cmd/compile: diagnose constant division by complex zero

When casting an ideal to complex{64,128}, for example during the
evaluation of

  var a = complex64(0) / 1e-50

we want the compiler to report a division-by-zero error if a divisor
would be zero after the cast.

We already do this for floats; for example

  var b = float32(0) / 1e-50

generates a 'division by zero' error at compile time (because
float32(1e-50) is zero, and the cast is done before performing the
division).

There's no such check in the path for complex{64,128} expressions, and
no cast is performed before the division in the evaluation of

  var a = complex64(0) / 1e-50

which compiles just fine.

This patch changes the convlit1 function so that complex ideals
components (real and imag) are correctly truncated to float{32,64}
when doing an ideal -> complex{64, 128} cast.

Fixes #11674

Change-Id: Ic5f8ee3c8cfe4c3bb0621481792c96511723d151
Reviewed-on: https://go-review.googlesource.com/37891
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -160,6 +160,37 @@ func truncfltlit(oldv *Mpflt, t *types.Type) *Mpflt {
 	return fv
 }
 
+// truncate Real and Imag parts of Mpcplx to 32-bit or 64-bit
+// precision, according to type; return truncated value. In case of
+// overflow, calls yyerror but does not truncate the input value.
+func trunccmplxlit(oldv *Mpcplx, t *types.Type) *Mpcplx {
+	if t == nil {
+		return oldv
+	}
+
+	if overflow(Val{oldv}, t) {
+		// Avoid setting to Inf if there was an overflow. It's never
+		// useful, and it'll cause spourious and confusing 'constant Inf
+		// overflows float32' errors down the road.
+		return oldv
+	}
+
+	cv := newMpcmplx()
+
+	switch t.Etype {
+	case TCOMPLEX64:
+		cv.Real.SetFloat64(oldv.Real.Float32())
+		cv.Imag.SetFloat64(oldv.Imag.Float32())
+	case TCOMPLEX128:
+		cv.Real.SetFloat64(oldv.Real.Float64())
+		cv.Imag.SetFloat64(oldv.Imag.Float64())
+	default:
+		Fatalf("trunccplxlit: unexpected Etype %v", t.Etype)
+	}
+
+	return cv
+}
+
 // canReuseNode indicates whether it is known to be safe
 // to reuse a Node.
 type canReuseNode bool
@@ -361,7 +392,7 @@ func convlit1(n *Node, t *types.Type, explicit bool, reuse canReuseNode) *Node {
 				fallthrough
 
 			case CTCPLX:
-				overflow(n.Val(), t)
+				n.SetVal(Val{trunccmplxlit(n.Val().U.(*Mpcplx), t)})
 			}
 		} else if et == types.TSTRING && (ct == CTINT || ct == CTRUNE) && explicit {
 			n.SetVal(tostr(n.Val()))
@@ -519,21 +550,25 @@ func doesoverflow(v Val, t *types.Type) bool {
 	return false
 }
 
-func overflow(v Val, t *types.Type) {
+func overflow(v Val, t *types.Type) bool {
 	// v has already been converted
 	// to appropriate form for t.
 	if t == nil || t.Etype == TIDEAL {
-		return
+		return false
 	}
 
 	// Only uintptrs may be converted to unsafe.Pointer, which cannot overflow.
 	if t.Etype == TUNSAFEPTR {
-		return
+		return false
 	}
 
 	if doesoverflow(v, t) {
 		yyerror("constant %v overflows %v", v, t)
+		return true
 	}
+
+	return false
+
 }
 
 func tostr(v Val) Val {

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

@@ -37,6 +37,13 @@ func newMpflt() *Mpflt {
 	return &a
 }
 
+func newMpcmplx() *Mpcplx {
+	var a Mpcplx
+	a.Real = *newMpflt()
+	a.Imag = *newMpflt()
+	return &a
+}
+
 func (a *Mpflt) SetInt(b *Mpint) {
 	if b.checkOverflow(0) {
 		// sign doesn't really matter but copy anyway

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

@@ -0,0 +1,63 @@
+// Copyright 2017 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 gc
+
+import "testing"
+
+var f52want float64 = 1.0 / (1 << 52)
+var f53want float64 = 1.0 / (1 << 53)
+
+func TestTruncFlt(t *testing.T) {
+	const f52 = 1 + 1.0/(1<<52)
+	const f53 = 1 + 1.0/(1<<53)
+
+	if got := f52 - 1; got != f52want {
+		t.Errorf("f52-1 = %g, want %g", got, f52want)
+	}
+	if got := float64(f52) - 1; got != f52want {
+		t.Errorf("float64(f52)-1 = %g, want %g", got, f52want)
+	}
+	if got := f53 - 1; got != f53want {
+		t.Errorf("f53-1 = %g, want %g", got, f53want)
+	}
+	if got := float64(f53) - 1; got != 0 {
+		t.Errorf("float64(f53)-1 = %g, want 0", got)
+	}
+}
+
+func TestTruncCmplx(t *testing.T) {
+	const r52 = complex(1+1.0/(1<<52), 0)
+	const r53 = complex(1+1.0/(1<<53), 0)
+
+	if got := real(r52 - 1); got != f52want {
+		t.Errorf("real(r52-1) = %g, want %g", got, f52want)
+	}
+	if got := real(complex128(r52) - 1); got != f52want {
+		t.Errorf("real(complex128(r52)-1) = %g, want %g", got, f52want)
+	}
+	if got := real(r53 - 1); got != f53want {
+		t.Errorf("real(r53-1) = %g, want %g", got, f53want)
+	}
+	if got := real(complex128(r53) - 1); got != 0 {
+		t.Errorf("real(complex128(r53)-1) = %g, want 0", got)
+	}
+
+	const i52 = complex(0, 1+1.0/(1<<52))
+	const i53 = complex(0, 1+1.0/(1<<53))
+
+	if got := imag(i52 - 1i); got != f52want {
+		t.Errorf("imag(i52-1i) = %g, want %g", got, f52want)
+	}
+	if got := imag(complex128(i52) - 1i); got != f52want {
+		t.Errorf("imag(complex128(i52)-1i) = %g, want %g", got, f52want)
+	}
+	if got := imag(i53 - 1i); got != f53want {
+		t.Errorf("imag(i53-1i) = %g, want %g", got, f53want)
+	}
+	if got := imag(complex128(i53) - 1i); got != 0 {
+		t.Errorf("imag(complex128(i53)-1i) = %g, want 0", got)
+	}
+
+}

test/fixedbugs/issue11674.go

@@ -0,0 +1,40 @@
+// errorcheck
+
+// Copyright 2017 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.
+
+// Issue 11674: cmd/compile: does not diagnose constant division by
+// zero
+
+package p
+
+const x complex64 = 0
+const y complex128 = 0
+
+var _ = x / 1e-20
+var _ = x / 1e-50   // ERROR "complex division by zero"
+var _ = x / 1e-1000 // ERROR "complex division by zero"
+var _ = x / 1e-20i
+var _ = x / 1e-50i   // ERROR "complex division by zero"
+var _ = x / 1e-1000i // ERROR "complex division by zero"
+
+var _ = x / 1e-45 // smallest positive float32
+
+var _ = x / (1e-20 + 1e-20i)
+var _ = x / (1e-50 + 1e-20i)
+var _ = x / (1e-20 + 1e-50i)
+var _ = x / (1e-50 + 1e-50i)     // ERROR "complex division by zero"
+var _ = x / (1e-1000 + 1e-1000i) // ERROR "complex division by zero"
+
+var _ = y / 1e-50
+var _ = y / 1e-1000 // ERROR "complex division by zero"
+var _ = y / 1e-50i
+var _ = y / 1e-1000i // ERROR "complex division by zero"
+
+var _ = y / 5e-324 // smallest positive float64
+
+var _ = y / (1e-50 + 1e-50)
+var _ = y / (1e-1000 + 1e-50i)
+var _ = y / (1e-50 + 1e-1000i)
+var _ = y / (1e-1000 + 1e-1000i) // ERROR "complex division by zero"