proc: optimize parseG

runtime.g is a large and growing struct, we only need a few fields.
Instead of using loadValue to load the full contents of g cache its
memory and then only load the fields we care about.

Benchmark before:

BenchmarkConditionalBreakpoints-4              1        10013510647 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	9330025748 ns/op

Conditional breakpoint evaluation: 1.0ms -> 0.93ms

Updates go-delve#1549

pkg/proc/stack.go

@@ -534,11 +534,14 @@ func (it *stackIterator) loadG0SchedSP() {
 	}
 	it.g0_sched_sp_loaded = true
 	if it.g != nil {
-		g0var, _ := it.g.variable.fieldVariable("m").structMember("g0")
-		if g0var != nil {
-			g0, _ := g0var.parseG()
-			if g0 != nil {
-				it.g0_sched_sp = g0.SP
+		mvar, _ := it.g.variable.structMember("m")
+		if mvar != nil {
+			g0var, _ := mvar.structMember("g0")
+			if g0var != nil {
+				g0, _ := g0var.parseG()
+				if g0 != nil {
+					it.g0_sched_sp = g0.SP
+				}
 			}
 		}
 	}

pkg/proc/threads.go

@@ -503,7 +503,11 @@ func GetG(thread Thread) (*G, error) {
 		// For our purposes it's better if we always return the real goroutine
 		// since the rest of the code assumes the goroutine ID is univocal.
 		// The real 'current goroutine' is stored in g0.m.curg
-		curgvar, err := g.variable.fieldVariable("m").structMember("curg")
+		mvar, err := g.variable.structMember("m")
+		if err != nil {
+			return nil, err
+		}
+		curgvar, err := mvar.structMember("curg")
 		if err != nil {
 			return nil, err
 		}

pkg/proc/variables.go

@@ -186,19 +186,18 @@ const (
 // G represents a runtime G (goroutine) structure (at least the
 // fields that Delve is interested in).
 type G struct {
-	ID         int    // Goroutine ID
-	PC         uint64 // PC of goroutine when it was parked.
-	SP         uint64 // SP of goroutine when it was parked.
-	BP         uint64 // BP of goroutine when it was parked (go >= 1.7).
-	LR         uint64 // LR of goroutine when it was parked.
-	GoPC       uint64 // PC of 'go' statement that created this goroutine.
-	StartPC    uint64 // PC of the first function run on this goroutine.
-	WaitReason string // Reason for goroutine being parked.
-	Status     uint64
-	stkbarVar  *Variable // stkbar field of g struct
-	stkbarPos  int       // stkbarPos field of g struct
-	stackhi    uint64    // value of stack.hi
-	stacklo    uint64    // value of stack.lo
+	ID        int    // Goroutine ID
+	PC        uint64 // PC of goroutine when it was parked.
+	SP        uint64 // SP of goroutine when it was parked.
+	BP        uint64 // BP of goroutine when it was parked (go >= 1.7).
+	LR        uint64 // LR of goroutine when it was parked.
+	GoPC      uint64 // PC of 'go' statement that created this goroutine.
+	StartPC   uint64 // PC of the first function run on this goroutine.
+	Status    uint64
+	stkbarVar *Variable // stkbar field of g struct
+	stkbarPos int       // stkbarPos field of g struct
+	stackhi   uint64    // value of stack.hi
+	stacklo   uint64    // value of stack.lo
 
 	SystemStack bool // SystemStack is true if this goroutine is currently executing on a system stack.
 
@@ -220,7 +219,11 @@ func (g *G) Defer() *Defer {
 	if g.variable.Unreadable != nil {
 		return nil
 	}
-	dvar := g.variable.fieldVariable("_defer").maybeDereference()
+	dvar, _ := g.variable.structMember("_defer")
+	if dvar == nil {
+		return nil
+	}
+	dvar = dvar.maybeDereference()
 	if dvar.Addr == 0 {
 		return nil
 	}
@@ -518,6 +521,8 @@ func (ng ErrNoGoroutine) Error() string {
 	return fmt.Sprintf("no G executing on thread %d", ng.tid)
 }
 
+var ErrUnreadableG = errors.New("could not read G struct")
+
 func (v *Variable) parseG() (*G, error) {
 	mem := v.mem
 	gaddr := uint64(v.Addr)
@@ -544,11 +549,13 @@ func (v *Variable) parseG() (*G, error) {
 		}
 		v = v.maybeDereference()
 	}
-	v.loadValue(LoadConfig{false, 2, 64, 0, -1, 0})
-	if v.Unreadable != nil {
-		return nil, v.Unreadable
+
+	v.mem = cacheMemory(v.mem, v.Addr, int(v.RealType.Size()))
+
+	schedVar := v.loadFieldNamed("sched")
+	if schedVar == nil {
+		return nil, ErrUnreadableG
 	}
-	schedVar := v.fieldVariable("sched")
 	pc, _ := constant.Int64Val(schedVar.fieldVariable("pc").Value)
 	sp, _ := constant.Int64Val(schedVar.fieldVariable("sp").Value)
 	var bp, lr int64
@@ -558,21 +565,24 @@ func (v *Variable) parseG() (*G, error) {
 	if bpvar := schedVar.fieldVariable("lr"); bpvar != nil && bpvar.Value != nil {
 		lr, _ = constant.Int64Val(bpvar.Value)
 	}
-	id, _ := constant.Int64Val(v.fieldVariable("goid").Value)
-	gopc, _ := constant.Int64Val(v.fieldVariable("gopc").Value)
-	startpc, _ := constant.Int64Val(v.fieldVariable("startpc").Value)
-	waitReason := ""
-	if wrvar := v.fieldVariable("waitreason"); wrvar.Value != nil {
-		switch wrvar.Kind {
-		case reflect.String:
-			waitReason = constant.StringVal(wrvar.Value)
-		case reflect.Uint:
-			waitReason = wrvar.ConstDescr()
-		}
 
+	unreadable := false
+
+	loadInt64Maybe := func(name string) int64 {
+		vv := v.loadFieldNamed(name)
+		if vv == nil {
+			unreadable = true
+			return 0
+		}
+		n, _ := constant.Int64Val(vv.Value)
+		return n
 	}
+
+	id := loadInt64Maybe("goid")
+	gopc := loadInt64Maybe("gopc")
+	startpc := loadInt64Maybe("startpc")
 	var stackhi, stacklo uint64
-	if stackVar := v.fieldVariable("stack"); stackVar != nil {
+	if stackVar := v.loadFieldNamed("stack"); stackVar != nil {
 		if stackhiVar := stackVar.fieldVariable("hi"); stackhiVar != nil {
 			stackhi, _ = constant.Uint64Val(stackhiVar.Value)
 		}
@@ -581,14 +591,19 @@ func (v *Variable) parseG() (*G, error) {
 		}
 	}
 
-	stkbarVar, _ := v.structMember("stkbar")
-	stkbarVarPosFld := v.fieldVariable("stkbarPos")
+	stkbarVar := v.loadFieldNamed("stkbar")
+	stkbarVarPosFld := v.loadFieldNamed("stkbarPos")
 	var stkbarPos int64
 	if stkbarVarPosFld != nil { // stack barriers were removed in Go 1.9
 		stkbarPos, _ = constant.Int64Val(stkbarVarPosFld.Value)
 	}
 
-	status, _ := constant.Int64Val(v.fieldVariable("atomicstatus").Value)
+	status := loadInt64Maybe("atomicstatus")
+
+	if unreadable {
+		return nil, ErrUnreadableG
+	}
+
 	f, l, fn := v.bi.PCToLine(uint64(pc))
 
 	g := &G{
@@ -599,7 +614,6 @@ func (v *Variable) parseG() (*G, error) {
 		SP:         uint64(sp),
 		BP:         uint64(bp),
 		LR:         uint64(lr),
-		WaitReason: waitReason,
 		Status:     uint64(status),
 		CurrentLoc: Location{PC: uint64(pc), File: f, Line: l, Fn: fn},
 		variable:   v,
@@ -624,6 +638,9 @@ func (v *Variable) loadFieldNamed(name string) *Variable {
 }
 
 func (v *Variable) fieldVariable(name string) *Variable {
+	if !v.loaded {
+		panic("fieldVariable called on a variable that wasn't loaded")
+	}
 	for i := range v.Children {
 		if child := &v.Children[i]; child.Name == name {
 			return child