test/float_lit2.go: compute test values from first principles

These constants pass go/types constant conversions as well.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/91590047

test/float_lit2.go

@@ -12,20 +12,75 @@ import (
 	"fmt"
 )
 
+const (
+	m32bits   = 23  // number of float32 mantissa bits
+	e32max    = 127 // max. float32 exponent
+	maxExp32  = e32max - m32bits
+	maxMant32 = 1<<(m32bits+1) - 1
+
+	maxFloat32_0 = (maxMant32 - 0) << maxExp32
+	maxFloat32_1 = (maxMant32 - 1) << maxExp32
+	maxFloat32_2 = (maxMant32 - 2) << maxExp32
+)
+
+func init() {
+	if maxExp32 != 104 {
+		panic("incorrect maxExp32")
+	}
+	if maxMant32 != 16777215 {
+		panic("incorrect maxMant32")
+	}
+	if maxFloat32_0 != 340282346638528859811704183484516925440 {
+		panic("incorrect maxFloat32_0")
+	}
+}
+
+const (
+	m64bits   = 52   // number of float64 mantissa bits
+	e64max    = 1023 // max. float64 exponent
+	maxExp64  = e64max - m64bits
+	maxMant64 = 1<<(m64bits+1) - 1
+
+	// These expressions are not permitted due to implementation restrictions.
+	// maxFloat64_0 = (maxMant64-0) << maxExp64
+	// maxFloat64_1 = (maxMant64-1) << maxExp64
+	// maxFloat64_2 = (maxMant64-2) << maxExp64
+
+	// These equivalent values were computed using math/big.
+	maxFloat64_0 = 1.7976931348623157e308
+	maxFloat64_1 = 1.7976931348623155e308
+	maxFloat64_2 = 1.7976931348623153e308
+)
+
+func init() {
+	if maxExp64 != 971 {
+		panic("incorrect maxExp64")
+	}
+	if maxMant64 != 9007199254740991 {
+		panic("incorrect maxMant64")
+	}
+}
+
 var cvt = []struct {
 	val    interface{}
 	binary string
 }{
-	{float32(-340282356779733661637539395458142568447), "-16777215p+104"},
-	{float32(-340282326356119256160033759537265639424), "-16777214p+104"},
-	{float32(340282326356119256160033759537265639424), "16777214p+104"},
-	{float32(340282356779733661637539395458142568447), "16777215p+104"},
-	{float64(-1.797693134862315807937289714053e+308), "-9007199254740991p+971"},
-	{float64(-1.797693134862315708145274237317e+308), "-9007199254740991p+971"},
-	{float64(-1.797693134862315608353258760581e+308), "-9007199254740990p+971"},
-	{float64(1.797693134862315608353258760581e+308), "9007199254740990p+971"},
-	{float64(1.797693134862315708145274237317e+308), "9007199254740991p+971"},
-	{float64(1.797693134862315807937289714053e+308), "9007199254740991p+971"},
+
+	{float32(maxFloat32_0), fmt.Sprintf("%dp+%d", maxMant32-0, maxExp32)},
+	{float32(maxFloat32_1), fmt.Sprintf("%dp+%d", maxMant32-1, maxExp32)},
+	{float32(maxFloat32_2), fmt.Sprintf("%dp+%d", maxMant32-2, maxExp32)},
+
+	{float64(maxFloat64_0), fmt.Sprintf("%dp+%d", maxMant64-0, maxExp64)},
+	{float64(maxFloat64_1), fmt.Sprintf("%dp+%d", maxMant64-1, maxExp64)},
+	{float64(maxFloat64_2), fmt.Sprintf("%dp+%d", maxMant64-2, maxExp64)},
+
+	{float32(-maxFloat32_0), fmt.Sprintf("-%dp+%d", maxMant32-0, maxExp32)},
+	{float32(-maxFloat32_1), fmt.Sprintf("-%dp+%d", maxMant32-1, maxExp32)},
+	{float32(-maxFloat32_2), fmt.Sprintf("-%dp+%d", maxMant32-2, maxExp32)},
+
+	{float64(-maxFloat64_0), fmt.Sprintf("-%dp+%d", maxMant64-0, maxExp64)},
+	{float64(-maxFloat64_1), fmt.Sprintf("-%dp+%d", maxMant64-1, maxExp64)},
+	{float64(-maxFloat64_2), fmt.Sprintf("-%dp+%d", maxMant64-2, maxExp64)},
 }
 
 func main() {