[PATCH v4 next 8/9] lib: mul_u64_u64_div_u64() Optimise the divide code

[David Laight <david.laight.linux@gmail.com>]

Replace the bit by bit algorithm with one that generates 16 bits
per iteration on 32bit architectures and 32 bits on 64bit ones.

On my zen 5 this reduces the time for the tests (using the generic
code) from ~3350ns to ~1000ns.

Running the 32bit algorithm on 64bit x86 takes ~1500ns.
It'll be slightly slower on a real 32bit system, mostly due
to register pressure.

The savings for 32bit x86 are much higher (tested in userspace).
The worst case (lots of bits in the quotient) drops from ~900 clocks
to ~130 (pretty much independant of the arguments).
Other 32bit architectures may see better savings.

It is possibly to optimise for divisors that span less than
__LONG_WIDTH__/2 bits. However I suspect they don't happen that often
and it doesn't remove any slow cpu divide instructions which dominate
the result.

Typical improvements for 64bit random divides:
               old     new
sandy bridge:  470     150
haswell:       400     144
piledriver:    960     467   I think rdpmc is very slow.
zen5:          244      80
(Timing is 'rdpmc; mul_div(); rdpmc' with the multiply depending on the
first rdpmc and the second rdpmc depending on the quotient.)

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

Algorithm unchanged from v3.

 lib/math/div64.c | 124 ++++++++++++++++++++++++++++++++---------------
 1 file changed, 85 insertions(+), 39 deletions(-)

diff --git a/lib/math/div64.c b/lib/math/div64.c
index f6da7b5fb69e..4e4e962261c3 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -190,7 +190,6 @@ EXPORT_SYMBOL(iter_div_u64_rem);
 #define mul_add(a, b, c) add_u64_u32(mul_u32_u32(a, b), c)
 
 #if defined(__SIZEOF_INT128__) && !defined(test_mul_u64_add_u64_div_u64)
-
 static inline u64 mul_u64_u64_add_u64(u64 *p_lo, u64 a, u64 b, u64 c)
 {
 	/* native 64x64=128 bits multiplication */
@@ -199,9 +198,7 @@ static inline u64 mul_u64_u64_add_u64(u64 *p_lo, u64 a, u64 b, u64 c)
 	*p_lo = prod;
 	return prod >> 64;
 }
-
 #else
-
 static inline u64 mul_u64_u64_add_u64(u64 *p_lo, u64 a, u64 b, u64 c)
 {
 	/* perform a 64x64=128 bits multiplication in 32bit chunks */
@@ -216,12 +213,37 @@ static inline u64 mul_u64_u64_add_u64(u64 *p_lo, u64 a, u64 b, u64 c)
 	*p_lo = (y << 32) + (u32)x;
 	return add_u64_u32(z, y >> 32);
 }
+#endif
+
+#ifndef BITS_PER_ITER
+#define BITS_PER_ITER (__LONG_WIDTH__ >= 64 ? 32 : 16)
+#endif
+
+#if BITS_PER_ITER == 32
+#define mul_u64_long_add_u64(p_lo, a, b, c) mul_u64_u64_add_u64(p_lo, a, b, c)
+#define add_u64_long(a, b) ((a) + (b))
+#else
+#undef BITS_PER_ITER
+#define BITS_PER_ITER 16
+static inline u32 mul_u64_long_add_u64(u64 *p_lo, u64 a, u32 b, u64 c)
+{
+	u64 n_lo = mul_add(a, b, c);
+	u64 n_med = mul_add(a >> 32, b, c >> 32);
+
+	n_med = add_u64_u32(n_med, n_lo >> 32);
+	*p_lo = n_med << 32 | (u32)n_lo;
+	return n_med >> 32;
+}
 
+#define add_u64_long(a, b) add_u64_u32(a, b)
 #endif
 
 u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
 {
-	u64 n_lo, n_hi;
+	unsigned long d_msig, q_digit;
+	unsigned int reps, d_z_hi;
+	u64 quotient, n_lo, n_hi;
+	u32 overflow;
 
 	n_hi = mul_u64_u64_add_u64(&n_lo, a, b, c);
 
@@ -240,46 +262,70 @@ u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
 	if (!n_hi)
 		return div64_u64(n_lo, d);
 
-	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, d >> shift);
-		/*
-		 * The remainder value if needed would be:
-		 *   res = div64_u64_rem(n, d >> shift, &rem);
-		 *   rem = (rem << shift) + (n_lo - (n << shift));
-		 */
+	/* Left align the divisor, shifting the dividend to match */
+	d_z_hi = __builtin_clzll(d);
+	if (d_z_hi) {
+		d <<= d_z_hi;
+		n_hi = n_hi << d_z_hi | n_lo >> (64 - d_z_hi);
+		n_lo <<= d_z_hi;
 	}
 
-	/* Do the full 128 by 64 bits division */
-
-	shift = __builtin_clzll(d);
-	d <<= shift;
-
-	int p = 64 + shift;
-	u64 res = 0;
-	bool carry;
+	reps = 64 / BITS_PER_ITER;
+	/* Optimise loop count for small dividends */
+	if (!(u32)(n_hi >> 32)) {
+		reps -= 32 / BITS_PER_ITER;
+		n_hi = n_hi << 32 | n_lo >> 32;
+		n_lo <<= 32;
+	}
+#if BITS_PER_ITER == 16
+	if (!(u32)(n_hi >> 48)) {
+		reps--;
+		n_hi = add_u64_u32(n_hi << 16, n_lo >> 48);
+		n_lo <<= 16;
+	}
+#endif
 
-	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 >= d)) {
-			n_hi -= d;
-			res |= 1ULL << p;
+	/* Invert the dividend so we can use add instead of subtract. */
+	n_lo = ~n_lo;
+	n_hi = ~n_hi;
+
+	/*
+	 * Get the most significant BITS_PER_ITER bits of the divisor.
+	 * This is used to get a low 'guestimate' of the quotient digit.
+	 */
+	d_msig = (d >> (64 - BITS_PER_ITER)) + 1;
+
+	/*
+	 * Now do a 'long division' with BITS_PER_ITER bit 'digits'.
+	 * The 'guess' quotient digit can be low and BITS_PER_ITER+1 bits.
+	 * The worst case is dividing ~0 by 0x8000 which requires two subtracts.
+	 */
+	quotient = 0;
+	while (reps--) {
+		q_digit = (unsigned long)(~n_hi >> (64 - 2 * BITS_PER_ITER)) / d_msig;
+		/* Shift 'n' left to align with the product q_digit * d */
+		overflow = n_hi >> (64 - BITS_PER_ITER);
+		n_hi = add_u64_u32(n_hi << BITS_PER_ITER, n_lo >> (64 - BITS_PER_ITER));
+		n_lo <<= BITS_PER_ITER;
+		/* Add product to negated divisor */
+		overflow += mul_u64_long_add_u64(&n_hi, d, q_digit, n_hi);
+		/* Adjust for the q_digit 'guestimate' being low */
+		while (overflow < 0xffffffff >> (32 - BITS_PER_ITER)) {
+			q_digit++;
+			n_hi += d;
+			overflow += n_hi < d;
 		}
-	} while (n_hi);
-	/* The remainder value if needed would be n_hi << p */
+		quotient = add_u64_long(quotient << BITS_PER_ITER, q_digit);
+	}
 
-	return res;
+	/*
+	 * The above only ensures the remainder doesn't overflow,
+	 * it can still be possible to add (aka subtract) another copy
+	 * of the divisor.
+	 */
+	if ((n_hi + d) > n_hi)
+		quotient++;
+	return quotient;
 }
 #if !defined(test_mul_u64_add_u64_div_u64)
 EXPORT_SYMBOL(mul_u64_add_u64_div_u64);
-- 
2.39.5