It looks like __builtin_prefetch() appeared in gcc-3.1 as there's no
mention of it in 3.0's doc. Let's replace it with eb_prefetch() which
maps to __builtin_prefetch() on supported compilers and falls back to
the usual do{}while(0) on other ones. It was tested to properly build
with tcc as well as gcc-2.95.
This is ebtree commit 7ee6ede56a57a046cb552ed31302b93ff1a21b1a.
In the loop we can help the compiler build slightly more efficient code
by placing an unlikely() around the leaf test. This shows a consistent
0.5% performance gain both on eb32 and eb64.
This is ebtree commit 6c9cdbda496837bac1e0738c14e42faa0d1b92c4.
The current code calculates the next troot based on a calculation.
This was efficient when the algorithm was developed many years ago
on K6 and K7 CPUs running at low frequencies with few registers and
limited branch prediction units but nowadays with ultra-deep pipelines
and high latency memory that's no longer efficient, because the CPU
needs to have completed multiple operations before knowing which
address to start fetching from. It's sad because we only have two
branches each time but the CPU cannot know it. In addition, the
calculation is performed late in the loop, which does not help the
address generation unit to start prefetching next data.
Instead we should help the CPU by preloading data early from the node
and calculing troot as soon as possible. The CPU will be able to
postpone that processing until the dependencies are available and it
really needs to dereference it. In addition we must absolutely avoid
serializing instructions such as "(a >> b) & 1" because there's no
way for the compiler to parallelize that code nor for the CPU to pre-
process some early data.
What this patch does is relatively simple:
- we try to prefetch the next two branches as soon as the
node is known, which will help dereference the selected node in
the next iteration; it was shown that it only works with the next
changes though, otherwise it can reduce the performance instead.
In practice the prefetching will start a bit later once the node
is really in the cache, but since there's no dependency between
these instructions and any other one, we let the CPU optimize as
it wants.
- we preload all important data from the node (next two branches,
key and node.bit) very early even if not immediately needed.
This is cheap, it doesn't cause any pipeline stall and speeds
up later operations.
- we pre-calculate 1<<bit that we assign into a register, so as
to avoid serializing instructions when deciding which branch to
take.
- we assign the troot based on a ternary operation (or if/else) so
that the CPU knows upfront the two possible next addresses without
waiting for the end of a calculation and can prefetch their contents
every time the branch prediction unit guesses right.
Just doing this provides significant gains at various tree sizes on
random keys (in million lookups per second):
eb32 1k: 29.07 -> 33.17 +14.1%
10k: 14.27 -> 15.74 +10.3%
100k: 6.64 -> 8.00 +20.5%
eb64 1k: 27.51 -> 34.40 +25.0%
10k: 13.54 -> 16.17 +19.4%
100k: 7.53 -> 8.38 +11.3%
The performance is now much closer to the sequential keys. This was
done for all variants ({32,64}{,i,le,ge}).
Another point, the equality test in the loop improves the performance
when looking up random keys (since we don't need to reach the leaf),
but is counter-productive for sequential keys, which can gain ~17%
without that test. However sequential keys are normally not used with
exact lookups, but rather with lookup_ge() that spans a time frame,
and which does not have that test for this precise reason, so in the
end both use cases are served optimally.
It's interesting to note that everything here is solely based on data
dependencies, and that trying to perform *less* operations upfront
always ends up with lower performance (typically the original one).
This is ebtree commit 05a0613e97f51b6665ad5ae2801199ad55991534.
This is where other imported components are located. All files which
used to directly include ebtree were touched to update their include
path so that "import/" is now prefixed before the ebtree-related files.
The ebtree.h file was slightly adjusted to read compiler.h from the
common/ subdirectory (this is the only change).
A build issue was encountered when eb32sctree.h is loaded before
eb32tree.h because only the former checks for the latter before
defining type u32. This was addressed by adding the reverse ifdef
in eb32tree.h.
No further cleanup was done yet in order to keep changes minimal.
As part of the include files cleanup, we're going to kill the ebtree
directory. For this we need to host its C files in a different location
and src/ is the right one.
