cmd/compile: remove duplicate nilchecks

Mark nil check operations as faulting if their arg is zero.
This lets the late nilcheck pass remove duplicates.

Fixes #17242.

Change-Id: I4c9938d8a5a1e43edd85b4a66f0b34004860bcd9
Reviewed-on: https://go-review.googlesource.com/29952
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/386Ops.go

@@ -439,7 +439,7 @@ func init() {
 		// use of DX (the closure pointer)
 		{name: "LoweredGetClosurePtr", reg: regInfo{outputs: []regMask{buildReg("DX")}}},
 		//arg0=ptr,arg1=mem, returns void.  Faults if ptr is nil.
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpsp}}, clobberFlags: true, nilCheck: true},
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpsp}}, clobberFlags: true, nilCheck: true, faultOnNilArg0: true},
 
 		// MOVLconvert converts between pointers and integers.
 		// We have a special op for this so as to not confuse GC

src/cmd/compile/internal/ssa/gen/AMD64Ops.go

@@ -486,7 +486,7 @@ func init() {
 		// use of DX (the closure pointer)
 		{name: "LoweredGetClosurePtr", reg: regInfo{outputs: []regMask{buildReg("DX")}}},
 		//arg0=ptr,arg1=mem, returns void.  Faults if ptr is nil.
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpsp}}, clobberFlags: true, nilCheck: true},
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpsp}}, clobberFlags: true, nilCheck: true, faultOnNilArg0: true},
 
 		// MOVQconvert converts between pointers and integers.
 		// We have a special op for this so as to not confuse GC

src/cmd/compile/internal/ssa/gen/ARM64Ops.go

@@ -325,7 +325,7 @@ func init() {
 		{name: "CALLinter", argLength: 2, reg: regInfo{inputs: []regMask{gp}, clobbers: callerSave}, aux: "Int64", clobberFlags: true, call: true},                         // call fn by pointer.  arg0=codeptr, arg1=mem, auxint=argsize, returns mem
 
 		// pseudo-ops
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpg}}, nilCheck: true}, // panic if arg0 is nil.  arg1=mem.
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpg}}, nilCheck: true, faultOnNilArg0: true}, // panic if arg0 is nil.  arg1=mem.
 
 		{name: "Equal", argLength: 1, reg: readflags},         // bool, true flags encode x==y false otherwise.
 		{name: "NotEqual", argLength: 1, reg: readflags},      // bool, true flags encode x!=y false otherwise.

src/cmd/compile/internal/ssa/gen/ARMOps.go

@@ -381,7 +381,7 @@ func init() {
 		{name: "CALLinter", argLength: 2, reg: regInfo{inputs: []regMask{gp}, clobbers: callerSave}, aux: "Int64", clobberFlags: true, call: true},                        // call fn by pointer.  arg0=codeptr, arg1=mem, auxint=argsize, returns mem
 
 		// pseudo-ops
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpg}}, nilCheck: true}, // panic if arg0 is nil.  arg1=mem.
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpg}}, nilCheck: true, faultOnNilArg0: true}, // panic if arg0 is nil.  arg1=mem.
 
 		{name: "Equal", argLength: 1, reg: readflags},         // bool, true flags encode x==y false otherwise.
 		{name: "NotEqual", argLength: 1, reg: readflags},      // bool, true flags encode x!=y false otherwise.

src/cmd/compile/internal/ssa/gen/MIPS64Ops.go

@@ -336,7 +336,7 @@ func init() {
 		},
 
 		// pseudo-ops
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpg}}, nilCheck: true}, // panic if arg0 is nil.  arg1=mem.
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gpg}}, nilCheck: true, faultOnNilArg0: true}, // panic if arg0 is nil.  arg1=mem.
 
 		{name: "FPFlagTrue", argLength: 1, reg: readflags},  // bool, true if FP flag is true
 		{name: "FPFlagFalse", argLength: 1, reg: readflags}, // bool, true if FP flag is false

src/cmd/compile/internal/ssa/gen/PPC64Ops.go

@@ -288,7 +288,7 @@ func init() {
 		{name: "LoweredGetClosurePtr", reg: regInfo{outputs: []regMask{ctxt}}},
 
 		//arg0=ptr,arg1=mem, returns void.  Faults if ptr is nil.
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gp | sp | sb}, clobbers: tmp}, clobberFlags: true, nilCheck: true},
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{gp | sp | sb}, clobbers: tmp}, clobberFlags: true, nilCheck: true, faultOnNilArg0: true},
 
 		// Convert pointer to integer, takes a memory operand for ordering.
 		{name: "MOVDconvert", argLength: 2, reg: gp11, asm: "MOVD"},

src/cmd/compile/internal/ssa/gen/S390XOps.go

@@ -370,7 +370,7 @@ func init() {
 		// use of R12 (the closure pointer)
 		{name: "LoweredGetClosurePtr", reg: regInfo{outputs: []regMask{buildReg("R12")}}},
 		// arg0=ptr,arg1=mem, returns void.  Faults if ptr is nil.
-		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{ptrsp}}, clobberFlags: true, nilCheck: true},
+		{name: "LoweredNilCheck", argLength: 2, reg: regInfo{inputs: []regMask{ptrsp}}, clobberFlags: true, nilCheck: true, faultOnNilArg0: true},
 
 		// MOVDconvert converts between pointers and integers.
 		// We have a special op for this so as to not confuse GC

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

@@ -4023,10 +4023,11 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:         "LoweredNilCheck",
-		argLen:       2,
-		clobberFlags: true,
-		nilCheck:     true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		clobberFlags:   true,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 255}, // AX CX DX BX SP BP SI DI
@@ -6992,10 +6993,11 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:         "LoweredNilCheck",
-		argLen:       2,
-		clobberFlags: true,
-		nilCheck:     true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		clobberFlags:   true,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 65535}, // AX CX DX BX SP BP SI DI R8 R9 R10 R11 R12 R13 R14 R15
@@ -10081,9 +10083,10 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:     "LoweredNilCheck",
-		argLen:   2,
-		nilCheck: true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 6143}, // R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 g R12
@@ -12200,9 +12203,10 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:     "LoweredNilCheck",
-		argLen:   2,
-		nilCheck: true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 268173311}, // R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R19 R20 R21 R22 R23 R24 R25 R26 g
@@ -13817,9 +13821,10 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:     "LoweredNilCheck",
-		argLen:   2,
-		nilCheck: true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 100663294}, // R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R24 R25 g
@@ -15195,10 +15200,11 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:         "LoweredNilCheck",
-		argLen:       2,
-		clobberFlags: true,
-		nilCheck:     true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		clobberFlags:   true,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 1073733630}, // SP SB R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29
@@ -17511,10 +17517,11 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:         "LoweredNilCheck",
-		argLen:       2,
-		clobberFlags: true,
-		nilCheck:     true,
+		name:           "LoweredNilCheck",
+		argLen:         2,
+		clobberFlags:   true,
+		nilCheck:       true,
+		faultOnNilArg0: true,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 37886}, // R1 R2 R3 R4 R5 R6 R7 R8 R9 R12 SP

test/nilptr3.go

@@ -233,3 +233,17 @@ func c1() {
 	var x Struct
 	func() { x.m() }() // ERROR "removed nil check"
 }
+
+type SS struct {
+	x byte
+}
+
+type TT struct {
+	SS
+}
+
+func f(t *TT) *byte {
+	// See issue 17242.
+	s := &t.SS  // ERROR "removed nil check"
+	return &s.x // ERROR "generated nil check"
+}