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cmd/compile: diagnose constant division by complex zero
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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 golang#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>
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ALTree authored and lparth committed Apr 13, 2017
1 parent 7287d01 commit cfdb4a9
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Showing 4 changed files with 149 additions and 4 deletions.
43 changes: 39 additions & 4 deletions src/cmd/compile/internal/gc/const.go
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -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()))
Expand Down Expand Up @@ -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 {
Expand Down
7 changes: 7 additions & 0 deletions src/cmd/compile/internal/gc/mpfloat.go
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down
63 changes: 63 additions & 0 deletions src/cmd/compile/internal/gc/truncconst_test.go
Original file line number Diff line number Diff line change
@@ -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)
}

}
40 changes: 40 additions & 0 deletions test/fixedbugs/issue11674.go
Original file line number Diff line number Diff line change
@@ -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"

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