cmd/compile: don't truncate tiny float constants to 0 in error messages

Fixes #13559.

Change-Id: I6fe8b5083192e8eb6c1b3ca1919fde81a00ccb7e
Reviewed-on: https://go-review.googlesource.com/17695
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/compile/internal/gc/mparith3.go

@@ -212,31 +212,40 @@ func Fconv(fvp *Mpflt, flag int) string {
 		return sign + "Inf"
 	}
 
-	// Use fmt formatting if in float64 range (common case).
-	if x, _ := f.Float64(); !math.IsInf(x, 0) {
+	// Use exact fmt formatting if in float64 range (common case):
+	// proceed if f doesn't underflow to 0 or overflow to inf.
+	if x, _ := f.Float64(); f.Sign() == 0 == (x == 0) && !math.IsInf(x, 0) {
 		return fmt.Sprintf("%s%.6g", sign, x)
 	}
 
 	// Out of float64 range. Do approximate manual to decimal
 	// conversion to avoid precise but possibly slow Float
-	// formatting. The exponent is > 0 since a negative out-
-	// of-range exponent would have underflowed and led to 0.
+	// formatting.
 	// f = mant * 2**exp
 	var mant big.Float
-	exp := float64(f.MantExp(&mant)) // 0.5 <= mant < 1.0, exp > 0
+	exp := f.MantExp(&mant) // 0.5 <= mant < 1.0
 
 	// approximate float64 mantissa m and decimal exponent d
 	// f ~ m * 10**d
-	m, _ := mant.Float64()            // 0.5 <= m < 1.0
-	d := exp * (math.Ln2 / math.Ln10) // log_10(2)
+	m, _ := mant.Float64()                     // 0.5 <= m < 1.0
+	d := float64(exp) * (math.Ln2 / math.Ln10) // log_10(2)
 
 	// adjust m for truncated (integer) decimal exponent e
 	e := int64(d)
 	m *= math.Pow(10, d-float64(e))
-	for m >= 10 {
+
+	// ensure 1 <= m < 10
+	switch {
+	case m < 1-0.5e-6:
+		// The %.6g format below rounds m to 5 digits after the
+		// decimal point. Make sure that m*10 < 10 even after
+		// rounding up: m*10 + 0.5e-5 < 10 => m < 1 - 0.5e6.
+		m *= 10
+		e--
+	case m >= 10:
 		m /= 10
 		e++
 	}
 
-	return fmt.Sprintf("%s%.5fe+%d", sign, m, e)
+	return fmt.Sprintf("%s%.6ge%+d", sign, m, e)
 }

test/fixedbugs/issue11326.go

@@ -18,14 +18,14 @@ func main() {
 
 	// Any implementation must be able to handle these constants at
 	// compile time (even though they cannot be assigned to a float64).
-	var _ = 1e646456992  // ERROR "1.00000e\+646456992 overflows float64"
-	var _ = 1e64645699   // ERROR "1.00000e\+64645699 overflows float64"
-	var _ = 1e6464569    // ERROR "1.00000e\+6464569 overflows float64"
-	var _ = 1e646456     // ERROR "1.00000e\+646456 overflows float64"
-	var _ = 1e64645      // ERROR "1.00000e\+64645 overflows float64"
-	var _ = 1e6464       // ERROR "1.00000e\+6464 overflows float64"
-	var _ = 1e646        // ERROR "1.00000e\+646 overflows float64"
-	var _ = 1e309        // ERROR "1.00000e\+309 overflows float64"
+	var _ = 1e646456992  // ERROR "1e\+646456992 overflows float64"
+	var _ = 1e64645699   // ERROR "1e\+64645699 overflows float64"
+	var _ = 1e6464569    // ERROR "1e\+6464569 overflows float64"
+	var _ = 1e646456     // ERROR "1e\+646456 overflows float64"
+	var _ = 1e64645      // ERROR "1e\+64645 overflows float64"
+	var _ = 1e6464       // ERROR "1e\+6464 overflows float64"
+	var _ = 1e646        // ERROR "1e\+646 overflows float64"
+	var _ = 1e309        // ERROR "1e\+309 overflows float64"
 
 	var _ = 1e308
 }

test/fixedbugs/issue11590.go

@@ -8,4 +8,4 @@ package p
 
 var _ = int8(4) * 300         // ERROR "constant 300 overflows int8" "constant 1200 overflows int8"
 var _ = complex64(1) * 1e200  // ERROR "constant 1e\+200 overflows complex64"
-var _ = complex128(1) * 1e500 // ERROR "constant 1\.00000e\+500 overflows complex128"
+var _ = complex128(1) * 1e500 // ERROR "constant 1e\+500 overflows complex128"

test/fixedbugs/issue13471.go

@@ -9,17 +9,17 @@
 package main
 
 func main() {
-	const _ int64 = 1e646456992 // ERROR "1.00000e\+646456992 overflows integer"
-	const _ int32 = 1e64645699  // ERROR "1.00000e\+64645699 overflows integer"
-	const _ int16 = 1e6464569   // ERROR "1.00000e\+6464569 overflows integer"
-	const _ int8 = 1e646456     // ERROR "1.00000e\+646456 overflows integer"
-	const _ int = 1e64645       // ERROR "1.00000e\+64645 overflows integer"
+	const _ int64 = 1e646456992 // ERROR "1e\+646456992 overflows integer"
+	const _ int32 = 1e64645699  // ERROR "1e\+64645699 overflows integer"
+	const _ int16 = 1e6464569   // ERROR "1e\+6464569 overflows integer"
+	const _ int8 = 1e646456     // ERROR "1e\+646456 overflows integer"
+	const _ int = 1e64645       // ERROR "1e\+64645 overflows integer"
 
-	const _ uint64 = 1e646456992 // ERROR "1.00000e\+646456992 overflows integer"
-	const _ uint32 = 1e64645699  // ERROR "1.00000e\+64645699 overflows integer"
-	const _ uint16 = 1e6464569   // ERROR "1.00000e\+6464569 overflows integer"
-	const _ uint8 = 1e646456     // ERROR "1.00000e\+646456 overflows integer"
-	const _ uint = 1e64645       // ERROR "1.00000e\+64645 overflows integer"
+	const _ uint64 = 1e646456992 // ERROR "1e\+646456992 overflows integer"
+	const _ uint32 = 1e64645699  // ERROR "1e\+64645699 overflows integer"
+	const _ uint16 = 1e6464569   // ERROR "1e\+6464569 overflows integer"
+	const _ uint8 = 1e646456     // ERROR "1e\+646456 overflows integer"
+	const _ uint = 1e64645       // ERROR "1e\+64645 overflows integer"
 
-	const _ rune = 1e64645 // ERROR "1.00000e\+64645 overflows integer"
+	const _ rune = 1e64645 // ERROR "1e\+64645 overflows integer"
 }

test/fixedbugs/issue13559.go

+// errorcheck
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Verify that error messages print meaningful values
+// for various extreme floating-point constants.
+
+package p
+
+// failure case in issue
+const _ int64 = 1e-10000 // ERROR "1e\-10000 truncated"
+
+const (
+	_ int64 = 1e10000000 // ERROR "1e\+10000000 overflows"
+	_ int64 = 1e1000000  // ERROR "1e\+1000000 overflows"
+	_ int64 = 1e100000   // ERROR "1e\+100000 overflows"
+	_ int64 = 1e10000    // ERROR "1e\+10000 overflows"
+	_ int64 = 1e1000     // ERROR "1e\+1000 overflows"
+	_ int64 = 1e100      // ERROR "10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 overflows"
+	_ int64 = 1e10
+	_ int64 = 1e1
+	_ int64 = 1e0
+	_ int64 = 1e-1       // ERROR "0\.1 truncated"
+	_ int64 = 1e-10      // ERROR "1e\-10 truncated"
+	_ int64 = 1e-100     // ERROR "1e\-100 truncated"
+	_ int64 = 1e-1000    // ERROR "1e\-1000 truncated"
+	_ int64 = 1e-10000   // ERROR "1e\-10000 truncated"
+	_ int64 = 1e-100000  // ERROR "1e\-100000 truncated"
+	_ int64 = 1e-1000000 // ERROR "1e\-1000000 truncated"
+)
+
+const (
+	_ int64 = -1e10000000 // ERROR "\-1e\+10000000 overflows"
+	_ int64 = -1e1000000  // ERROR "\-1e\+1000000 overflows"
+	_ int64 = -1e100000   // ERROR "\-1e\+100000 overflows"
+	_ int64 = -1e10000    // ERROR "\-1e\+10000 overflows"
+	_ int64 = -1e1000     // ERROR "\-1e\+1000 overflows"
+	_ int64 = -1e100      // ERROR "\-10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 overflows"
+	_ int64 = -1e10
+	_ int64 = -1e1
+	_ int64 = -1e0
+	_ int64 = -1e-1       // ERROR "\-0\.1 truncated"
+	_ int64 = -1e-10      // ERROR "\-1e\-10 truncated"
+	_ int64 = -1e-100     // ERROR "\-1e\-100 truncated"
+	_ int64 = -1e-1000    // ERROR "\-1e\-1000 truncated"
+	_ int64 = -1e-10000   // ERROR "\-1e\-10000 truncated"
+	_ int64 = -1e-100000  // ERROR "\-1e\-100000 truncated"
+	_ int64 = -1e-1000000 // ERROR "\-1e\-1000000 truncated"
+)
+
+const (
+	_ int64 = 1.23456789e10000000 // ERROR "1\.23457e\+10000000 overflows"
+	_ int64 = 1.23456789e1000000  // ERROR "1\.23457e\+1000000 overflows"
+	_ int64 = 1.23456789e100000   // ERROR "1\.23457e\+100000 overflows"
+	_ int64 = 1.23456789e10000    // ERROR "1\.23457e\+10000 overflows"
+	_ int64 = 1.23456789e1000     // ERROR "1\.23457e\+1000 overflows"
+	_ int64 = 1.23456789e100      // ERROR "12345678900000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 overflows"
+	_ int64 = 1.23456789e10
+	_ int64 = 1.23456789e1        // ERROR "12\.3457 truncated"
+	_ int64 = 1.23456789e0        // ERROR "1\.23457 truncated"
+	_ int64 = 1.23456789e-1       // ERROR "0\.123457 truncated"
+	_ int64 = 1.23456789e-10      // ERROR "1\.23457e\-10 truncated"
+	_ int64 = 1.23456789e-100     // ERROR "1\.23457e\-100 truncated"
+	_ int64 = 1.23456789e-1000    // ERROR "1\.23457e\-1000 truncated"
+	_ int64 = 1.23456789e-10000   // ERROR "1\.23457e\-10000 truncated"
+	_ int64 = 1.23456789e-100000  // ERROR "1\.23457e\-100000 truncated"
+	_ int64 = 1.23456789e-1000000 // ERROR "1\.23457e\-1000000 truncated"
+)
+
+const (
+	_ int64 = -1.23456789e10000000 // ERROR "\-1\.23457e\+10000000 overflows"
+	_ int64 = -1.23456789e1000000  // ERROR "\-1\.23457e\+1000000 overflows"
+	_ int64 = -1.23456789e100000   // ERROR "\-1\.23457e\+100000 overflows"
+	_ int64 = -1.23456789e10000    // ERROR "\-1\.23457e\+10000 overflows"
+	_ int64 = -1.23456789e1000     // ERROR "\-1\.23457e\+1000 overflows"
+	_ int64 = -1.23456789e100      // ERROR "\-12345678900000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 overflows"
+	_ int64 = -1.23456789e10
+	_ int64 = -1.23456789e1        // ERROR "\-12\.3457 truncated"
+	_ int64 = -1.23456789e0        // ERROR "\-1\.23457 truncated"
+	_ int64 = -1.23456789e-1       // ERROR "\-0\.123457 truncated"
+	_ int64 = -1.23456789e-10      // ERROR "\-1\.23457e\-10 truncated"
+	_ int64 = -1.23456789e-100     // ERROR "\-1\.23457e\-100 truncated"
+	_ int64 = -1.23456789e-1000    // ERROR "\-1\.23457e\-1000 truncated"
+	_ int64 = -1.23456789e-10000   // ERROR "\-1\.23457e\-10000 truncated"
+	_ int64 = -1.23456789e-100000  // ERROR "\-1\.23457e\-100000 truncated"
+	_ int64 = -1.23456789e-1000000 // ERROR "\-1\.23457e\-1000000 truncated"
+)