math/big: streamline divLarge initialization

The divLarge code contained "todo"s about avoiding alias
and clear calls in the initialization of variables.  By
rearranging the order of initialization and always using
an auxiliary variable for the shifted divisor, all of these
calls can be safely avoided.  On average, normalizing
the divisor (shift>0) is required 31/32 or 63/64 of the
time.  If one always performs the shift into an auxiliary
variable first, this avoids the need to check for aliasing of
vIn in the output variables u and z.  The remainder u is
initialized via a left shift of uIn and thus needs no
alias check against uIn.  Since uIn and vIn were both used,
z needs no alias checks except against u which is used for
storage of the remainder. This change has a minimal impact
on performance (see below), but cleans up the initialization
code and eliminates the "todo"s.

name                 old time/op  new time/op  delta
Div/20/10-4          86.7ns ± 6%  85.7ns ± 5%    ~     (p=0.841 n=5+5)
Div/200/100-4         523ns ± 5%   502ns ± 3%  -4.13%  (p=0.024 n=5+5)
Div/2000/1000-4      2.55µs ± 3%  2.59µs ± 5%    ~     (p=0.548 n=5+5)
Div/20000/10000-4    80.4µs ± 4%  80.0µs ± 2%    ~     (p=1.000 n=5+5)
Div/200000/100000-4  6.43ms ± 6%  6.35ms ± 4%    ~     (p=0.548 n=5+5)

Fixes #22928

Change-Id: I30d8498ef1cf8b69b0f827165c517bc25a5c32d7
Reviewed-on: https://go-review.googlesource.com/130775
Reviewed-by: Robert Griesemer <gri@golang.org>

2 files changed, 48 insertions, 30 deletions

diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go
index 9930ed016a..7ef2b3907f 100644
--- a/src/math/big/int_test.go
+++ b/src/math/big/int_test.go
@@ -1727,3 +1727,29 @@ func BenchmarkIntSqr(b *testing.B) {
 		})
 	}
 }
+
+func benchmarkDiv(b *testing.B, aSize, bSize int) {
+	var r = rand.New(rand.NewSource(1234))
+	aa := randInt(r, uint(aSize))
+	bb := randInt(r, uint(bSize))
+	if aa.Cmp(bb) < 0 {
+		aa, bb = bb, aa
+	}
+	x := new(Int)
+	y := new(Int)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		x.DivMod(aa, bb, y)
+	}
+}
+
+func BenchmarkDiv(b *testing.B) {
+	min, max, step := 10, 100000, 10
+	for i := min; i <= max; i *= step {
+		j := 2 * i
+		b.Run(fmt.Sprintf("%d/%d", j, i), func(b *testing.B) {
+			benchmarkDiv(b, j, i)
+		})
+	}
+}
diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index a6f79edccc..5f5cf5c3e4 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -680,43 +680,36 @@ func putNat(x *nat) {
 
 var natPool sync.Pool
 
-// q = (uIn-r)/v, with 0 <= r < y
+// q = (uIn-r)/vIn, with 0 <= r < y
 // Uses z as storage for q, and u as storage for r if possible.
 // See Knuth, Volume 2, section 4.3.1, Algorithm D.
 // Preconditions:
-//    len(v) >= 2
-//    len(uIn) >= len(v)
-func (z nat) divLarge(u, uIn, v nat) (q, r nat) {
-	n := len(v)
+//    len(vIn) >= 2
+//    len(uIn) >= len(vIn)
+//    u must not alias z
+func (z nat) divLarge(u, uIn, vIn nat) (q, r nat) {
+	n := len(vIn)
 	m := len(uIn) - n
 
-	// determine if z can be reused
-	// TODO(gri) should find a better solution - this if statement
-	//           is very costly (see e.g. time pidigits -s -n 10000)
-	if alias(z, u) || alias(z, uIn) || alias(z, v) {
-		z = nil // z is an alias for u or uIn or v - cannot reuse
+	// D1.
+	shift := nlz(vIn[n-1])
+	// do not modify vIn, it may be used by another goroutine simultaneously
+	vp := getNat(n)
+	v := *vp
+	shlVU(v, vIn, shift)
+
+	// u may safely alias uIn or vIn, the value of uIn is used to set u and vIn was already used
+	u = u.make(len(uIn) + 1)
+	u[len(uIn)] = shlVU(u[0:len(uIn)], uIn, shift)
+
+	// z may safely alias uIn or vIn, both values were used already
+	if alias(z, u) {
+		z = nil // z is an alias for u - cannot reuse
 	}
 	q = z.make(m + 1)
 
 	qhatvp := getNat(n + 1)
 	qhatv := *qhatvp
-	if alias(u, uIn) || alias(u, v) {
-		u = nil // u is an alias for uIn or v - cannot reuse
-	}
-	u = u.make(len(uIn) + 1)
-	u.clear() // TODO(gri) no need to clear if we allocated a new u
-
-	// D1.
-	var v1p *nat
-	shift := nlz(v[n-1])
-	if shift > 0 {
-		// do not modify v, it may be used by another goroutine simultaneously
-		v1p = getNat(n)
-		v1 := *v1p
-		shlVU(v1, v, shift)
-		v = v1
-	}
-	u[len(uIn)] = shlVU(u[0:len(uIn)], uIn, shift)
 
 	// D2.
 	vn1 := v[n-1]
@@ -756,9 +749,8 @@ func (z nat) divLarge(u, uIn, v nat) (q, r nat) {
 
 		q[j] = qhat
 	}
-	if v1p != nil {
-		putNat(v1p)
-	}
+
+	putNat(vp)
 	putNat(qhatvp)
 
 	q = q.norm()