ASSERT_ENABLED builds.
--HG--
extra : rebase_source : fd7ef2d58ec601186647799e9dcf8146e723241c
First and foremost, the code and corresponding comment weren't in
agreement on what's going on.
The code checks:
RUN_MAX_OVRHD * (bin->mSizeClass << 3) <= RUN_MAX_OVRHD_RELAX
which is equivalent to:
(bin->mSizeClass << 3) <= RUN_MAX_OVRHD_RELAX / RUN_MAX_OVRHD
replacing constants:
(bin->mSizeClass << 3) <= 0x1800 / 0x3d
The left hand side is just bin->mSizeClass * 8, and the right hand side
is about 100, so this can be roughly summarized as:
bin->mSizeClass <= 12
The comment says the overhead constraint is relaxed for runs with a
per-region overhead greater than RUN_MAX_OVRHD / (mSizeClass << (3+RUN_BFP)).
Which, on itself, doesn't make sense, because it translates to
61 / (mSizeClass * 32768), which, even for a size class of 1 would mean
less than 0.2%, and this value would be even smaller for bigger classes.
The comment would make more sense with RUN_MAX_OVRHD_RELAX, but would
still not match what the code was doing.
So we change how the relaxed rule works, as per the comment in the new
code, and make it happen after the standard run overhead constraint has
been checked.
--HG--
extra : rebase_source : cec35b5bfec416761fbfbcffdc2b39f0098af849
The description above the RUN_* constant definitions talks about binary
fixed point math, which is one way to look at the problem, but a clearer
one is to look at it as comparing ratios in a way that doesn't use
divisions.
So, starting from the current expression:
(try_reg0_offset << RUN_BFP) <= RUN_MAX_OVRHD * try_run_size
This can be rewritten as
try_reg0_offset * (1 << RUN_BFP) <= RUN_MAX_OVRHD * try_run_size
Dividing both sides with ((1 << RUN_BFP) * try_run_size), and
simplifying, gives us:
try_reg0_offset / try_run_size <= RUN_MAX_OVRHD / (1 << RUN_BFP)
Replacing the constants:
try_reg0_offset / try_run_size <= 0x3d / (1 << 12)
or
try_reg0_offset / try_run_size <= 61 / 4096
61 / 4096 is roughly 1.5%.
So what the check really intends to do is check that the overhead is
below 1.5%.
So we introduce a helper class and a user-defined literal that makes the
test more self-descriptive, while producing identical machine code.
This is a lot of code to add, but I think it's one of those cases where
abstraction can help make the code clearer.
--HG--
extra : rebase_source : 3d4a94f524a60e40ba75859c4f761f59d689e81a
This is, practically speaking, a no-op, and will hopefully help make the
following changes clearer.
--HG--
extra : rebase_source : b704bdf2ae46c2408e0061363822b9744ef449cb
QUANTUM_2POW_MIN is exactly 4, and we are unlikely to ever make it
smaller. Also turn a MOZ_ASSERT into a static_assert, because it only
uses constants, and will fail if QUANTUM_2POW_MIN is lowered without
touching size_invs.
--HG--
extra : rebase_source : 7c8ee3c0ea30a88bddba816c41c6f63914f7a03c
There is a set of "constants" that are actually globals that depend on
the page size that we get at runtime, when compiling without
MALLOC_STATIC_PAGESIZE, but that are actual constants when compiling
with it. Their value was unfortunately duplicated.
We setup a set of macros allowing to make the declarations unique.
--HG--
extra : rebase_source : 56557b7ba01ee60fe85f2cd3c2a0aa910c4c93c6
At the same time, add user-defined literals to make those constants more
legible.
--HG--
extra : rebase_source : ce143ad9d8a6603179042d8cf432f00c815156c5
At the moment, while they are used before their declaration, it's from a
macro. It is desirable to replace the macros with C++ constants, which
will require the structures being defined first.
--HG--
extra : rebase_source : 7a351dafea04a7d75b6eec50fa52fb49c135e569
We create a new helper class that rounds up allocations sizes and
categorizes them. Compilers are smart enough to elide what they don't
need, like in malloc_good_size, they elide the code related to the
class type enum.
--HG--
extra : rebase_source : 61381e600587b045e720a85a7b46673edeb691b9
Because of alignment issues due to the system glibc when running the
SSE2 gcov code generated during the PGO profile gen phase, Firefox
crashes when running the PGO profile. We work around the issue by
disabling SSE2 when building mozjemalloc during that phase. That
shouldn't affect the coverage data anyways, which is bound to the
original C++ code, and the profile-use code generation will still emit
SSE2 based on the coverage data if it needs to.
--HG--
extra : rebase_source : 3596fdc795cdef0789f3a2dd8f10b42cde00430f
We introduce the notion of private arenas, separate from other arenas
(main and thread-local). They are kept in a separate arena tree, and
arena lookups from moz_arena_* functions only access the tree of
private arenas. Iteration still goes through all arenas, private and
non-private.
--HG--
extra : rebase_source : 86c43c7c920b01eb6fa1fa214d612fd9220eac3e
We create the ArenaCollection class to handle operations on the
arena tree. Ideally, iter() would trigger locking, but the
prefork/postfork code complicates things, so we leave this for later.
--HG--
extra : rebase_source : bd7021098baf0ec01c14063294098edea4473d36
Note we use a local variable for fallible allocator because using plain
`new (fallible)` would require some figuring out for non-Firefox builds
(e.g. standalone js).
--HG--
extra : rebase_source : 2132f98ebc7e37a139b673f80631e672bcf8ed15
RedBlackTree::{Insert,Remove} allocate an object on the stack for its
RedBlackTreeNode, and that shouldn't have side effects if the type
happens to have a constructor. This will allow to add constructors to
some of the mozjemalloc types.
--HG--
extra : rebase_source : 14dbb7d73c86921701d83156186df5d645530dda
