runtime: use internal/strconv

Runtime doing its own number formatting dates back to
when runtime was the bottom-most Go package.
Those days are long gone. Use internal/strconv to avoid
duplicating code and also to get better floating-point
formatting:

% go1.24.6 run x.go
+1.234568e+004
% go run x.go
12345.678
%

With accurate floating point it becomes necessary to
introduce separate printers for float32 vs float64 and
for complex64 vs complex128. Otherwise float32(93.7)
prints as 93.69999694824219.

Change-Id: I25ae3f09519342dc3d1dcabf4711651423e00128
Reviewed-on: https://go-review.googlesource.com/c/go/+/716002
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 23 insertions, 89 deletions

diff --git a/src/runtime/panic_test.go b/src/runtime/panic_test.go
index 994abfdd45..4a30c30db6 100644
--- a/src/runtime/panic_test.go
+++ b/src/runtime/panic_test.go
@@ -18,10 +18,10 @@ func TestPanicWithDirectlyPrintableCustomTypes(t *testing.T) {
 		wantPanicPrefix string
 	}{
 		{"panicCustomBool", `panic: main.MyBool(true)`},
-		{"panicCustomComplex128", `panic: main.MyComplex128(+3.210000e+001+1.000000e+001i)`},
-		{"panicCustomComplex64", `panic: main.MyComplex64(+1.100000e-001+3.000000e+000i)`},
-		{"panicCustomFloat32", `panic: main.MyFloat32(-9.370000e+001)`},
-		{"panicCustomFloat64", `panic: main.MyFloat64(-9.370000e+001)`},
+		{"panicCustomComplex128", `panic: main.MyComplex128(32.1+10i)`},
+		{"panicCustomComplex64", `panic: main.MyComplex64(0.11+3i)`},
+		{"panicCustomFloat32", `panic: main.MyFloat32(-93.7)`},
+		{"panicCustomFloat64", `panic: main.MyFloat64(-93.7)`},
 		{"panicCustomInt", `panic: main.MyInt(93)`},
 		{"panicCustomInt8", `panic: main.MyInt8(93)`},
 		{"panicCustomInt16", `panic: main.MyInt16(93)`},
diff --git a/src/runtime/print.go b/src/runtime/print.go
index 0b05aedad3..e32ecb9450 100644
--- a/src/runtime/print.go
+++ b/src/runtime/print.go
@@ -6,6 +6,7 @@ package runtime
 
 import (
 	"internal/goarch"
+	"internal/strconv"
 	"unsafe"
 )
 
@@ -118,101 +119,34 @@ func printbool(v bool) {
 	}
 }
 
-func printfloat(v float64) {
-	switch {
-	case v != v:
-		printstring("NaN")
-		return
-	case v+v == v && v > 0:
-		printstring("+Inf")
-		return
-	case v+v == v && v < 0:
-		printstring("-Inf")
-		return
-	}
-
-	const n = 7 // digits printed
-	var buf [n + 7]byte
-	buf[0] = '+'
-	e := 0 // exp
-	if v == 0 {
-		if 1/v < 0 {
-			buf[0] = '-'
-		}
-	} else {
-		if v < 0 {
-			v = -v
-			buf[0] = '-'
-		}
-
-		// normalize
-		for v >= 10 {
-			e++
-			v /= 10
-		}
-		for v < 1 {
-			e--
-			v *= 10
-		}
-
-		// round
-		h := 5.0
-		for i := 0; i < n; i++ {
-			h /= 10
-		}
-		v += h
-		if v >= 10 {
-			e++
-			v /= 10
-		}
-	}
-
-	// format +d.dddd+edd
-	for i := 0; i < n; i++ {
-		s := int(v)
-		buf[i+2] = byte(s + '0')
-		v -= float64(s)
-		v *= 10
-	}
-	buf[1] = buf[2]
-	buf[2] = '.'
+func printfloat64(v float64) {
+	var buf [20]byte
+	gwrite(strconv.AppendFloat(buf[:0], v, 'g', -1, 64))
+}
 
-	buf[n+2] = 'e'
-	buf[n+3] = '+'
-	if e < 0 {
-		e = -e
-		buf[n+3] = '-'
-	}
+func printfloat32(v float32) {
+	var buf [20]byte
+	gwrite(strconv.AppendFloat(buf[:0], float64(v), 'g', -1, 32))
+}
 
-	buf[n+4] = byte(e/100) + '0'
-	buf[n+5] = byte(e/10)%10 + '0'
-	buf[n+6] = byte(e%10) + '0'
-	gwrite(buf[:])
+func printcomplex128(c complex128) {
+	var buf [44]byte
+	gwrite(strconv.AppendComplex(buf[:0], c, 'g', -1, 128))
 }
 
-func printcomplex(c complex128) {
-	print("(", real(c), imag(c), "i)")
+func printcomplex64(c complex64) {
+	var buf [44]byte
+	gwrite(strconv.AppendComplex(buf[:0], complex128(c), 'g', -1, 64))
 }
 
 func printuint(v uint64) {
-	var buf [100]byte
-	i := len(buf)
-	for i--; i > 0; i-- {
-		buf[i] = byte(v%10 + '0')
-		if v < 10 {
-			break
-		}
-		v /= 10
-	}
-	gwrite(buf[i:])
+	var buf [20]byte
+	gwrite(strconv.AppendUint(buf[:0], v, 10))
 }
 
 func printint(v int64) {
-	if v < 0 {
-		printstring("-")
-		v = -v
-	}
-	printuint(uint64(v))
+	var buf [20]byte
+	gwrite(strconv.AppendInt(buf[:0], v, 10))
 }
 
 var minhexdigits = 0 // protected by printlock