gc: correct rounding of denormal constants

Fixes #1463.

R=rsc
CC=golang-dev
https://golang.org/cl/4079060

diff --git a/src/cmd/gc/mparith3.c b/src/cmd/gc/mparith3.c
index 7b7e66668eb68547df3441663617de0869f35d59..b11a4f5f1a7c4ec245df96e22d577d3c6b7f51df 100644 (file)
--- a/src/cmd/gc/mparith3.c
+++ b/src/cmd/gc/mparith3.c
@@ -179,7 +179,7 @@ mpdivfltflt(Mpflt *a, Mpflt *b)
 double
 mpgetflt(Mpflt *a)
 {
-       int s, i;
+       int s, i, e;
        uvlong v, vm;
        double f;
 
@@ -200,12 +200,12 @@ mpgetflt(Mpflt *a)
                a->exp -= 1;
        }
 
-       // the magic numbers (64, 63, 53, 10) are
+       // the magic numbers (64, 63, 53, 10, -1074) are
        // IEEE specific. this should be done machine
        // independently or in the 6g half of the compiler
 
-       // pick up the mantissa in a uvlong
-       s = 53;
+       // pick up the mantissa and a rounding bit in a uvlong
+       s = 53+1;
        v = 0;
        for(i=Mpnorm-1; s>=Mpscale; i--) {
                v = (v<<Mpscale) | a->val.a[i];
@@ -224,13 +224,26 @@ mpgetflt(Mpflt *a)
        if(s > 0)
                v = (v<<s) | (a->val.a[i]>>(Mpscale-s));
 
+       // gradual underflow
+       e = Mpnorm*Mpscale + a->exp - 53;
+       if(e < -1074) {
+               s = -e - 1074;
+               if(s > 54)
+                       s = 54;
+               v |= vm & ((1ULL<<s) - 1);
+               vm >>= s;
+               e = -1074;
+       }
+
 //print("vm=%.16llux v=%.16llux\n", vm, v);
        // round toward even
-       if(v != (1ULL<<63) || (vm&1ULL) != 0)
-               vm += v>>63;
+       if(v != 0 || (vm&2ULL) != 0)
+               vm = (vm>>1) + (vm&1ULL);
+       else
+               vm >>= 1;
 
        f = (double)(vm);
-       f = ldexp(f, Mpnorm*Mpscale + a->exp - 53);
+       f = ldexp(f, e);
 
        if(a->val.neg)
                f = -f;

diff --git a/src/pkg/strconv/ftoa_test.go b/src/pkg/strconv/ftoa_test.go
index bc327600e18b017f0541cd3b362d63c412dad297..6d361a138efe81638a69c247ed51a1afad219899 100644 (file)
--- a/src/pkg/strconv/ftoa_test.go
+++ b/src/pkg/strconv/ftoa_test.go
@@ -121,9 +121,8 @@ var ftoatests = []ftoaTest{
 
        // http://www.exploringbinary.com/java-hangs-when-converting-2-2250738585072012e-308/
        {2.2250738585072012e-308, 'g', -1, "2.2250738585072014e-308"},
-       // TODO: uncomment after fixing issue 1463.
        // http://www.exploringbinary.com/php-hangs-on-numeric-value-2-2250738585072011e-308/
-       // {2.2250738585072011e-308, 'g', -1, "2.225073858507201e-308"},
+       {2.2250738585072011e-308, 'g', -1, "2.225073858507201e-308"},
 }
 
 func TestFtoa(t *testing.T) {

diff --git a/test/fixedbugs/bug321.go b/test/fixedbugs/bug321.go
new file mode 100644 (file)
index 0000000..d0595ff
--- /dev/null
+++ b/test/fixedbugs/bug321.go
@@ -0,0 +1,30 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug321
+
+// Copyright 2011 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.
+
+// Troublesome floating point constants. Issue 1463.
+
+package main
+
+import "fmt"
+
+func check(test string, got, want float64) bool {
+       if got != want {
+               fmt.Println(test, "got", got, "want", want)
+               return false
+       }
+       return true
+}
+
+func main() {
+       good := true
+       // http://www.exploringbinary.com/java-hangs-when-converting-2-2250738585072012e-308/
+       good = good && check("2.2250738585072012e-308", 2.2250738585072012e-308, 2.2250738585072014e-308)
+       // http://www.exploringbinary.com/php-hangs-on-numeric-value-2-2250738585072011e-308/
+       good = good && check("2.2250738585072011e-308", 2.2250738585072011e-308, 2.225073858507201e-308)
+       if !good {
+               panic("fail")
+       }
+}