cmd/compile: remove more nil ptr checks after newobject

For code like the following (where x escapes):

   x := []int{1}

We're currently generating a nil check.  The line above is really 3 operations:

	t := new([1]int)
	t[0] = 1
	x := t[:]

We remove the nil check for t[0] = 1, but not for t[:].

Our current nil check removal rule is too strict about the possible
memory arguments of the nil check. Unlike zeroing or storing to the
result of runtime.newobject, the nilness of runtime.newobject is
always false, even after other stores have happened in the meantime.

Change-Id: I95fad4e3a59c27effdb37c43ea215e18f30b1e5f
Reviewed-on: https://go-review.googlesource.com/58711
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -1377,15 +1377,13 @@
 	-> mem
 // nil checks just need to rewrite to something useless.
 // they will be deadcode eliminated soon afterwards.
-(NilCheck (Load (OffPtr [c] (SP)) mem) mem)
-	&& mem.Op == OpStaticCall
-	&& isSameSym(mem.Aux, "runtime.newobject")
+(NilCheck (Load (OffPtr [c] (SP)) (StaticCall {sym} _)) _)
+	&& isSameSym(sym, "runtime.newobject")
 	&& c == config.ctxt.FixedFrameSize() + config.RegSize // offset of return value
 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
 	-> (Invalid)
-(NilCheck (OffPtr (Load (OffPtr [c] (SP)) mem)) mem)
-	&& mem.Op == OpStaticCall
-	&& isSameSym(mem.Aux, "runtime.newobject")
+(NilCheck (OffPtr (Load (OffPtr [c] (SP)) (StaticCall {sym} _))) _)
+	&& isSameSym(sym, "runtime.newobject")
 	&& c == config.ctxt.FixedFrameSize() + config.RegSize // offset of return value
 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
 	-> (Invalid)

src/cmd/compile/internal/ssa/rewritegeneric.go

@@ -16833,8 +16833,8 @@ func rewriteValuegeneric_OpNilCheck_0(v *Value) bool {
 		v.AddArg(mem)
 		return true
 	}
-	// match: (NilCheck (Load (OffPtr [c] (SP)) mem) mem)
-	// cond: mem.Op == OpStaticCall 	&& isSameSym(mem.Aux, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
+	// match: (NilCheck (Load (OffPtr [c] (SP)) (StaticCall {sym} _)) _)
+	// cond: isSameSym(sym, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
 	// result: (Invalid)
 	for {
 		_ = v.Args[1]
@@ -16852,18 +16852,19 @@ func rewriteValuegeneric_OpNilCheck_0(v *Value) bool {
 		if v_0_0_0.Op != OpSP {
 			break
 		}
-		mem := v_0.Args[1]
-		if mem != v.Args[1] {
+		v_0_1 := v_0.Args[1]
+		if v_0_1.Op != OpStaticCall {
 			break
 		}
-		if !(mem.Op == OpStaticCall && isSameSym(mem.Aux, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize && warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")) {
+		sym := v_0_1.Aux
+		if !(isSameSym(sym, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize && warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")) {
 			break
 		}
 		v.reset(OpInvalid)
 		return true
 	}
-	// match: (NilCheck (OffPtr (Load (OffPtr [c] (SP)) mem)) mem)
-	// cond: mem.Op == OpStaticCall 	&& isSameSym(mem.Aux, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
+	// match: (NilCheck (OffPtr (Load (OffPtr [c] (SP)) (StaticCall {sym} _))) _)
+	// cond: isSameSym(sym, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
 	// result: (Invalid)
 	for {
 		_ = v.Args[1]
@@ -16885,11 +16886,12 @@ func rewriteValuegeneric_OpNilCheck_0(v *Value) bool {
 		if v_0_0_0_0.Op != OpSP {
 			break
 		}
-		mem := v_0_0.Args[1]
-		if mem != v.Args[1] {
+		v_0_0_1 := v_0_0.Args[1]
+		if v_0_0_1.Op != OpStaticCall {
 			break
 		}
-		if !(mem.Op == OpStaticCall && isSameSym(mem.Aux, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize && warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")) {
+		sym := v_0_0_1.Aux
+		if !(isSameSym(sym, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize && warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")) {
 			break
 		}
 		v.reset(OpInvalid)

test/nilptr3.go

@@ -259,3 +259,10 @@ func f7() (*Struct, float64) {
 func f8(t *[8]int) [8]int {
 	return *t // ERROR "removed nil check"
 }
+
+func f9() []int {
+	x := new([1]int)
+	x[0] = 1  // ERROR "removed nil check"
+	y := x[:] // ERROR "removed nil check"
+	return y
+}