lib: mul_u64_u64_div_u64(): simplify check for a 64bit product

lib: mul_u64_u64_div_u64(): simplify check for a 64bit product

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>