floating point bug

R=rsc
DELTA=35  (35 added, 0 deleted, 0 changed)
OCL=29442
CL=29444

test/bugs/bug154.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: should not panic
+
+// Copyright 2009 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.
+
+package main
+
+import "fmt"
+
+func f0() string {
+	const f = 3.141592;
+	return fmt.Sprintf("%v", float64(f));
+}
+
+
+func f1() string {
+	const f = 3.141592;
+	_ := float64(float32(f));  // appears to change the precision of f
+	return fmt.Sprintf("%v", float64(f));
+}
+
+
+func main() {
+	r0 := f0();
+	r1 := f1();
+	if r0 != r1 {
+		println("r0 =", r0);
+		println("r1 =", r1);
+		panic("r0 and r1 should be the same");
+	}
+}

test/golden.out

@@ -112,6 +112,13 @@ BUG: should compile
 BUG: errchk: bugs/bug153.go:9: error message does not match 'nil'
 bugs/bug153.go:9: fatal error: dowidth: unknown type: E-34
 
+=========== bugs/bug154.go
+r0 = 3.141592
+r1 = 3.141592025756836
+r0 and r1 should be the same
+panic PC=xxx
+BUG: should not panic
+
 =========== fixedbugs/bug016.go
 fixedbugs/bug016.go:7: constant -3 overflows uint