cmd/compile: use magic multiply for unsigned values less than 1<<16 on 32-bit architectures

This is done by decomposing the number to be divided in 32-bit
components and using the 32-bit magic multiply. For the lowering to be
effective the constant must fit in 16 bits.

On ARM the expression n / 5 compiles to 25 instructions.

Benchmark for GOARCH=arm (Cortex-A53)

name                     old time/op  new time/op  delta
DivconstU64/3-6          1.19µs ± 0%  0.03µs ± 1%  -97.40%  (p=0.000 n=9+9)
DivconstU64/5-6          1.18µs ± 1%  0.03µs ± 1%  -97.38%  (p=0.000 n=10+8)
DivconstU64/37-6         1.13µs ± 1%  0.04µs ± 1%  -96.51%  (p=0.000 n=10+8)
DivconstU64/1234567-6     852ns ± 0%   901ns ± 1%   +5.73%  (p=0.000 n=8+9)

Benchmark for GOARCH=386 (Haswell)

name                     old time/op  new time/op  delta
DivconstU64/3-4          18.0ns ± 2%   5.6ns ± 1%  -69.06%  (p=0.000 n=10+10)
DivconstU64/5-4          17.8ns ± 1%   5.5ns ± 1%  -68.87%  (p=0.000 n=9+10)
DivconstU64/37-4         17.8ns ± 1%   7.3ns ± 0%  -58.90%  (p=0.000 n=10+10)
DivconstU64/1234567-4    17.5ns ± 1%  16.0ns ± 0%   -8.55%  (p=0.000 n=10+9)

Change-Id: I38a19b4d59093ec021ef2e5241364a3dad4eae73
Reviewed-on: https://go-review.googlesource.com/c/go/+/264683
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

1 files changed, 78 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/test/divconst_test.go b/src/cmd/compile/internal/test/divconst_test.go
index b03550058f..9358a60374 100644
--- a/src/cmd/compile/internal/test/divconst_test.go
+++ b/src/cmd/compile/internal/test/divconst_test.go
@@ -44,10 +44,85 @@ func BenchmarkDivisibleWDivconstI64(b *testing.B) {
 
 var u64res uint64
 
-func BenchmarkDivconstU64(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		u64res = uint64(i) / 7
+func TestDivmodConstU64(t *testing.T) {
+	// Test division by c. Function f must be func(n) { return n/c, n%c }
+	testdiv := func(c uint64, f func(uint64) (uint64, uint64)) func(*testing.T) {
+		return func(t *testing.T) {
+			x := uint64(12345)
+			for i := 0; i < 10000; i++ {
+				x += x << 2
+				q, r := f(x)
+				if r < 0 || r >= c || q*c+r != x {
+					t.Errorf("divmod(%d, %d) returned incorrect (%d, %d)", x, c, q, r)
+				}
+			}
+			max := uint64(1<<64-1) / c * c
+			xs := []uint64{0, 1, c - 1, c, c + 1, 2*c - 1, 2 * c, 2*c + 1,
+				c*c - 1, c * c, c*c + 1, max - 1, max, max + 1, 1<<64 - 1}
+			for _, x := range xs {
+				q, r := f(x)
+				if r < 0 || r >= c || q*c+r != x {
+					t.Errorf("divmod(%d, %d) returned incorrect (%d, %d)", x, c, q, r)
+				}
+			}
+		}
 	}
+	t.Run("2", testdiv(2, func(n uint64) (uint64, uint64) { return n / 2, n % 2 }))
+	t.Run("3", testdiv(3, func(n uint64) (uint64, uint64) { return n / 3, n % 3 }))
+	t.Run("4", testdiv(4, func(n uint64) (uint64, uint64) { return n / 4, n % 4 }))
+	t.Run("5", testdiv(5, func(n uint64) (uint64, uint64) { return n / 5, n % 5 }))
+	t.Run("6", testdiv(6, func(n uint64) (uint64, uint64) { return n / 6, n % 6 }))
+	t.Run("7", testdiv(7, func(n uint64) (uint64, uint64) { return n / 7, n % 7 }))
+	t.Run("8", testdiv(8, func(n uint64) (uint64, uint64) { return n / 8, n % 8 }))
+	t.Run("9", testdiv(9, func(n uint64) (uint64, uint64) { return n / 9, n % 9 }))
+	t.Run("10", testdiv(10, func(n uint64) (uint64, uint64) { return n / 10, n % 10 }))
+	t.Run("11", testdiv(11, func(n uint64) (uint64, uint64) { return n / 11, n % 11 }))
+	t.Run("12", testdiv(12, func(n uint64) (uint64, uint64) { return n / 12, n % 12 }))
+	t.Run("13", testdiv(13, func(n uint64) (uint64, uint64) { return n / 13, n % 13 }))
+	t.Run("14", testdiv(14, func(n uint64) (uint64, uint64) { return n / 14, n % 14 }))
+	t.Run("15", testdiv(15, func(n uint64) (uint64, uint64) { return n / 15, n % 15 }))
+	t.Run("16", testdiv(16, func(n uint64) (uint64, uint64) { return n / 16, n % 16 }))
+	t.Run("17", testdiv(17, func(n uint64) (uint64, uint64) { return n / 17, n % 17 }))
+	t.Run("255", testdiv(255, func(n uint64) (uint64, uint64) { return n / 255, n % 255 }))
+	t.Run("256", testdiv(256, func(n uint64) (uint64, uint64) { return n / 256, n % 256 }))
+	t.Run("257", testdiv(257, func(n uint64) (uint64, uint64) { return n / 257, n % 257 }))
+	t.Run("65535", testdiv(65535, func(n uint64) (uint64, uint64) { return n / 65535, n % 65535 }))
+	t.Run("65536", testdiv(65536, func(n uint64) (uint64, uint64) { return n / 65536, n % 65536 }))
+	t.Run("65537", testdiv(65537, func(n uint64) (uint64, uint64) { return n / 65537, n % 65537 }))
+	t.Run("1<<32-1", testdiv(1<<32-1, func(n uint64) (uint64, uint64) { return n / (1<<32 - 1), n % (1<<32 - 1) }))
+	t.Run("1<<32+1", testdiv(1<<32+1, func(n uint64) (uint64, uint64) { return n / (1<<32 + 1), n % (1<<32 + 1) }))
+	t.Run("1<<64-1", testdiv(1<<64-1, func(n uint64) (uint64, uint64) { return n / (1<<64 - 1), n % (1<<64 - 1) }))
+}
+
+func BenchmarkDivconstU64(b *testing.B) {
+	b.Run("3", func(b *testing.B) {
+		x := uint64(123456789123456789)
+		for i := 0; i < b.N; i++ {
+			x += x << 4
+			u64res = uint64(x) / 3
+		}
+	})
+	b.Run("5", func(b *testing.B) {
+		x := uint64(123456789123456789)
+		for i := 0; i < b.N; i++ {
+			x += x << 4
+			u64res = uint64(x) / 5
+		}
+	})
+	b.Run("37", func(b *testing.B) {
+		x := uint64(123456789123456789)
+		for i := 0; i < b.N; i++ {
+			x += x << 4
+			u64res = uint64(x) / 37
+		}
+	})
+	b.Run("1234567", func(b *testing.B) {
+		x := uint64(123456789123456789)
+		for i := 0; i < b.N; i++ {
+			x += x << 4
+			u64res = uint64(x) / 1234567
+		}
+	})
 }
 
 func BenchmarkModconstU64(b *testing.B) {