proc: avoid constructing unnecessary strings when evaluating variables

Avoids constructing:

1. name of runtime.curg fields while executing parseG
2. the location expression while evaluating any variable.

Benchmark before:

BenchmarkConditionalBreakpoints-4   	       1	4953889884 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	4419775128 ns/op

Updates #1549

pkg/proc/bininfo.go

@@ -809,29 +809,51 @@ func (bi *BinaryInfo) LoadImageFromData(dwdata *dwarf.Data, debugFrameBytes, deb
 	bi.Images = append(bi.Images, image)
 }
 
-func (bi *BinaryInfo) locationExpr(entry godwarf.Entry, attr dwarf.Attr, pc uint64) ([]byte, string, error) {
+func (bi *BinaryInfo) locationExpr(entry godwarf.Entry, attr dwarf.Attr, pc uint64) ([]byte, *locationExpr, error) {
 	a := entry.Val(attr)
 	if a == nil {
-		return nil, "", fmt.Errorf("no location attribute %s", attr)
+		return nil, nil, fmt.Errorf("no location attribute %s", attr)
 	}
 	if instr, ok := a.([]byte); ok {
-		var descr bytes.Buffer
-		fmt.Fprintf(&descr, "[block] ")
-		op.PrettyPrint(&descr, instr)
-		return instr, descr.String(), nil
+		return instr, &locationExpr{isBlock: true, instr: instr}, nil
 	}
 	off, ok := a.(int64)
 	if !ok {
-		return nil, "", fmt.Errorf("could not interpret location attribute %s", attr)
+		return nil, nil, fmt.Errorf("could not interpret location attribute %s", attr)
 	}
 	instr := bi.loclistEntry(off, pc)
 	if instr == nil {
-		return nil, "", fmt.Errorf("could not find loclist entry at %#x for address %#x", off, pc)
+		return nil, nil, fmt.Errorf("could not find loclist entry at %#x for address %#x", off, pc)
+	}
+	return instr, &locationExpr{pc: pc, off: off, instr: instr}, nil
+}
+
+type locationExpr struct {
+	isBlock   bool
+	isEscaped bool
+	off       int64
+	pc        uint64
+	instr     []byte
+}
+
+func (le *locationExpr) String() string {
+	if le == nil {
+		return ""
 	}
 	var descr bytes.Buffer
-	fmt.Fprintf(&descr, "[%#x:%#x] ", off, pc)
-	op.PrettyPrint(&descr, instr)
-	return instr, descr.String(), nil
+
+	if le.isBlock {
+		fmt.Fprintf(&descr, "[block] ")
+		op.PrettyPrint(&descr, le.instr)
+	} else {
+		fmt.Fprintf(&descr, "[%#x:%#x] ", le.off, le.pc)
+		op.PrettyPrint(&descr, le.instr)
+	}
+
+	if le.isEscaped {
+		fmt.Fprintf(&descr, " (escaped)")
+	}
+	return descr.String()
 }
 
 // LocationCovers returns the list of PC addresses that is covered by the
@@ -877,10 +899,10 @@ func (bi *BinaryInfo) LocationCovers(entry *dwarf.Entry, attr dwarf.Attr) ([][2]
 // This will either be an int64 address or a slice of Pieces for locations
 // that don't correspond to a single memory address (registers, composite
 // locations).
-func (bi *BinaryInfo) Location(entry godwarf.Entry, attr dwarf.Attr, pc uint64, regs op.DwarfRegisters) (int64, []op.Piece, string, error) {
+func (bi *BinaryInfo) Location(entry godwarf.Entry, attr dwarf.Attr, pc uint64, regs op.DwarfRegisters) (int64, []op.Piece, *locationExpr, error) {
 	instr, descr, err := bi.locationExpr(entry, attr, pc)
 	if err != nil {
-		return 0, nil, "", err
+		return 0, nil, nil, err
 	}
 	addr, pieces, err := op.ExecuteStackProgram(regs, instr, bi.Arch.PtrSize())
 	return addr, pieces, descr, err

pkg/proc/eval.go

@@ -266,7 +266,8 @@ func (scope *EvalScope) Locals() ([]*Variable, error) {
 			}
 			v.Name = name[1:]
 			v.Flags |= VariableEscaped
-			v.LocationExpr = locationExpr + " (escaped)"
+			locationExpr.isEscaped = true
+			v.LocationExpr = locationExpr
 			v.DeclLine = declLine
 			vars[i] = v
 		}

pkg/proc/variables.go

@@ -118,8 +118,8 @@ type Variable struct {
 	loaded     bool
 	Unreadable error
 
-	LocationExpr string // location expression
-	DeclLine     int64  // line number of this variable's declaration
+	LocationExpr *locationExpr // location expression
+	DeclLine     int64         // line number of this variable's declaration
 }
 
 // LoadConfig controls how variables are loaded from the targets memory.
@@ -441,8 +441,6 @@ func newGVariable(thread Thread, gaddr uintptr, deref bool) (*Variable, error) {
 		return nil, err
 	}
 
-	name := ""
-
 	if deref {
 		typ = &godwarf.PtrType{
 			CommonType: godwarf.CommonType{
@@ -453,11 +451,9 @@ func newGVariable(thread Thread, gaddr uintptr, deref bool) (*Variable, error) {
 			},
 			Type: typ,
 		}
-	} else {
-		name = "runtime.curg"
 	}
 
-	return newVariableFromThread(thread, name, gaddr, typ), nil
+	return newVariableFromThread(thread, "", gaddr, typ), nil
 }
 
 // Defer returns the top-most defer of the goroutine.
@@ -851,6 +847,8 @@ func (v *Variable) parseG() (*G, error) {
 
 	f, l, fn := v.bi.PCToLine(uint64(pc))
 
+	v.Name = "runtime.curg"
+
 	g := &G{
 		ID:         int(id),
 		GoPC:       uint64(gopc),

service/api/conversions.go

@@ -148,7 +148,7 @@ func ConvertVar(v *proc.Variable) *Variable {
 		Flags:    VariableFlags(v.Flags),
 		Base:     v.Base,
 
-		LocationExpr: v.LocationExpr,
+		LocationExpr: v.LocationExpr.String(),
 		DeclLine:     v.DeclLine,
 	}