cmd/compile: fix nilcheck for AIX

This commit adapts compile tool to create correct nilchecks for AIX.

AIX allows to load a nil pointer. Therefore, the default nilcheck
which issues a load must be replaced by a CMP instruction followed by a
store at 0x0 if the value is nil. The store will trigger a SIGSEGV as on
others OS.

The nilcheck algorithm must be adapted to do not remove nilcheck if it's
only a read. Stores are detected with v.Type.IsMemory().

Tests related to nilptr must be adapted to the previous changements.
nilptr.go cannot be used as it's because the AIX address space starts at
1<<32.

Change-Id: I9f5aaf0b7e185d736a9b119c0ed2fe4e5bd1e7af
Reviewed-on: https://go-review.googlesource.com/c/144538
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 185 insertions, 0 deletions

diff --git a/test/nilptr_aix.go b/test/nilptr_aix.go
new file mode 100644
index 0000000000..ea5fcc3f4e
--- /dev/null
+++ b/test/nilptr_aix.go
@@ -0,0 +1,185 @@
+// run
+
+// Copyright 2018 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.
+
+// Test that the implementation catches nil ptr indirection
+// in a large address space.
+
+// +build aix
+
+package main
+
+import "unsafe"
+
+// Having a big address space means that indexing
+// at a 1G + 256 MB offset from a nil pointer might not
+// cause a memory access fault. This test checks
+// that Go is doing the correct explicit checks to catch
+// these nil pointer accesses, not just relying on the hardware.
+// The reason of the 1G offset is because AIX addresses start after 1G.
+var dummy [256 << 20]byte // give us a big address space
+
+func main() {
+	// the test only tests what we intend to test
+	// if dummy starts in the first 256 MB of memory.
+	// otherwise there might not be anything mapped
+	// at the address that might be accidentally
+	// dereferenced below.
+	if uintptr(unsafe.Pointer(&dummy)) < 1<<32 {
+		panic("dummy not far enough")
+	}
+
+	shouldPanic(p1)
+	shouldPanic(p2)
+	shouldPanic(p3)
+	shouldPanic(p4)
+	shouldPanic(p5)
+	shouldPanic(p6)
+	shouldPanic(p7)
+	shouldPanic(p8)
+	shouldPanic(p9)
+	shouldPanic(p10)
+	shouldPanic(p11)
+	shouldPanic(p12)
+	shouldPanic(p13)
+	shouldPanic(p14)
+	shouldPanic(p15)
+	shouldPanic(p16)
+}
+
+func shouldPanic(f func()) {
+	defer func() {
+		if recover() == nil {
+			panic("memory reference did not panic")
+		}
+	}()
+	f()
+}
+
+func p1() {
+	// Array index.
+	var p *[1 << 33]byte = nil
+	println(p[1<<32+256<<20]) // very likely to be inside dummy, but should panic
+}
+
+var xb byte
+
+func p2() {
+	var p *[1 << 33]byte = nil
+	xb = 123
+
+	// Array index.
+	println(p[uintptr(unsafe.Pointer(&xb))]) // should panic
+}
+
+func p3() {
+	// Array to slice.
+	var p *[1 << 33]byte = nil
+	var x []byte = p[0:] // should panic
+	_ = x
+}
+
+var q *[1 << 33]byte
+
+func p4() {
+	// Array to slice.
+	var x []byte
+	var y = &x
+	*y = q[0:] // should crash (uses arraytoslice runtime routine)
+}
+
+func fb([]byte) {
+	panic("unreachable")
+}
+
+func p5() {
+	// Array to slice.
+	var p *[1 << 33]byte = nil
+	fb(p[0:]) // should crash
+}
+
+func p6() {
+	// Array to slice.
+	var p *[1 << 33]byte = nil
+	var _ []byte = p[10 : len(p)-10] // should crash
+}
+
+type T struct {
+	x [1<<32 + 256<<20]byte
+	i int
+}
+
+func f() *T {
+	return nil
+}
+
+var y *T
+var x = &y
+
+func p7() {
+	// Struct field access with large offset.
+	println(f().i) // should crash
+}
+
+func p8() {
+	// Struct field access with large offset.
+	println((*x).i) // should crash
+}
+
+func p9() {
+	// Struct field access with large offset.
+	var t *T
+	println(&t.i) // should crash
+}
+
+func p10() {
+	// Struct field access with large offset.
+	var t *T
+	println(t.i) // should crash
+}
+
+type T1 struct {
+	T
+}
+
+type T2 struct {
+	*T1
+}
+
+func p11() {
+	t := &T2{}
+	p := &t.i
+	println(*p)
+}
+
+// ADDR(DOT(IND(p))) needs a check also
+func p12() {
+	var p *T = nil
+	println(*(&((*p).i)))
+}
+
+// Tests suggested in golang.org/issue/6080.
+
+func p13() {
+	var x *[10]int
+	y := x[:]
+	_ = y
+}
+
+func p14() {
+	println((*[1]int)(nil)[:])
+}
+
+func p15() {
+	for i := range (*[1]int)(nil)[:] {
+		_ = i
+	}
+}
+
+func p16() {
+	for i, v := range (*[1]int)(nil)[:] {
+		_ = i + v
+	}
+}