admin/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2025-12-07 20:06:24 +00:00
Code Issues Packages Projects Releases Wiki Activity

lib: mul_u64_u64_div_u64(): rename parameter 'c' to 'd'

Browse Source 
Patch series "Implement mul_u64_u64_div_u64_roundup()", v5.

The pwm-stm32.c code wants a 'rounding up' version of
mul_u64_u64_div_u64().  This can be done simply by adding 'divisor - 1' to
the 128bit product.  Implement mul_u64_add_u64_div_u64(a, b, c, d) = (a *
b + c)/d based on the existing code.  Define mul_u64_u64_div_u64(a, b, d)
as mul_u64_add_u64_div_u64(a, b, 0, d) and mul_u64_u64_div_u64_roundup(a,
b, d) as mul_u64_add_u64_div_u64(a, b, d-1, d).

Only x86-64 has an optimsed (asm) version of the function.  That is
optimised to avoid the 'add c' when c is known to be zero.  In all other
cases the extra code will be noise compared to the software divide code.

The test module has been updated to test mul_u64_u64_div_u64_roundup() and
also enhanced it to verify the C division code on x86-64 and the 32bit
division code on 64bit.


This patch (of 9):

Change to prototype from mul_u64_u64_div_u64(u64 a, u64 b, u64 c) to
mul_u64_u64_div_u64(u64 a, u64 b, u64 d).  Using 'd' for 'divisor' makes
more sense.

An upcoming change adds a 'c' parameter to calculate (a * b + c)/d.

Link: https://lkml.kernel.org/r/20251105201035.64043-1-david.laight.linux@gmail.com
Link: https://lkml.kernel.org/r/20251105201035.64043-2-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>
This commit is contained in:
David Laight
2025-11-05 20:10:27 +00:00
committed by Andrew Morton
parent af9b65d686
commit 5944f875ac
1 changed files with 12 additions and 12 deletions
Download Patch File Download Diff File Expand all files Collapse all files

24
lib/math/div64.c
View File

@@ -184,10 +184,10 @@ u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
EXPORT_SYMBOL(iter_div_u64_rem);
#ifndef mul_u64_u64_div_u64
u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
{
if (ilog2(a) + ilog2(b) <= 62)
return div64_u64(a * b, c);
return div64_u64(a * b, d);
#if defined(__SIZEOF_INT128__)
@@ -212,37 +212,37 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
#endif
/* make sure c is not zero, trigger runtime exception otherwise */
if (unlikely(c == 0)) {
/* make sure d is not zero, trigger runtime exception otherwise */
if (unlikely(d == 0)) {
unsigned long zero = 0;
OPTIMIZER_HIDE_VAR(zero);
return ~0UL/zero;
}
int shift = __builtin_ctzll(c);
int shift = __builtin_ctzll(d);
/* try reducing the fraction in case the dividend becomes <= 64 bits */
if ((n_hi >> shift) == 0) {
u64 n = shift ? (n_lo >> shift) | (n_hi << (64 - shift)) : n_lo;
return div64_u64(n, c >> shift);
return div64_u64(n, d >> shift);
/*
* The remainder value if needed would be:
* res = div64_u64_rem(n, c >> shift, &rem);
* res = div64_u64_rem(n, d >> shift, &rem);
* rem = (rem << shift) + (n_lo - (n << shift));
*/
}
if (n_hi >= c) {
if (n_hi >= d) {
/* overflow: result is unrepresentable in a u64 */
return -1;
}
/* Do the full 128 by 64 bits division */
shift = __builtin_clzll(c);
c <<= shift;
shift = __builtin_clzll(d);
d <<= shift;
int p = 64 + shift;
u64 res = 0;
@@ -257,8 +257,8 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
n_hi <<= shift;
n_hi |= n_lo >> (64 - shift);
n_lo <<= shift;
if (carry || (n_hi >= c)) {
n_hi -= c;
if (carry || (n_hi >= d)) {
n_hi -= d;
res |= 1ULL << p;
}
} while (n_hi);