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If the product is only 64bits div64_u64() can be used for the divide. Replace the pre-multiply check (ilog2(a) + ilog2(b) <= 62) with a simple post-multiply check that the high 64bits are zero. This has the advantage of being simpler, more accurate and less code. It will always be faster when the product is larger than 64bits. Most 64bit cpu have a native 64x64=128 bit multiply, this is needed (for the low 64bits) even when div64_u64() is called - so the early check gains nothing and is just extra code. 32bit cpu will need a compare (etc) to generate the 64bit ilog2() from two 32bit bit scans - so that is non-trivial. (Never mind the mess of x86's 'bsr' and any oddball cpu without fast bit-scan instructions.) Whereas the additional instructions for the 128bit multiply result are pretty much one multiply and two adds (typically the 'adc $0,%reg' can be run in parallel with the instruction that follows). The only outliers are 64bit systems without 128bit mutiply and simple in order 32bit ones with fast bit scan but needing extra instructions to get the high bits of the multiply result. I doubt it makes much difference to either, the latter is definitely not mainstream. If anyone is worried about the analysis they can look at the generated code for x86 (especially when cmov isn't used). Link: https://lkml.kernel.org/r/20251105201035.64043-4-david.laight.linux@gmail.com Signed-off-by: David Laight <david.laight.linux@gmail.com> Reviewed-by: Nicolas Pitre <npitre@baylibre.com> Cc: Biju Das <biju.das.jz@bp.renesas.com> Cc: Borislav Betkov <bp@alien8.de> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Li RongQing <lirongqing@baidu.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleinxer <tglx@linutronix.de> Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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| README | ||
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.