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4b8fa1173c
This switches x86-64 over to using 'tzcount' instead of the integer multiply trick to turn the bytemask information into actual byte counts. We even had a comment saying that a fast bit count instruction is better than a multiply, but x86 bit counting has traditionally been "questionably fast", and so avoiding it was the right thing back in the days. Now, on any half-way modern core, using bit counting is cheaper and smaller than the large constant multiply, so let's just switch over. Note that as part of switching over to counting bits, we also do it at a different point. We used to create the byte count from the final byte mask, but once you use the 'tzcount' instruction (aka 'bsf' on older CPU's), you can actually count the leading zeroes using a value we have available earlier. In fact, we can just use the very first mask of bits that tells us whether we have any zero bytes at all. The zero bytes in the word will have the high bit set, so just doing 'tzcount' on that value and dividing by 8 will give the number of bytes that precede the first NUL character, which is exactly what we want. Note also that the input value to the tzcount is by definition not zero, since that is the condition that we already used to check the whole "do we have any zero bytes at all". So we don't need to worry about the legacy instruction behavior of pre-lzcount days when 'bsf' didn't have a result for zero input. The 32-bit code continues to use the bimple bit op trick that is faster even on newer cores, but particularly on the older 32-bit-only ones. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.