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 #6019.

R=ken2
CC=golang-dev
https://golang.org/cl/13334052

src/cmd/5g/ggen.c

@@ -889,7 +889,7 @@ expandchecks(Prog *firstp)
 		if(p->as != ACHECKNIL)
 			continue;
 		if(debug_checknil && p->lineno > 1) // p->lineno==1 in generated wrappers
-			warnl(p->lineno, "nil check %D", &p->from);
+			warnl(p->lineno, "generated nil check");
 		if(p->from.type != D_REG)
 			fatal("invalid nil check %P", p);
 		reg = p->from.reg;

src/cmd/5g/peep.c

@@ -1193,6 +1193,20 @@ copyas(Adr *a, Adr *v)
 	return 0;
 }
 
+int
+sameaddr(Adr *a, Adr *v)
+{
+	if(a->type != v->type)
+		return 0;
+	if(regtyp(v) && a->reg == v->reg)
+		return 1;
+	if(v->type == D_AUTO || v->type == D_PARAM) {
+		if(v->offset == a->offset)
+			return 1;
+	}
+	return 0;
+}
+
 /*
  * either direct or indirect
  */
@@ -1525,3 +1539,17 @@ isdconst(Addr *a)
 		return 1;
 	return 0;
 }
+
+int
+stackaddr(Addr *a)
+{
+	return regtyp(a) && a->reg == REGSP;
+}
+
+int
+smallindir(Addr *a, Addr *reg)
+{
+	return reg->type == D_REG && a->type == D_OREG &&
+		a->reg == reg->reg &&
+		0 <= a->offset && a->offset < 4096;
+}

src/cmd/5g/prog.c

@@ -100,7 +100,7 @@ static ProgInfo progtable[ALAST] = {
 	[AMOVHU]=		{SizeW | LeftRead | RightWrite | Conv},
 	
 	// Jumps.
-	[AB]=		{Jump},
+	[AB]=		{Jump | Break},
 	[ABL]=		{Call},
 	[ABEQ]=		{Cjmp},
 	[ABNE]=		{Cjmp},

src/cmd/5g/reg.c

@@ -1293,6 +1293,10 @@ dumpit(char *str, Flow *r0, int isreg)
 			print("	pred:");
 			for(; r1 != nil; r1 = r1->p2link)
 				print(" %.4ud", r1->prog->loc);
+			if(r->p1 != nil)
+				print(" (and %.4ud)", r->p1->prog->loc);
+			else
+				print(" (only)");
 			print("\n");
 		}
 //		r1 = r->s1;

src/cmd/6g/cgen.c

@@ -1015,6 +1015,7 @@ igen(Node *n, Node *a, Node *res)
 			fixlargeoffset(a);
 			return;
 		}
+		break;
 	}
 
 	agenr(n, a, res);

src/cmd/6g/ggen.c

@@ -1081,7 +1081,7 @@ expandchecks(Prog *firstp)
 		if(p->as != ACHECKNIL)
 			continue;
 		if(debug_checknil && p->lineno > 1) // p->lineno==1 in generated wrappers
-			warnl(p->lineno, "nil check %D", &p->from);
+			warnl(p->lineno, "generated nil check");
 		// check is
 		//	CMP arg, $0
 		//	JNE 2(PC) (likely)

src/cmd/6g/peep.c

@@ -844,6 +844,19 @@ copyas(Adr *a, Adr *v)
 	return 0;
 }
 
+int
+sameaddr(Addr *a, Addr *v)
+{
+	if(a->type != v->type)
+		return 0;
+	if(regtyp(v))
+		return 1;
+	if(v->type == D_AUTO || v->type == D_PARAM)
+		if(v->offset == a->offset)
+			return 1;
+	return 0;
+}
+
 /*
  * either direct or indirect
  */
@@ -951,3 +964,18 @@ loop:
 		break;
 	}
 }
+
+int
+smallindir(Addr *a, Addr *reg)
+{
+	return regtyp(reg) &&
+		a->type == D_INDIR + reg->type &&
+		a->index == D_NONE &&
+		0 <= a->offset && a->offset < 4096;
+}
+
+int
+stackaddr(Addr *a)
+{
+	return regtyp(a) && a->type == D_SP;
+}

src/cmd/8g/cgen.c

@@ -857,7 +857,35 @@ igen(Node *n, Node *a, Node *res)
 		a->xoffset = fp->width;
 		a->type = n->type;
 		return;
+
+	case OINDEX:
+		// Index of fixed-size array by constant can
+		// put the offset in the addressing.
+		// Could do the same for slice except that we need
+		// to use the real index for the bounds checking.
+		if(isfixedarray(n->left->type) ||
+		   (isptr[n->left->type->etype] && isfixedarray(n->left->left->type)))
+		if(isconst(n->right, CTINT)) {
+			// Compute &a.
+			if(!isptr[n->left->type->etype])
+				igen(n->left, a, res);
+			else {
+				igen(n->left, &n1, res);
+				cgen_checknil(&n1);
+				regalloc(a, types[tptr], res);
+				gmove(&n1, a);
+				regfree(&n1);
+				a->op = OINDREG;
+			}
+
+			// Compute &a[i] as &a + i*width.
+			a->type = n->type;
+			a->xoffset += mpgetfix(n->right->val.u.xval)*n->type->width;
+			return;
+		}
+		break;
 	}
+
 	// release register for now, to avoid
 	// confusing tempname.
 	if(res != N && res->op == OREGISTER)

src/cmd/8g/ggen.c

@@ -1232,7 +1232,7 @@ expandchecks(Prog *firstp)
 		if(p->as != ACHECKNIL)
 			continue;
 		if(debug_checknil && p->lineno > 1) // p->lineno==1 in generated wrappers
-			warnl(p->lineno, "nil check %D", &p->from);
+			warnl(p->lineno, "generated nil check");
 		// check is
 		//	CMP arg, $0
 		//	JNE 2(PC) (likely)

src/cmd/8g/peep.c

@@ -640,6 +640,19 @@ copyas(Adr *a, Adr *v)
 	return 0;
 }
 
+int
+sameaddr(Addr *a, Addr *v)
+{
+	if(a->type != v->type)
+		return 0;
+	if(regtyp(v))
+		return 1;
+	if(v->type == D_AUTO || v->type == D_PARAM)
+		if(v->offset == a->offset)
+			return 1;
+	return 0;
+}
+
 /*
  * either direct or indirect
  */
@@ -738,3 +751,18 @@ loop:
 		break;
 	}
 }
+
+int
+smallindir(Addr *a, Addr *reg)
+{
+	return regtyp(reg) &&
+		a->type == D_INDIR + reg->type &&
+		a->index == D_NONE &&
+		0 <= a->offset && a->offset < 4096;
+}
+
+int
+stackaddr(Addr *a)
+{
+	return regtyp(a) && a->type == D_SP;
+}

src/cmd/gc/gen.c

@@ -629,6 +629,10 @@ cgen_discard(Node *nr)
 	case OPLUS:
 		cgen_discard(nr->left);
 		break;
+	
+	case OIND:
+		cgen_checknil(nr->left);
+		break;
 
 	// special enough to just evaluate
 	default:

src/cmd/gc/pgen.c

@@ -181,6 +181,7 @@ compile(Node *fn)
 
 	if(!debug['N'] || debug['R'] || debug['P']) {
 		regopt(ptxt);
+		nilopt(ptxt);
 	}
 	expandchecks(ptxt);
 
@@ -537,8 +538,11 @@ cgen_checknil(Node *n)
 
 	if(disable_checknil)
 		return;
-	while(n->op == ODOT || (n->op == OINDEX && isfixedarray(n->left->type->type))) // NOTE: not ODOTPTR
-		n = n->left;
+	// Ideally we wouldn't see any TUINTPTR here, but we do.
+	if(n->type == T || (!isptr[n->type->etype] && n->type->etype != TUINTPTR && n->type->etype != TUNSAFEPTR)) {
+		dump("checknil", n);
+		fatal("bad checknil");
+	}
 	if((thechar == '5' && n->op != OREGISTER) || !n->addable) {
 		regalloc(&reg, types[tptr], n);
 		cgen(n, &reg);

src/cmd/gc/popt.c

@@ -764,3 +764,187 @@ mergewalk(TempVar *v, TempFlow *r0, uint32 gen)
 		for(r2 = (TempFlow*)r->f.p2; r2 != nil; r2 = (TempFlow*)r2->f.p2link)
 			mergewalk(v, r2, gen);
 }
+
+// Eliminate redundant nil pointer checks.
+//
+// The code generation pass emits a CHECKNIL for every possibly nil pointer.
+// This pass removes a CHECKNIL if every predecessor path has already
+// checked this value for nil.
+//
+// Simple backwards flood from check to definition.
+// Run prog loop backward from end of program to beginning to avoid quadratic
+// behavior removing a run of checks.
+//
+// Assume that stack variables with address not taken can be loaded multiple times
+// from memory without being rechecked. Other variables need to be checked on
+// each load.
+	
+typedef struct NilVar NilVar;
+typedef struct NilFlow NilFlow;
+
+struct NilFlow {
+	Flow f;
+	int kill;
+};
+
+static void nilwalkback(NilFlow *rcheck);
+static void nilwalkfwd(NilFlow *rcheck);
+
+void
+nilopt(Prog *firstp)
+{
+	NilFlow *r;
+	Prog *p;
+	uint32 gen;
+	Graph *g;
+	int ncheck, nkill;
+
+	g = flowstart(firstp, sizeof(NilFlow));
+	if(g == nil)
+		return;
+
+	if(debug_checknil > 1 /* || strcmp(curfn->nname->sym->name, "f1") == 0 */)
+		dumpit("nilopt", g->start, 0);
+
+	gen = 0;
+	ncheck = 0;
+	nkill = 0;
+	for(r = (NilFlow*)g->start; r != nil; r = (NilFlow*)r->f.link) {
+		p = r->f.prog;
+		if(p->as != ACHECKNIL || !regtyp(&p->from))
+			continue;
+		ncheck++;
+		if(stackaddr(&p->from)) {
+			if(debug_checknil && p->lineno > 1)
+				warnl(p->lineno, "removed nil check of SP address");
+			r->kill = 1;
+			continue;
+		}
+		nilwalkfwd(r);
+		if(r->kill) {
+			if(debug_checknil && p->lineno > 1)
+				warnl(p->lineno, "removed nil check before indirect");
+			continue;
+		}
+		nilwalkback(r);
+		if(r->kill) {
+			if(debug_checknil && p->lineno > 1)
+				warnl(p->lineno, "removed repeated nil check");
+			continue;
+		}
+	}
+	
+	for(r = (NilFlow*)g->start; r != nil; r = (NilFlow*)r->f.link) {
+		if(r->kill) {
+			nkill++;
+			excise(&r->f);
+		}
+	}
+
+	flowend(g);
+	
+	if(debug_checknil > 1)
+		print("%S: removed %d of %d nil checks\n", curfn->nname->sym, nkill, ncheck);
+}
+
+static void
+nilwalkback(NilFlow *rcheck)
+{
+	Prog *p;
+	ProgInfo info;
+	NilFlow *r;
+	
+	for(r = rcheck; r != nil; r = (NilFlow*)uniqp(&r->f)) {
+		p = r->f.prog;
+		proginfo(&info, p);
+		if((info.flags & RightWrite) && sameaddr(&p->to, &rcheck->f.prog->from)) {
+			// Found initialization of value we're checking for nil.
+			// without first finding the check, so this one is unchecked.
+			return;
+		}
+		if(r != rcheck && p->as == ACHECKNIL && sameaddr(&p->from, &rcheck->f.prog->from)) {
+			rcheck->kill = 1;
+			return;
+		}
+	}
+
+	// Here is a more complex version that scans backward across branches.
+	// It assumes rcheck->kill = 1 has been set on entry, and its job is to find a reason
+	// to keep the check (setting rcheck->kill = 0).
+	// It doesn't handle copying of aggregates as well as I would like,
+	// nor variables with their address taken,
+	// and it's too subtle to turn on this late in Go 1.2. Perhaps for Go 1.3.
+	/*
+	for(r1 = r0; r1 != nil; r1 = (NilFlow*)r1->f.p1) {
+		if(r1->f.active == gen)
+			break;
+		r1->f.active = gen;
+		p = r1->f.prog;
+		
+		// If same check, stop this loop but still check
+		// alternate predecessors up to this point.
+		if(r1 != rcheck && p->as == ACHECKNIL && sameaddr(&p->from, &rcheck->f.prog->from))
+			break;
+
+		proginfo(&info, p);
+		if((info.flags & RightWrite) && sameaddr(&p->to, &rcheck->f.prog->from)) {
+			// Found initialization of value we're checking for nil.
+			// without first finding the check, so this one is unchecked.
+			rcheck->kill = 0;
+			return;
+		}
+		
+		if(r1->f.p1 == nil && r1->f.p2 == nil) {
+			print("lost pred for %P\n", rcheck->f.prog);
+			for(r1=r0; r1!=nil; r1=(NilFlow*)r1->f.p1) {
+				proginfo(&info, r1->f.prog);
+				print("\t%P %d %d %D %D\n", r1->f.prog, info.flags&RightWrite, sameaddr(&r1->f.prog->to, &rcheck->f.prog->from), &r1->f.prog->to, &rcheck->f.prog->from);
+			}
+			fatal("lost pred trail");
+		}
+	}
+
+	for(r = r0; r != r1; r = (NilFlow*)r->f.p1)
+		for(r2 = (NilFlow*)r->f.p2; r2 != nil; r2 = (NilFlow*)r2->f.p2link)
+			nilwalkback(rcheck, r2, gen);
+	*/
+}
+
+static void
+nilwalkfwd(NilFlow *rcheck)
+{
+	NilFlow *r;
+	Prog *p;
+	ProgInfo info;
+	
+	// If the path down from rcheck dereferences the address
+	// (possibly with a small offset) before writing to memory
+	// and before any subsequent checks, it's okay to wait for
+	// that implicit check. Only consider this basic block to
+	// avoid problems like:
+	//	_ = *x // should panic
+	//	for {} // no writes but infinite loop may be considered visible
+	for(r = (NilFlow*)uniqs(&rcheck->f); r != nil; r = (NilFlow*)uniqs(&r->f)) {
+		p = r->f.prog;
+		proginfo(&info, p);
+		
+		if((info.flags & LeftRead) && smallindir(&p->from, &rcheck->f.prog->from)) {
+			rcheck->kill = 1;
+			return;
+		}
+		if((info.flags & (RightRead|RightWrite)) && smallindir(&p->to, &rcheck->f.prog->from)) {
+			rcheck->kill = 1;
+			return;
+		}
+		
+		// Stop if another nil check happens.
+		if(p->as == ACHECKNIL)
+			return;
+		// Stop if value is lost.
+		if((info.flags & RightWrite) && sameaddr(&p->to, &rcheck->f.prog->from))
+			return;
+		// Stop if memory write.
+		if((info.flags & RightWrite) && !regtyp(&p->to))
+			return;
+	}
+}

src/cmd/gc/popt.h

@@ -36,7 +36,11 @@ Graph*	flowstart(Prog*, int);
 void	flowrpo(Graph*);
 void	flowend(Graph*);
 void	mergetemp(Prog*);
+void	nilopt(Prog*);
 int	noreturn(Prog*);
 int	regtyp(Addr*);
+int	sameaddr(Addr*, Addr*);
+int	smallindir(Addr*, Addr*);
+int	stackaddr(Addr*);
 Flow*	uniqp(Flow*);
 Flow*	uniqs(Flow*);

src/cmd/gc/walk.c

@@ -407,11 +407,6 @@ walkexpr(Node **np, NodeList **init)
 		goto ret;
 
 	case OIND:
-		if(n->left->type->type->width == 0) {
-			// No actual copy will be generated, so emit an explicit nil check.
-			n->left = cheapexpr(n->left, init);
-			checknil(n->left, init);
-		}
 		walkexpr(&n->left, init);
 		goto ret;
 

test/nilptr3.go

+// 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"
+}
+