Re: [PATCH v5 next 3/9] lib: mul_u64_u64_div_u64() simplify check for a 64bit product

Nicolas Pitre
2025-11-05T20:59:06Z

On Wed, 5 Nov 2025, David Laight wrote:

> 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).
>
> Signed-off-by: David Laight <david.laight.linux@gmail.com>

Reviewed-by: Nicolas Pitre <npitre@baylibre.com>

> ---
>
> Split from patch 3 for v2.
>
> Changes for v5:
> - Test for small dividends before overflow.
>
> lib/math/div64.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 1092f41e878e..4a4b1aa9e6e1 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -186,9 +186,6 @@ EXPORT_SYMBOL(iter_div_u64_rem);
> #ifndef mul_u64_u64_div_u64
> u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> {
> - if (ilog2(a) + ilog2(b) <= 62)
> - return div64_u64(a * b, d);
> -
> #if defined(__SIZEOF_INT128__)
>
> /* native 64x64=128 bits multiplication */
> @@ -212,6 +209,9 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>
> #endif
>
> + if (!n_hi)
> + return div64_u64(n_lo, d);
> +
> if (unlikely(n_hi >= d)) {
> /* trigger runtime exception if divisor is zero */
> if (d == 0) {
> --
> 2.39.5
>
>