These new functions is_char4_outside() and is_char8_outside() are meant
to be used to verify if any of the 4 or 8 chars represented respectively
by a uint32_t or a uint64_t is outside of the min,max byte range passed
in argument. This is the simplified, fast version of the function so it
is restricted to less than 0x80 distance between min and max (sufficient
to validate chars). Extra functions are also provided to check for min
or max alone as well, with the same restriction.
The use case typically is to check that the output of read_u32() or
read_u64() contains exclusively certain bytes.
Sometimes we need to be able to signal one thread among a mask, without
caring much about which bit will be picked. At the moment we use ffsl()
for this but this sometimes results in imbalance at certain specific
places where the same first thread in a set is always the same one that
is selected.
Another approach would consist in using the rank finding function but it
requires a popcount and a setup phase, and possibly a modulo operation
depending on the popcount, which starts to be very expensive.
Here we take a different approach. The idea is an input bit value is
passed, from 0 to LONGBITS-1, and that as much as possible we try to
pick the bit matching it if it is set. Otherwise we look at a mirror
position based on a decreasing power of two, and jump to the side
that still has bits left. In 6 iterations it ends up spotting one bit
among 64 and the operations are very cheap and optimizable. This method
has the benefit that we don't care where the holes are located in the
mask, thus it shows a good distribution of output bits based on the
input ones. A long-time test shows an average of 16 cycles, or ~4ns
per lookup at 3.8 GHz, which is about twice as fast as using the rank
finding function.
Just like for that one, the code was stored into tools.c since we don't
have a C file for intops.
At a few places in the code the switch/case ond flags are tested against
64-bit constants without explicitly being marked as long long. Some
32-bit compilers complain that the constant is too large for a long, and
other likely always use long long there. Better fix that as it's uncertain
what others which do not complain do. It may be backported to avoid doubts
on uncommon platforms if needed, as it touches very few areas.
mul32hi() multiples a constant a with a variable b from 0 to 0xffffffff
and shifts the result by 32 bits. It's visible that it's always impossible
to reach the constant a this way because the product always misses exactly
one unit of a to be preserved. And this cannot be corrected by the caller
either as adding one to the output will only shift the output range, and
it's not possible to pass 2^32 on the ratio <b>. The right approach is to
add "a" after the multiplication so that the input range is always
preserved for all ratio values from 0 to 0xffffffff:
(a=0x00000000 * b=0x00000000 + a=0x00000000) >> 32 = 0x00000000
(a=0x00000000 * b=0x00000001 + a=0x00000000) >> 32 = 0x00000000
(a=0x00000000 * b=0xffffffff + a=0x00000000) >> 32 = 0x00000000
(a=0x00000001 * b=0x00000000 + a=0x00000001) >> 32 = 0x00000000
(a=0x00000001 * b=0x00000001 + a=0x00000001) >> 32 = 0x00000000
(a=0x00000001 * b=0xffffffff + a=0x00000001) >> 32 = 0x00000001
(a=0xffffffff * b=0x00000000 + a=0xffffffff) >> 32 = 0x00000000
(a=0xffffffff * b=0x00000001 + a=0xffffffff) >> 32 = 0x00000001
(a=0xffffffff * b=0xffffffff + a=0xffffffff) >> 32 = 0xffffffff
This is only used in freq_ctr calculations and the slightly lower value
is unlikely to have ever been noticed by anyone. This may be backported
though it is not important.
There are quite a number of integer manipulation functions defined in
standard.h, which is one of the reasons why standard.h is included from
many places and participates to the dependencies loop.
Let's just have a new file, intops.h to place all these operations.
These are a few bitops, 32/64 bit mul/div/rotate, integer parsing and
encoding (including varints), the full avalanche hash function, and
the my_htonll/my_ntohll functions.
For now no new C file was created for these yet.
