strconv: optimize Atoi for common case

Benchmark results on GOOS=linux:

GOARCH=amd64

name              old time/op  new time/op  delta
Atoi/Pos/7bit-4   20.1ns ± 2%   8.6ns ± 1%  -57.34%  (p=0.000 n=10+10)
Atoi/Pos/26bit-4  25.8ns ± 7%  11.9ns ± 0%  -53.91%  (p=0.000 n=10+8)
Atoi/Pos/31bit-4  27.3ns ± 2%  13.2ns ± 1%  -51.56%  (p=0.000 n=10+10)
Atoi/Pos/56bit-4  37.2ns ± 5%  18.2ns ± 1%  -51.26%  (p=0.000 n=10+10)
Atoi/Pos/63bit-4  38.7ns ± 1%  38.6ns ± 1%     ~     (p=0.297 n=9+10)
Atoi/Neg/7bit-4   17.6ns ± 1%   7.2ns ± 0%  -59.22%  (p=0.000 n=10+10)
Atoi/Neg/26bit-4  24.4ns ± 1%  12.4ns ± 1%  -49.28%  (p=0.000 n=10+10)
Atoi/Neg/31bit-4  26.9ns ± 0%  14.0ns ± 1%  -47.88%  (p=0.000 n=7+10)
Atoi/Neg/56bit-4  36.2ns ± 1%  19.5ns ± 0%  -46.24%  (p=0.000 n=10+9)
Atoi/Neg/63bit-4  38.9ns ± 1%  38.8ns ± 1%     ~     (p=0.385 n=9+10)

GOARCH=386

name              old time/op  new time/op  delta
Atoi/Pos/7bit-4   89.6ns ± 1%   8.2ns ± 1%  -90.84%  (p=0.000 n=9+10)
Atoi/Pos/26bit-4   187ns ± 2%    12ns ± 1%  -93.71%  (p=0.000 n=10+9)
Atoi/Pos/31bit-4   225ns ± 1%   225ns ± 1%     ~     (p=0.995 n=10+10)
Atoi/Neg/7bit-4   86.2ns ± 1%   8.5ns ± 1%  -90.14%  (p=0.000 n=10+10)
Atoi/Neg/26bit-4   183ns ± 1%    13ns ± 1%  -92.77%  (p=0.000 n=9+10)
Atoi/Neg/31bit-4   223ns ± 0%   223ns ± 0%     ~     (p=0.247 n=8+9)

Fixes #20557

Change-Id: Ib6245d88cffd4b037419e2bf8e4a71b86c6d773f
Reviewed-on: https://go-review.googlesource.com/44692
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

2 files changed, 115 insertions, 14 deletions

diff --git a/src/strconv/atoi.go b/src/strconv/atoi.go
index e1ac42716c..bebed04820 100644
--- a/src/strconv/atoi.go
+++ b/src/strconv/atoi.go
@@ -201,6 +201,34 @@ func ParseInt(s string, base int, bitSize int) (i int64, err error) {
 // Atoi returns the result of ParseInt(s, 10, 0) converted to type int.
 func Atoi(s string) (int, error) {
 	const fnAtoi = "Atoi"
+
+	sLen := len(s)
+	if intSize == 32 && (0 < sLen && sLen < 10) ||
+		intSize == 64 && (0 < sLen && sLen < 19) {
+		// Fast path for small integers that fit int type.
+		s0 := s
+		if s[0] == '-' || s[0] == '+' {
+			s = s[1:]
+			if len(s) < 1 {
+				return 0, &NumError{fnAtoi, s0, ErrSyntax}
+			}
+		}
+
+		n := 0
+		for _, ch := range []byte(s) {
+			ch -= '0'
+			if ch > 9 {
+				return 0, &NumError{fnAtoi, s0, ErrSyntax}
+			}
+			n = n*10 + int(ch)
+		}
+		if s0[0] == '-' {
+			n = -n
+		}
+		return n, nil
+	}
+
+	// Slow path for invalid or big integers.
 	i64, err := ParseInt(s, 10, 0)
 	if nerr, ok := err.(*NumError); ok {
 		nerr.Func = fnAtoi
diff --git a/src/strconv/atoi_test.go b/src/strconv/atoi_test.go
index 94844c7e10..e2f505a665 100644
--- a/src/strconv/atoi_test.go
+++ b/src/strconv/atoi_test.go
@@ -6,6 +6,7 @@ package strconv_test
 
 import (
 	"errors"
+	"fmt"
 	"reflect"
 	. "strconv"
 	"testing"
@@ -354,6 +355,37 @@ func TestParseInt(t *testing.T) {
 	}
 }
 
+func TestAtoi(t *testing.T) {
+	switch IntSize {
+	case 32:
+		for i := range parseInt32Tests {
+			test := &parseInt32Tests[i]
+			out, err := Atoi(test.in)
+			var testErr error
+			if test.err != nil {
+				testErr = &NumError{"Atoi", test.in, test.err.(*NumError).Err}
+			}
+			if int(test.out) != out || !reflect.DeepEqual(testErr, err) {
+				t.Errorf("Atoi(%q) = %v, %v want %v, %v",
+					test.in, out, err, test.out, testErr)
+			}
+		}
+	case 64:
+		for i := range parseInt64Tests {
+			test := &parseInt64Tests[i]
+			out, err := Atoi(test.in)
+			var testErr error
+			if test.err != nil {
+				testErr = &NumError{"Atoi", test.in, test.err.(*NumError).Err}
+			}
+			if test.out != int64(out) || !reflect.DeepEqual(testErr, err) {
+				t.Errorf("Atoi(%q) = %v, %v want %v, %v",
+					test.in, out, err, test.out, testErr)
+			}
+		}
+	}
+}
+
 func bitSizeErrStub(name string, bitSize int) error {
 	return BitSizeError(name, "0", bitSize)
 }
@@ -448,26 +480,67 @@ func TestNumError(t *testing.T) {
 	}
 }
 
-func BenchmarkAtoi(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		ParseInt("12345678", 10, 0)
-	}
+func BenchmarkParseInt(b *testing.B) {
+	b.Run("Pos", func(b *testing.B) {
+		benchmarkParseInt(b, 1)
+	})
+	b.Run("Neg", func(b *testing.B) {
+		benchmarkParseInt(b, -1)
+	})
 }
 
-func BenchmarkAtoiNeg(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		ParseInt("-12345678", 10, 0)
-	}
+type benchCase struct {
+	name string
+	num  int64
 }
 
-func BenchmarkAtoi64(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		ParseInt("12345678901234", 10, 64)
+func benchmarkParseInt(b *testing.B, neg int) {
+	cases := []benchCase{
+		{"7bit", 1<<7 - 1},
+		{"26bit", 1<<26 - 1},
+		{"31bit", 1<<31 - 1},
+		{"56bit", 1<<56 - 1},
+		{"63bit", 1<<63 - 1},
+	}
+	for _, cs := range cases {
+		b.Run(cs.name, func(b *testing.B) {
+			s := fmt.Sprintf("%d", cs.num*int64(neg))
+			for i := 0; i < b.N; i++ {
+				out, _ := ParseInt(s, 10, 64)
+				BenchSink += int(out)
+			}
+		})
 	}
 }
 
-func BenchmarkAtoi64Neg(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		ParseInt("-12345678901234", 10, 64)
+func BenchmarkAtoi(b *testing.B) {
+	b.Run("Pos", func(b *testing.B) {
+		benchmarkAtoi(b, 1)
+	})
+	b.Run("Neg", func(b *testing.B) {
+		benchmarkAtoi(b, -1)
+	})
+}
+
+func benchmarkAtoi(b *testing.B, neg int) {
+	cases := []benchCase{
+		{"7bit", 1<<7 - 1},
+		{"26bit", 1<<26 - 1},
+		{"31bit", 1<<31 - 1},
+	}
+	if IntSize == 64 {
+		cases = append(cases, []benchCase{
+			{"56bit", 1<<56 - 1},
+			{"63bit", 1<<63 - 1},
+		}...)
+	}
+	for _, cs := range cases {
+		b.Run(cs.name, func(b *testing.B) {
+			s := fmt.Sprintf("%d", cs.num*int64(neg))
+			for i := 0; i < b.N; i++ {
+				out, _ := Atoi(s)
+				BenchSink += out
+			}
+		})
 	}
 }