diff --git a/src/cmd/compile/internal/gc/const.go b/src/cmd/compile/internal/gc/const.go
index 1403a2be11e..d87c4980d06 100644
--- a/src/cmd/compile/internal/gc/const.go
+++ b/src/cmd/compile/internal/gc/const.go
@@ -476,7 +476,7 @@ func toflt(v Val) Val {
 		f := newMpflt()
 		f.Set(&u.Real)
 		if u.Imag.CmpFloat64(0) != 0 {
-			yyerror("constant %v%vi truncated to real", fconv(&u.Real, FmtSharp), fconv(&u.Imag, FmtSharp|FmtSign))
+			yyerror("constant %v truncated to real", cconv(u))
 		}
 		v.U = f
 	}
@@ -509,11 +509,11 @@ func toint(v Val) Val {
 				// value from the error message.
 				// (See issue #11371).
 				var t big.Float
-				t.Parse(fconv(u, FmtSharp), 10)
+				t.Parse(fconv(u), 10)
 				if t.IsInt() {
 					yyerror("constant truncated to integer")
 				} else {
-					yyerror("constant %v truncated to integer", fconv(u, FmtSharp))
+					yyerror("constant %v truncated to integer", fconv(u))
 				}
 			}
 		}
@@ -522,7 +522,7 @@ func toint(v Val) Val {
 	case *Mpcplx:
 		i := new(Mpint)
 		if !i.SetFloat(&u.Real) || u.Imag.CmpFloat64(0) != 0 {
-			yyerror("constant %v%vi truncated to integer", fconv(&u.Real, FmtSharp), fconv(&u.Imag, FmtSharp|FmtSign))
+			yyerror("constant %v truncated to integer", cconv(u))
 		}
 
 		v.U = i
diff --git a/src/cmd/compile/internal/gc/fmt.go b/src/cmd/compile/internal/gc/fmt.go
index 5b7445d4dba..be8a7ef6f58 100644
--- a/src/cmd/compile/internal/gc/fmt.go
+++ b/src/cmd/compile/internal/gc/fmt.go
@@ -537,10 +537,10 @@ func (v Val) vconv(s fmt.State, flag FmtFlag) {
 
 	case *Mpflt:
 		if flag&FmtSharp != 0 {
-			fmt.Fprint(s, fconv(u, 0))
+			fmt.Fprint(s, u.String())
 			return
 		}
-		fmt.Fprint(s, fconv(u, FmtSharp))
+		fmt.Fprint(s, fconv(u))
 		return
 
 	case *Mpcplx:
@@ -549,16 +549,13 @@ func (v Val) vconv(s fmt.State, flag FmtFlag) {
 			fmt.Fprintf(s, "(%v+%vi)", &u.Real, &u.Imag)
 
 		case v.U.(*Mpcplx).Real.CmpFloat64(0) == 0:
-			fmt.Fprintf(s, "%vi", fconv(&u.Imag, FmtSharp))
+			fmt.Fprintf(s, "%vi", fconv(&u.Imag))
 
 		case v.U.(*Mpcplx).Imag.CmpFloat64(0) == 0:
-			fmt.Fprint(s, fconv(&u.Real, FmtSharp))
-
-		case v.U.(*Mpcplx).Imag.CmpFloat64(0) < 0:
-			fmt.Fprintf(s, "(%v%vi)", fconv(&u.Real, FmtSharp), fconv(&u.Imag, FmtSharp))
+			fmt.Fprint(s, fconv(&u.Real))
 
 		default:
-			fmt.Fprintf(s, "(%v+%vi)", fconv(&u.Real, FmtSharp), fconv(&u.Imag, FmtSharp))
+			fmt.Fprintf(s, "(%v)", cconv(u))
 		}
 
 	case string:
diff --git a/src/cmd/compile/internal/gc/mpfloat.go b/src/cmd/compile/internal/gc/mpfloat.go
index 5977ef9748e..8837628d866 100644
--- a/src/cmd/compile/internal/gc/mpfloat.go
+++ b/src/cmd/compile/internal/gc/mpfloat.go
@@ -201,24 +201,16 @@ func (a *Mpflt) SetString(as string) {
 }
 
 func (f *Mpflt) String() string {
-	return fconv(f, 0)
+	return f.Val.Text('b', 0)
 }
 
-func fconv(fvp *Mpflt, flag FmtFlag) string {
-	if flag&FmtSharp == 0 {
-		return fvp.Val.Text('b', 0)
-	}
-
-	// use decimal format for error messages
-
+func fconv(fvp *Mpflt) string {
 	// determine sign
+	sign := ""
 	f := &fvp.Val
-	var sign string
 	if f.Sign() < 0 {
 		sign = "-"
 		f = new(big.Float).Abs(f)
-	} else if flag&FmtSign != 0 {
-		sign = "+"
 	}
 
 	// Don't try to convert infinities (will not terminate).
@@ -334,3 +326,12 @@ func (v *Mpcplx) Div(rv *Mpcplx) bool {
 
 	return true
 }
+
+func cconv(v *Mpcplx) string {
+	re := fconv(&v.Real)
+	im := fconv(&v.Imag)
+	if im[0] == '-' {
+		return re + im + "i"
+	}
+	return re + "+" + im + "i"
+}