cmd/gc: eliminate redundant &x.Field nil checks

This eliminates ~75% of the nil checks being emitted,
on all architectures. We can do better, but we need
a bit more general support from the compiler, and
I don't want to do that so close to Go 1.2.
What's here is simple but effective and safe.

A few small code generation cleanups were required
to make the analysis consistent on all systems about
which nil checks are omitted, at least in the test.

Fixes issue 6019.

R=ken2
CC=golang-dev
https://codereview.appspot.com/13334052

1 files changed, 191 insertions, 0 deletions

diff --git a/test/nilptr3.go b/test/nilptr3.go
new file mode 100644
index 000000000..08597a02d
--- /dev/null
+++ b/test/nilptr3.go
@@ -0,0 +1,191 @@
+// errorcheck -0 -d=nil
+
+// Copyright 2013 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 nil checks are removed.
+// Optimization is enabled.
+
+package p
+
+type Struct struct {
+	X int
+	Y float64
+}
+
+type BigStruct struct {
+	X int
+	Y float64
+	A [1<<20]int
+	Z string
+}
+
+type Empty struct {
+}
+
+type Empty1 struct {
+	Empty
+}
+
+var (
+	intp *int
+	arrayp *[10]int
+	array0p *[0]int
+	bigarrayp *[1<<26]int
+	structp *Struct
+	bigstructp *BigStruct
+	emptyp *Empty
+	empty1p *Empty1
+)
+
+func f1() {
+	_ = *intp // ERROR "generated nil check"
+	
+	// This one should be removed but the block copy needs
+	// to be turned into its own pseudo-op in order to see
+	// the indirect.
+	_ = *arrayp // ERROR "generated nil check"
+	
+	// 0-byte indirect doesn't suffice
+	_ = *array0p // ERROR "generated nil check"
+	_ = *array0p // ERROR "removed repeated nil check" 386
+
+	_ = *intp // ERROR "removed repeated nil check"
+	_ = *arrayp // ERROR "removed repeated nil check"
+	_ = *structp // ERROR "generated nil check"
+	_ = *emptyp // ERROR "generated nil check"
+	_ = *arrayp // ERROR "removed repeated nil check"
+}
+
+func f2() {
+	var (
+		intp *int
+		arrayp *[10]int
+		array0p *[0]int
+		bigarrayp *[1<<20]int
+		structp *Struct
+		bigstructp *BigStruct
+		emptyp *Empty
+		empty1p *Empty1
+	)
+
+	_ = *intp // ERROR "generated nil check"
+	_ = *arrayp // ERROR "generated nil check"
+	_ = *array0p // ERROR "generated nil check"
+	_ = *array0p // ERROR "removed repeated nil check"
+	_ = *intp // ERROR "removed repeated nil check"
+	_ = *arrayp // ERROR "removed repeated nil check"
+	_ = *structp // ERROR "generated nil check"
+	_ = *emptyp // ERROR "generated nil check"
+	_ = *arrayp // ERROR "removed repeated nil check"
+	_ = *bigarrayp // ERROR "generated nil check" ARM removed nil check before indirect!!
+	_ = *bigstructp // ERROR "generated nil check"
+	_ = *empty1p // ERROR "generated nil check"
+}
+
+func fx10k() *[10000]int
+var b bool
+
+
+func f3(x *[10000]int) {
+	// Using a huge type and huge offsets so the compiler
+	// does not expect the memory hardware to fault.
+	_ = x[9999] // ERROR "generated nil check"
+	
+	for {
+		if x[9999] != 0 { // ERROR "generated nil check"
+			break
+		}
+	}
+	
+	x = fx10k() 
+	_ = x[9999] // ERROR "generated nil check"
+	if b {
+		_ = x[9999] // ERROR "removed repeated nil check"
+	} else {
+		_ = x[9999] // ERROR "removed repeated nil check"
+	}	
+	_ = x[9999] // ERROR "generated nil check"
+
+	x = fx10k() 
+	if b {
+		_ = x[9999] // ERROR "generated nil check"
+	} else {
+		_ = x[9999] // ERROR "generated nil check"
+	}	
+	_ = x[9999] // ERROR "generated nil check"
+	
+	fx10k()
+	// This one is a bit redundant, if we figured out that
+	// x wasn't going to change across the function call.
+	// But it's a little complex to do and in practice doesn't
+	// matter enough.
+	_ = x[9999] // ERROR "generated nil check"
+}
+
+func f3a() {
+	x := fx10k()
+	y := fx10k()
+	z := fx10k()
+	_ = &x[9] // ERROR "generated nil check"
+	y = z
+	_ = &x[9] // ERROR "removed repeated nil check"
+	x = y
+	_ = &x[9] // ERROR "generated nil check"
+}
+
+func f3b() {
+	x := fx10k()
+	y := fx10k()
+	_ = &x[9] // ERROR "generated nil check"
+	y = x
+	_ = &x[9] // ERROR "removed repeated nil check"
+	x = y
+	_ = &x[9] // ERROR "removed repeated nil check"
+}
+
+func fx10() *[10]int 
+
+func f4(x *[10]int) {
+	// Most of these have no checks because a real memory reference follows,
+	// and the offset is small enough that if x is nil, the address will still be
+	// in the first unmapped page of memory.
+
+	_ = x[9] // ERROR "removed nil check before indirect"
+	
+	for {
+		if x[9] != 0 { // ERROR "removed nil check before indirect"
+			break
+		}
+	}
+	
+	x = fx10() 
+	_ = x[9] // ERROR "removed nil check before indirect"
+	if b {
+		_ = x[9] // ERROR "removed nil check before indirect"
+	} else {
+		_ = x[9] // ERROR "removed nil check before indirect"
+	}
+	_ = x[9] // ERROR "removed nil check before indirect"
+
+	x = fx10() 
+	if b {
+		_ = x[9] // ERROR "removed nil check before indirect"
+	} else {
+		_ = &x[9] // ERROR "generated nil check"
+	}	
+	_ = x[9] // ERROR "removed nil check before indirect"
+	
+	fx10()
+	_ = x[9] // ERROR "removed nil check before indirect"
+	
+	x = fx10()
+	y := fx10()
+	_ = &x[9] // ERROR "generated nil check"
+	y = x
+	_ = &x[9] // ERROR "removed repeated nil check"
+	x = y
+	_ = &x[9] // ERROR "removed repeated nil check"
+}
+