[dev.ssa] cmd/compile: make failed nil checks panic

Introduce pseudo-ops PanicMem and LoweredPanicMem.
PanicMem could be rewritten directly into MOVL
during lowering, but then we couldn't log nil checks.

With this change, runnable nil check tests pass:

GOSSAPKG=main go run run.go -- nil*.go

Compiler output nil check tests fail:

GOSSAPKG=p go run run.go -- nil*.go

This is due to several factors:

* SSA has improved elimination of unnecessary nil checks.
* SSA is missing elimination of implicit nil checks.
* SSA is missing extra logging about why nil checks were removed.

I'm not sure how best to resolve these failures,
particularly in a world in which the two backends
will live side by side for some time.
For now, punt on the problem.

Change-Id: Ib2ca6824551671f92e0e1800b036f5ca0905e2a3
Reviewed-on: https://go-review.googlesource.com/13474Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1499,20 +1499,27 @@ func canSSA(n *Node) bool {
 }
 
 // nilCheck generates nil pointer checking code.
-// Starts a new block on return.
+// Starts a new block on return, unless nil checks are disabled.
 // Used only for automatically inserted nil checks,
 // not for user code like 'x != nil'.
 func (s *state) nilCheck(ptr *ssa.Value) {
+	if Disable_checknil != 0 {
+		return
+	}
 	c := s.newValue1(ssa.OpIsNonNil, Types[TBOOL], ptr)
 	b := s.endBlock()
-	b.Kind = ssa.BlockIf
+	b.Kind = ssa.BlockIf // TODO: likeliness hint
 	b.Control = c
 	bNext := s.f.NewBlock(ssa.BlockPlain)
+	bPanic := s.f.NewBlock(ssa.BlockPlain)
 	addEdge(b, bNext)
-	addEdge(b, s.exit)
-	s.startBlock(bNext)
-	// TODO(khr): Don't go directly to exit.  Go to a stub that calls panicmem first.
+	addEdge(b, bPanic)
+	addEdge(bPanic, s.exit)
+	s.startBlock(bPanic)
 	// TODO: implicit nil checks somehow?
+	s.vars[&memvar] = s.newValue2(ssa.OpPanicNilCheck, ssa.TypeMem, ptr, s.mem())
+	s.endBlock()
+	s.startBlock(bNext)
 }
 
 // boundsCheck generates bounds checking code.  Checks if 0 <= idx < len, branches to exit if not.
@@ -2145,6 +2152,25 @@ func genValue(v *ssa.Value) {
 	case ssa.OpArg:
 		// memory arg needs no code
 		// TODO: check that only mem arg goes here.
+	case ssa.OpAMD64LoweredPanicNilCheck:
+		if Debug_checknil != 0 && v.Line > 1 { // v.Line==1 in generated wrappers
+			Warnl(int(v.Line), "generated nil check")
+		}
+		// Write to memory address 0. It doesn't matter what we write; use AX.
+		// XORL AX, AX; MOVL AX, (AX) is shorter than MOVL AX, 0.
+		// TODO: If we had the pointer (v.Args[0]) in a register r,
+		// we could use MOVL AX, (r) instead of having to zero AX.
+		// But it isn't worth loading r just to accomplish that.
+		p := Prog(x86.AXORL)
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = x86.REG_AX
+		p.To.Type = obj.TYPE_REG
+		p.To.Reg = x86.REG_AX
+		q := Prog(x86.AMOVL)
+		q.From.Type = obj.TYPE_REG
+		q.From.Reg = x86.REG_AX
+		q.To.Type = obj.TYPE_MEM
+		q.To.Reg = x86.REG_AX
 	case ssa.OpAMD64CALLstatic:
 		p := Prog(obj.ACALL)
 		p.To.Type = obj.TYPE_MEM

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -216,6 +216,8 @@
 (IsNonNil p) -> (SETNE (TESTQ <TypeFlags> p p))
 (IsInBounds idx len) -> (SETB (CMPQ <TypeFlags> idx len))
 
+(PanicNilCheck ptr mem) -> (LoweredPanicNilCheck ptr mem)
+
 (Move [size] dst src mem) -> (REPMOVSB dst src (MOVQconst <config.Frontend().TypeUInt64()> [size]) mem)
 
 (Not x) -> (XORBconst [1] x)

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

@@ -287,6 +287,9 @@ func init() {
 		// Rewrites will convert this to (SETG (CMPQ b a)).
 		// InvertFlags is a pseudo-op which can't appear in assembly output.
 		{name: "InvertFlags"}, // reverse direction of arg0
+
+		// LoweredPanicNilCheck is a pseudo-op.
+		{name: "LoweredPanicNilCheck"},
 	}
 
 	var AMD64blocks = []blockData{

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

@@ -252,6 +252,8 @@ var genericOps = []opData{
 	{name: "IsNonNil"},   // arg0 != nil
 	{name: "IsInBounds"}, // 0 <= arg0 < arg1
 
+	{name: "PanicNilCheck"}, // trigger a dereference fault; arg0=nil ptr, arg1=mem
+
 	// Indexing operations
 	{name: "ArrayIndex"},   // arg0=array, arg1=index.  Returns a[i]
 	{name: "PtrIndex"},     // arg0=ptr, arg1=index. Computes ptr+sizeof(*v.type)*index, where index is extended to ptrwidth type

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

@@ -17,9 +17,14 @@ func checkLower(f *Func) {
 	// rules may leave dead generic ops behind).
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
-			if opcodeTable[v.Op].generic && v.Op != OpSP && v.Op != OpSB && v.Op != OpArg && v.Op != OpCopy && v.Op != OpPhi {
-				f.Unimplementedf("%s not lowered", v.LongString())
+			if !opcodeTable[v.Op].generic {
+				continue // lowered
 			}
+			switch v.Op {
+			case OpSP, OpSB, OpArg, OpCopy, OpPhi:
+				continue // ok not to lower
+			}
+			f.Unimplementedf("%s not lowered", v.LongString())
 		}
 	}
 }

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

@@ -194,6 +194,7 @@ const (
 	OpAMD64CALLclosure
 	OpAMD64REPMOVSB
 	OpAMD64InvertFlags
+	OpAMD64LoweredPanicNilCheck
 
 	OpAdd8
 	OpAdd16
@@ -367,6 +368,7 @@ const (
 	OpTrunc64to32
 	OpIsNonNil
 	OpIsInBounds
+	OpPanicNilCheck
 	OpArrayIndex
 	OpPtrIndex
 	OpOffPtr
@@ -2113,6 +2115,10 @@ var opcodeTable = [...]opInfo{
 		name: "InvertFlags",
 		reg:  regInfo{},
 	},
+	{
+		name: "LoweredPanicNilCheck",
+		reg:  regInfo{},
+	},
 
 	{
 		name:    "Add8",
@@ -2802,6 +2808,10 @@ var opcodeTable = [...]opInfo{
 		name:    "IsInBounds",
 		generic: true,
 	},
+	{
+		name:    "PanicNilCheck",
+		generic: true,
+	},
 	{
 		name:    "ArrayIndex",
 		generic: true,

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -4836,6 +4836,24 @@ func rewriteValueAMD64(v *Value, config *Config) bool {
 		goto end6f8a8c559a167d1f0a5901d09a1fb248
 	end6f8a8c559a167d1f0a5901d09a1fb248:
 		;
+	case OpPanicNilCheck:
+		// match: (PanicNilCheck ptr mem)
+		// cond:
+		// result: (LoweredPanicNilCheck ptr mem)
+		{
+			ptr := v.Args[0]
+			mem := v.Args[1]
+			v.Op = OpAMD64LoweredPanicNilCheck
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AddArg(ptr)
+			v.AddArg(mem)
+			return true
+		}
+		goto enda02b1ad5a6f929b782190145f2c8628b
+	enda02b1ad5a6f929b782190145f2c8628b:
+		;
 	case OpRsh16Ux16:
 		// match: (Rsh16Ux16 <t> x y)
 		// cond: