runtime: use dedicated printfloat32

It can be unexpected that printing a float32 involves 64-bit floating
point routines, see for example:
#1415

This commit adds a dedicated printfloat32 instead just for printing
float32 values. It comes with a possible code size increase, but only if
both float32 and float64 values are printed. Therefore, this should be
an improvement in almost all cases.

I also tried using printfloat32 for everything (and casting a float64 to
float32 to print) but the printed values are slightly different,
breaking the testdata/math.go test for example.

Author: aykevl

src/runtime/print.go

@@ -98,10 +98,90 @@ func printint64(n int64) {
 	printuint64(uint64(n))
 }
 
+// printfloat32() was copied from the relevant source in the original Go
+// implementation and modified to work with float32 instead of float64. It is
+// copyright by the Go authors, licensed under the same BSD 3-clause license.
+// See https://golang.org/LICENSE for details.
+//
+// It is a near-duplicate of printfloat64. This is done so that printing a
+// float32 value doesn't involve float64 routines, which can be unexpected and a
+// problem sometimes. It comes with a possible code size reduction if both
+// printfloat32 and printfloat64 are used, which seems uncommon.
+//
+// Source:
+// https://github.com/golang/go/blob/master/src/runtime/print.go
 func printfloat32(v float32) {
-	// TODO: write an implementation like printfloat64, as some systems have
-	// 32-bit floats but only software emulation for 64-bit floats.
-	printfloat64(float64(v))
+	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 := float32(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 -= float32(s)
+		v *= 10
+	}
+	buf[1] = buf[2]
+	buf[2] = '.'
+
+	buf[n+2] = 'e'
+	buf[n+3] = '+'
+	if e < 0 {
+		e = -e
+		buf[n+3] = '-'
+	}
+
+	buf[n+4] = byte(e/100) + '0'
+	buf[n+5] = byte(e/10)%10 + '0'
+	buf[n+6] = byte(e%10) + '0'
+	for _, c := range buf {
+		putchar(c)
+	}
 }
 
 // printfloat64() was copied from the relevant source in the original Go

testdata/print.go

@@ -29,6 +29,9 @@ func main() {
 	// print float64
 	println(3.14)
 
+	// print float32
+	println(float32(3.14))
+
 	// print complex128
 	println(5 + 1.2345i)
 

testdata/print.txt

@@ -16,6 +16,7 @@ a b c
 123456789012
 -123456789012
 +3.140000e+000
++3.140000e+000
 (+5.000000e+000+1.234500e+000i)
 (0:nil)
 map[2]