lib/math/div64.c | 123 ++++++++++++++++++++++++++++++----------------- 1 file changed, 80 insertions(+), 43 deletions(-)
Library facilities must always return exact results. If the caller may
be contented with approximations then it should do the approximation on
its own.
In this particular case the comment in the code says "the algorithm
... might lose some precision". Well, if you try it with e.g.:
a = 18446462598732840960
b = 18446462598732840960
c = 18446462598732840961
then the produced answer is 0 whereas the exact answer should be
18446462598732840959. This is _some_ precision loss indeed!
Let's reimplement this function so it always produces the exact result
regardless of its inputs while preserving existing fast paths
when possible.
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
---
lib/math/div64.c | 123 ++++++++++++++++++++++++++++++-----------------
1 file changed, 80 insertions(+), 43 deletions(-)
diff --git a/lib/math/div64.c b/lib/math/div64.c
index 191761b1b6..03881ab418 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -186,55 +186,92 @@ 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 res = 0, div, rem;
- int shift;
+ if (ilog2(a) + ilog2(b) <= 62)
+ return div64_u64(a * b, c);
- /* can a * b overflow ? */
- if (ilog2(a) + ilog2(b) > 62) {
- /*
- * Note that the algorithm after the if block below might lose
- * some precision and the result is more exact for b > a. So
- * exchange a and b if a is bigger than b.
- *
- * For example with a = 43980465100800, b = 100000000, c = 1000000000
- * the below calculation doesn't modify b at all because div == 0
- * and then shift becomes 45 + 26 - 62 = 9 and so the result
- * becomes 4398035251080. However with a and b swapped the exact
- * result is calculated (i.e. 4398046510080).
- */
- if (a > b)
- swap(a, b);
+#if defined(__SIZEOF_INT128__)
+
+ /* native 64x64=128 bits multiplication */
+ unsigned __int128 prod = (unsigned __int128)a * b;
+ u64 n_lo = prod, n_hi = prod >> 64;
+
+#else
+
+ /* perform a 64x64=128 bits multiplication manually */
+ union {
+ u64 v;
+ struct {
+#if defined(CONFIG_CPU_LITTLE_ENDIAN)
+ u32 l;
+ u32 h;
+#elif defined(CONFIG_CPU_BIG_ENDIAN)
+ u32 h;
+ u32 l;
+#else
+#error "unknown endianness"
+#endif
+ };
+ } A, B, X, Y, Z;
+
+ A.v = a;
+ B.v = b;
+
+ X.v = (u64)A.l * B.l;
+ Y.v = (u64)A.l * B.h + X.h;
+ Z.v = (u64)A.h * B.h + Y.h;
+ Y.v = (u64)A.h * B.l + Y.l;
+ X.h = Y.l;
+ Z.v += Y.h;
+
+ u64 n_lo = X.v, n_hi = Z.v;
+
+#endif
+ int shift = __builtin_ctzll(c);
+
+ /* try reducing the fraction in case the dividend becomes <= 64 bits */
+ if ((n_hi >> shift) == 0) {
+ u64 n = (n_lo >> shift) | (n_hi << (64 - shift));
+
+ return div64_u64(n, c >> shift);
/*
- * (b * a) / c is equal to
- *
- * (b / c) * a +
- * (b % c) * a / c
- *
- * if nothing overflows. Can the 1st multiplication
- * overflow? Yes, but we do not care: this can only
- * happen if the end result can't fit in u64 anyway.
- *
- * So the code below does
- *
- * res = (b / c) * a;
- * b = b % c;
+ * The remainder value if needed would be:
+ * res = div64_u64_rem(n, c >> shift, &rem);
+ * rem = (rem << shift) + (n_lo - (n << shift));
*/
- div = div64_u64_rem(b, c, &rem);
- res = div * a;
- b = rem;
-
- shift = ilog2(a) + ilog2(b) - 62;
- if (shift > 0) {
- /* drop precision */
- b >>= shift;
- c >>= shift;
- if (!c)
- return res;
- }
}
- return res + div64_u64(a * b, c);
+ if (n_hi >= c) {
+ /* overflow: result is unrepresentable in a u64 */
+ return -1;
+ }
+
+ /* Do the full 128 by 64 bits division */
+
+ shift = __builtin_clzll(c);
+ c <<= shift;
+
+ int p = 64 + shift;
+ u64 res = 0;
+ bool carry;
+
+ do {
+ carry = n_hi >> 63;
+ shift = carry ? 1 : __builtin_clzll(n_hi);
+ if (p < shift)
+ break;
+ p -= shift;
+ n_hi <<= shift;
+ n_hi |= n_lo >> (64 - shift);
+ n_lo <<= shift;
+ if (carry || (n_hi >= c)) {
+ n_hi -= c;
+ res |= 1ULL << p;
+ }
+ } while (n_hi);
+ /* The remainder value if needed would be n_hi << p */
+
+ return res;
}
EXPORT_SYMBOL(mul_u64_u64_div_u64);
#endif
--
2.45.2On Fri, 28 Jun 2024 15:06:20 -0400 (EDT) Nicolas Pitre <npitre@baylibre.com> wrote: > Library facilities must always return exact results. If the caller may > be contented with approximations then it should do the approximation on > its own. > > In this particular case the comment in the code says "the algorithm > ... might lose some precision". Well, if you try it with e.g.: > > a = 18446462598732840960 > b = 18446462598732840960 > c = 18446462598732840961 > > then the produced answer is 0 whereas the exact answer should be > 18446462598732840959. This is _some_ precision loss indeed! > > Let's reimplement this function so it always produces the exact result > regardless of its inputs while preserving existing fast paths > when possible. I assume this was tested with some userspace harness? It would be interesting to see that so that reviewers can see that suitable cases have been covered.
On Fri, 28 Jun 2024, Andrew Morton wrote: > On Fri, 28 Jun 2024 15:06:20 -0400 (EDT) Nicolas Pitre <npitre@baylibre.com> wrote: > > > Library facilities must always return exact results. If the caller may > > be contented with approximations then it should do the approximation on > > its own. > > > > In this particular case the comment in the code says "the algorithm > > ... might lose some precision". Well, if you try it with e.g.: > > > > a = 18446462598732840960 > > b = 18446462598732840960 > > c = 18446462598732840961 > > > > then the produced answer is 0 whereas the exact answer should be > > 18446462598732840959. This is _some_ precision loss indeed! > > > > Let's reimplement this function so it always produces the exact result > > regardless of its inputs while preserving existing fast paths > > when possible. > > I assume this was tested with some userspace harness? It would be > interesting to see that so that reviewers can see that suitable cases > have been covered. I do have a user space test tool but it isn't pretty looking. How should this be acceptably presented for public consumption? Nicolas
On Fri, 28 Jun 2024 17:46:33 -0400 (EDT) Nicolas Pitre <npitre@baylibre.com> wrote: > On Fri, 28 Jun 2024, Andrew Morton wrote: > > > On Fri, 28 Jun 2024 15:06:20 -0400 (EDT) Nicolas Pitre <npitre@baylibre.com> wrote: > > > > > Library facilities must always return exact results. If the caller may > > > be contented with approximations then it should do the approximation on > > > its own. > > > > > > In this particular case the comment in the code says "the algorithm > > > ... might lose some precision". Well, if you try it with e.g.: > > > > > > a = 18446462598732840960 > > > b = 18446462598732840960 > > > c = 18446462598732840961 > > > > > > then the produced answer is 0 whereas the exact answer should be > > > 18446462598732840959. This is _some_ precision loss indeed! > > > > > > Let's reimplement this function so it always produces the exact result > > > regardless of its inputs while preserving existing fast paths > > > when possible. > > > > I assume this was tested with some userspace harness? It would be > > interesting to see that so that reviewers can see that suitable cases > > have been covered. > > I do have a user space test tool but it isn't pretty looking. > How should this be acceptably presented for public consumption? Well, the fancy way would be as a new lib/test_XXX.c. There's a ton of precedent for that. A less fancy way would be to paste the code into this email thread ;)
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