strconv: optimize Parse for []byte arguments

When one has a []byte on hand, but desires to call the Parse functions,
the conversion from []byte to string would allocate.

    var b []byte = ...
    v, err := strconv.ParseXXX(string(b), ...)

This changes it such that the input string never escapes from
any of the Parse functions. Together with the compiler optimization
where the compiler stack allocates any string smaller than 32B
this makes most valid inputs for strconv.ParseXXX(string(b), ...)
not require an allocation for the input string.
For example, the longest int64 or uint64 encoded in decimal is 20B.
Also, the longest decimal formatting of a float64 in appendix B
of RFC 8785 is 25B.

Previously, this was not possible since the input leaked to the error,
which causes the prover to give up and instead heap copy the []byte.
We fix this by copying the input string in the error case.
The advantage of this change is that you can now call strconv.ParseXXX
with a []byte without allocations (most times) in the non-error case.
The detriment is that the error-case now has an extra allocation.
We should optimize for the non-error path, rather than the error path.

The effects of this change is transitively seen through packages
that must use strconv.ParseXXX on a []byte such as "encoding/json":

    name              old time/op    new time/op    delta
    UnmarshalFloat64  186ns          157ns          -15.89%  (p=0.000 n=10+10)

    name              old alloc/op   new alloc/op   delta
    UnmarshalFloat64  148B           144B            -2.70%  (p=0.000 n=10+10)

    name              old allocs/op  new allocs/op  delta
    UnmarshalFloat64  2.00           1.00           -50.00%  (p=0.000 n=10+10)

In order for "encoding/json" to benefit, there needs to be a
small change made to how "encoding/json" calls strconv.ParseXXX.
That will be a future change.

Credit goes to Jeff Wendling for a similar patch.

Fixes #42429

Change-Id: I512d6927f965f82e95bd7ec14a28a587f23b7203
Reviewed-on: https://go-review.googlesource.com/c/go/+/345488
Reviewed-by: Martin Möhrmann <martin@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/strconv/atoc.go

@@ -11,7 +11,7 @@ const fnParseComplex = "ParseComplex"
 func convErr(err error, s string) (syntax, range_ error) {
 	if x, ok := err.(*NumError); ok {
 		x.Func = fnParseComplex
-		x.Num = s
+		x.Num = cloneString(s)
 		if x.Err == ErrRange {
 			return nil, x
 		}

src/strconv/atoi.go

@@ -33,20 +33,36 @@ func (e *NumError) Error() string {
 
 func (e *NumError) Unwrap() error { return e.Err }
 
+// cloneString returns a string copy of x.
+//
+// All ParseXXX functions allow the input string to escape to the error value.
+// This hurts strconv.ParseXXX(string(b)) calls where b is []byte since
+// the conversion from []byte must allocate a string on the heap.
+// If we assume errors are infrequent, then we can avoid escaping the input
+// back to the output by copying it first. This allows the compiler to call
+// strconv.ParseXXX without a heap allocation for most []byte to string
+// conversions, since it can now prove that the string cannot escape Parse.
+//
+// TODO: Use strings.Clone instead? However, we cannot depend on "strings"
+// since it incurs a transitive dependency on "unicode".
+// Either move strings.Clone to an internal/bytealg or make the
+// "strings" to "unicode" dependency lighter (see https://go.dev/issue/54098).
+func cloneString(x string) string { return string([]byte(x)) }
+
 func syntaxError(fn, str string) *NumError {
-	return &NumError{fn, str, ErrSyntax}
+	return &NumError{fn, cloneString(str), ErrSyntax}
 }
 
 func rangeError(fn, str string) *NumError {
-	return &NumError{fn, str, ErrRange}
+	return &NumError{fn, cloneString(str), ErrRange}
 }
 
 func baseError(fn, str string, base int) *NumError {
-	return &NumError{fn, str, errors.New("invalid base " + Itoa(base))}
+	return &NumError{fn, cloneString(str), errors.New("invalid base " + Itoa(base))}
 }
 
 func bitSizeError(fn, str string, bitSize int) *NumError {
-	return &NumError{fn, str, errors.New("invalid bit size " + Itoa(bitSize))}
+	return &NumError{fn, cloneString(str), errors.New("invalid bit size " + Itoa(bitSize))}
 }
 
 const intSize = 32 << (^uint(0) >> 63)
@@ -205,7 +221,7 @@ func ParseInt(s string, base int, bitSize int) (i int64, err error) {
 	un, err = ParseUint(s, base, bitSize)
 	if err != nil && err.(*NumError).Err != ErrRange {
 		err.(*NumError).Func = fnParseInt
-		err.(*NumError).Num = s0
+		err.(*NumError).Num = cloneString(s0)
 		return 0, err
 	}
 
@@ -239,7 +255,7 @@ func Atoi(s string) (int, error) {
 		if s[0] == '-' || s[0] == '+' {
 			s = s[1:]
 			if len(s) < 1 {
-				return 0, &NumError{fnAtoi, s0, ErrSyntax}
+				return 0, syntaxError(fnAtoi, s0)
 			}
 		}
 
@@ -247,7 +263,7 @@ func Atoi(s string) (int, error) {
 		for _, ch := range []byte(s) {
 			ch -= '0'
 			if ch > 9 {
-				return 0, &NumError{fnAtoi, s0, ErrSyntax}
+				return 0, syntaxError(fnAtoi, s0)
 			}
 			n = n*10 + int(ch)
 		}

src/strconv/strconv_test.go

@@ -66,6 +66,68 @@ func TestCountMallocs(t *testing.T) {
 	}
 }
 
+// Sink makes sure the compiler cannot optimize away the benchmarks.
+var Sink struct {
+	Bool       bool
+	Int        int
+	Int64      int64
+	Uint64     uint64
+	Float64    float64
+	Complex128 complex128
+	Error      error
+	Bytes      []byte
+}
+
+func TestAllocationsFromBytes(t *testing.T) {
+	const runsPerTest = 100
+	bytes := struct{ Bool, Number, String, Buffer []byte }{
+		Bool:   []byte("false"),
+		Number: []byte("123456789"),
+		String: []byte("hello, world!"),
+		Buffer: make([]byte, 1024),
+	}
+
+	checkNoAllocs := func(f func()) func(t *testing.T) {
+		return func(t *testing.T) {
+			t.Helper()
+			if allocs := testing.AllocsPerRun(runsPerTest, f); allocs != 0 {
+				t.Errorf("got %v allocs, want 0 allocs", allocs)
+			}
+		}
+	}
+
+	t.Run("Atoi", checkNoAllocs(func() {
+		Sink.Int, Sink.Error = Atoi(string(bytes.Number))
+	}))
+	t.Run("ParseBool", checkNoAllocs(func() {
+		Sink.Bool, Sink.Error = ParseBool(string(bytes.Bool))
+	}))
+	t.Run("ParseInt", checkNoAllocs(func() {
+		Sink.Int64, Sink.Error = ParseInt(string(bytes.Number), 10, 64)
+	}))
+	t.Run("ParseUint", checkNoAllocs(func() {
+		Sink.Uint64, Sink.Error = ParseUint(string(bytes.Number), 10, 64)
+	}))
+	t.Run("ParseFloat", checkNoAllocs(func() {
+		Sink.Float64, Sink.Error = ParseFloat(string(bytes.Number), 64)
+	}))
+	t.Run("ParseComplex", checkNoAllocs(func() {
+		Sink.Complex128, Sink.Error = ParseComplex(string(bytes.Number), 128)
+	}))
+	t.Run("CanBackquote", checkNoAllocs(func() {
+		Sink.Bool = CanBackquote(string(bytes.String))
+	}))
+	t.Run("AppendQuote", checkNoAllocs(func() {
+		Sink.Bytes = AppendQuote(bytes.Buffer[:0], string(bytes.String))
+	}))
+	t.Run("AppendQuoteToASCII", checkNoAllocs(func() {
+		Sink.Bytes = AppendQuoteToASCII(bytes.Buffer[:0], string(bytes.String))
+	}))
+	t.Run("AppendQuoteToGraphic", checkNoAllocs(func() {
+		Sink.Bytes = AppendQuoteToGraphic(bytes.Buffer[:0], string(bytes.String))
+	}))
+}
+
 func TestErrorPrefixes(t *testing.T) {
 	_, errInt := Atoi("INVALID")
 	_, errBool := ParseBool("INVALID")