Similarly to the previous patch, it's better to keep a local copy of
the new node's key instead of accessing it every time. This slightly
reduces the code's size in the descent and further improves the load
time to 7.45s.
looking at a perf profile while loading a conf with a huge map, it
appeared that there was a hot spot on the access to the new node's
prefix, which is unexpectedly being reloaded for each visited node
during the tree descent. Better keep a copy of it because with large
trees that don't fit into the L3 cache the memory bandwidth is scarce.
Doing so reduces the load time from 8.0 to 7.5 seconds.
This function is designed to enlarge the scope of a lookup performed
by a caller via ebmb_lookup_longest() that was not satisfied with the
result. It will first visit next duplicates, and if none are found,
it will go up in the tree to visit similar keys with shorter prefixes
and will return them if they match. We only use the starting point's
value to perform the comparison since it was expected to be valid for
the looked up key, hence it has all bits in common with its own length.
The algorithm is a bit complex because when going up we may visit nodes
that are located beneath the level we just come from. However it is
guaranteed that keys having a shorter prefix will be present above the
current location, though they may be attached to the left branch of a
cover node, so we just visit all nodes as long as their prefix is too
large, possibly go down along the left branch on cover nodes, and stop
when either there's a match, or there's a non-matching prefix anymore.
The following tricky case now works fine and properly finds 10.0.0.0/7
when looking up 11.0.0.1 from tree version 1 though both belong to
different sub-trees:
prepare map #1
add map @1 #1 10.0.0.0/7 10.0.0.0/7
add map @1 #1 10.0.0.0/7 10.0.0.0/7
commit map @1 #1
prepare map #1
add map @2 #1 11.0.0.0/8 11.0.0.0/8
add map @2 #1 11.0.0.0/8 11.0.0.0/8
prepare map #1
add map @1 #1 10.0.0.0/7 10.0.0.0/7
commit map @1 #1
prepare map #1
add map @2 #1 10.0.0.0/7 10.0.0.0/7
add map @2 #1 11.0.0.0/8 11.0.0.0/8
add map @2 #1 11.0.0.0/8 11.0.0.0/8
ebtree is one piece using a lot of inlines and each tree root or node
definition needed by many of our structures requires to parse and
compile all these includes, which is large and painfully slow. Let's
move the very basic definitions to their own file and include it from
ebtree.h.
This is from the output of codespell. It's done at once over a bunch
of files and only affects comments, so there is nothing user-visible.
No backport needed.
As reported in issue #689, there is a subtle bug in the ebtree code used
to compared memory blocks. It stems from the platform-dependent memcmp()
implementation. Original implementations used to perform a byte-per-byte
comparison and to stop at the first non-matching byte, as in this old
example:
https://www.retro11.de/ouxr/211bsd/usr/src/lib/libc/compat-sys5/memcmp.c.html
The ebtree code has been relying on this to detect the first non-matching
byte when comparing keys. This is made so that a zero-terminated string
can fail to match against a longer string.
Over time, especially with large busses and SIMD instruction sets,
multi-byte comparisons have appeared, making the processor fetch bytes
past the first different byte, which could possibly be a trailing zero.
This means that it's possible to read past the allocated area for a
string if it was allocated by strdup().
This is not correct and definitely confuses address sanitizers. In real
life the problem doesn't have visible consequences. Indeed, multi-byte
comparisons are implemented so that aligned words are loaded (e.g. 512
bits at once to process a cache line at a time). So there is no way such
a multi-byte access will cross a page boundary and end up reading from
an unallocated zone. This is why it was never noticed before.
This patch addresses this by implementing a one-byte-at-a-time memcmp()
variant for ebtree, called eb_memcmp(). It's optimized for both small and
long strings and guarantees to stop after the first non-matching byte. It
only needs 5 instructions in the loop and was measured to be 3.2 times
faster than the glibc's AVX2-optimized memcmp() on short strings (1 to
257 bytes), since that latter one comes with a significant setup cost.
The break-even seems to be at 512 bytes where both version perform
equally, which is way longer than what's used in general here.
This fix should be backported to stable versions and reintegrated into
the ebtree code.
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.
