proc: Replaced FunctionEntryToFirstLine with disassembly alternative

- Unlike FunctionEntryToFirstLine can skip the prologue on functions
that are defined on a single line, either because they weren't
formatted or because they were autogenerated
- Can skip the prologue on most functions when setting a breakpoint
with the filename:line syntax

Fixes #396

_fixtures/callme.go

@@ -0,0 +1,27 @@
+package main
+
+import "fmt"
+
+func callme(i int) {
+	fmt.Println("got:", i)
+}
+
+const nBytes = 10
+var zeroarr [nBytes]byte
+
+func callme2() {
+	for i := 0; i < nBytes; i++ {
+		zeroarr[i] = '0'
+	}
+}
+
+func callme3() {
+	callme2()
+}
+
+func main() {
+	for i := 0; i < 5; i++ {
+		callme(i)
+	}
+	callme3()
+}

proc/disasm.go

@@ -31,10 +31,13 @@ func (thread *Thread) Disassemble(startPC, endPC uint64, currentGoroutine bool)
 	r := make([]AsmInstruction, 0, len(mem)/15)
 	pc := startPC
 
-	regs, _ := thread.Registers()
 	var curpc uint64
-	if regs != nil {
-		curpc = regs.PC()
+	var regs Registers
+	if currentGoroutine {
+		regs, _ = thread.Registers()
+		if regs != nil {
+			curpc = regs.PC()
+		}
 	}
 
 	for len(mem) > 0 {

proc/disasm_amd64.go

@@ -1,6 +1,7 @@
 package proc
 
 import (
+	"debug/gosym"
 	"encoding/binary"
 	"rsc.io/x86/x86asm"
 )
@@ -111,3 +112,50 @@ func (thread *Thread) resolveCallArg(inst *ArchInst, currentGoroutine bool, regs
 	}
 	return &Location{PC: pc, File: file, Line: line, Fn: fn}
 }
+
+type instrseq []x86asm.Op
+
+var windowsPrologue = instrseq{x86asm.MOV, x86asm.MOV, x86asm.LEA, x86asm.CMP, x86asm.JBE}
+var windowsPrologue2 = instrseq{x86asm.MOV, x86asm.MOV, x86asm.CMP, x86asm.JBE}
+var unixPrologue = instrseq{x86asm.MOV, x86asm.LEA, x86asm.CMP, x86asm.JBE}
+var unixPrologue2 = instrseq{x86asm.MOV, x86asm.CMP, x86asm.JBE}
+var prologues = []instrseq{windowsPrologue, windowsPrologue2, unixPrologue, unixPrologue2}
+
+// FirstPCAfterPrologue returns the address of the first instruction after the prologue for function fn
+// If sameline is set FirstPCAfterPrologue will always return an address associated with the same line as fn.Entry
+func (dbp *Process) FirstPCAfterPrologue(fn *gosym.Func, sameline bool) (uint64, error) {
+	text, err := dbp.CurrentThread.Disassemble(fn.Entry, fn.End, false)
+	if err != nil {
+		return fn.Entry, err
+	}
+
+	if len(text) <= 0 {
+		return fn.Entry, nil
+	}
+
+	for _, prologue := range prologues {
+		if len(prologue) >= len(text) {
+			continue
+		}
+		if checkPrologue(text, prologue) {
+			r := &text[len(prologue)]
+			if sameline {
+				if r.Loc.Line != text[0].Loc.Line {
+					return fn.Entry, nil
+				}
+			}
+			return r.Loc.PC, nil
+		}
+	}
+
+	return fn.Entry, nil
+}
+
+func checkPrologue(s []AsmInstruction, prologuePattern instrseq) bool {
+	for i, op := range prologuePattern {
+		if s[i].Inst.Op != op {
+			return false
+		}
+	}
+	return true
+}

proc/proc.go

@@ -9,7 +9,6 @@ import (
 	"go/constant"
 	"go/token"
 	"os"
-	"path/filepath"
 	"runtime"
 	"strconv"
 	"strings"
@@ -161,12 +160,15 @@ func (dbp *Process) LoadInformation(path string) error {
 }
 
 // FindFileLocation returns the PC for a given file:line.
-// Assumes that `file` is normailzed to lower case and '/' on Windows. 
+// Assumes that `file` is normailzed to lower case and '/' on Windows.
 func (dbp *Process) FindFileLocation(fileName string, lineno int) (uint64, error) {
-	pc, _, err := dbp.goSymTable.LineToPC(fileName, lineno)
+	pc, fn, err := dbp.goSymTable.LineToPC(fileName, lineno)
 	if err != nil {
 		return 0, err
 	}
+	if fn.Entry == pc {
+		pc, _ = dbp.FirstPCAfterPrologue(fn, true)
+	}
 	return pc, nil
 }
 
@@ -183,7 +185,7 @@ func (dbp *Process) FindFunctionLocation(funcName string, firstLine bool, lineOf
 	}
 
 	if firstLine {
-		return dbp.FunctionEntryToFirstLine(origfn.Entry)
+		return dbp.FirstPCAfterPrologue(origfn, false)
 	} else if lineOffset > 0 {
 		filename, lineno, _ := dbp.goSymTable.PCToLine(origfn.Entry)
 		breakAddr, _, err := dbp.goSymTable.LineToPC(filename, lineno+lineOffset)
@@ -193,34 +195,6 @@ func (dbp *Process) FindFunctionLocation(funcName string, firstLine bool, lineOf
 	return origfn.Entry, nil
 }
 
-func (dbp *Process) FunctionEntryToFirstLine(entry uint64) (uint64, error) {
-	filename, lineno, startfn := dbp.goSymTable.PCToLine(entry)
-	if filepath.Ext(filename) != ".go" {
-		return entry, nil
-	}
-	if startfn == nil {
-		return entry, nil
-	}
-	funcName := startfn.Name
-
-	for {
-		lineno++
-		pc, fn, _ := dbp.goSymTable.LineToPC(filename, lineno)
-		if fn != nil {
-			if fn.Name != funcName {
-				if strings.Contains(fn.Name, funcName) {
-					continue
-				}
-				break
-			}
-			if fn.Name == funcName {
-				return pc, nil
-			}
-		}
-	}
-	return entry, nil
-}
-
 // CurrentLocation returns the location of the current thread.
 func (dbp *Process) CurrentLocation() (*Location, error) {
 	return dbp.CurrentThread.Location()

proc/proc_test.go

@@ -311,6 +311,7 @@ func testnext(program string, testcases []nextTest, initialLocation string, t *t
 
 func TestNextGeneral(t *testing.T) {
 	testcases := []nextTest{
+		{17, 19},
 		{19, 20},
 		{20, 23},
 		{23, 24},
@@ -331,6 +332,7 @@ func TestNextGeneral(t *testing.T) {
 
 func TestNextConcurrent(t *testing.T) {
 	testcases := []nextTest{
+		{8, 9},
 		{9, 10},
 		{10, 11},
 	}
@@ -371,6 +373,7 @@ func TestNextConcurrent(t *testing.T) {
 func TestNextConcurrentVariant2(t *testing.T) {
 	// Just like TestNextConcurrent but instead of removing the initial breakpoint we check that when it happens is for other goroutines
 	testcases := []nextTest{
+		{8, 9},
 		{9, 10},
 		{10, 11},
 	}
@@ -419,6 +422,7 @@ func TestNextConcurrentVariant2(t *testing.T) {
 
 func TestNextFunctionReturn(t *testing.T) {
 	testcases := []nextTest{
+		{13, 14},
 		{14, 15},
 		{15, 35},
 	}
@@ -427,6 +431,7 @@ func TestNextFunctionReturn(t *testing.T) {
 
 func TestNextFunctionReturnDefer(t *testing.T) {
 	testcases := []nextTest{
+		{5, 8},
 		{8, 9},
 		{9, 10},
 		{10, 7},
@@ -1539,3 +1544,10 @@ func TestIssue332_Part2(t *testing.T) {
 		}
 	})
 }
+
+func TestIssue396(t *testing.T) {
+	withTestProcess("callme", t, func(p *Process, fixture protest.Fixture) {
+		_, err := p.FindFunctionLocation("main.init", true, -1)
+		assertNoError(err, t, "FindFunctionLocation()")
+	})
+}

service/debugger/locations.go

@@ -268,7 +268,8 @@ func (loc *AddrLocationSpec) Find(d *Debugger, scope *proc.EvalScope, locStr str
 			addr, _ := constant.Uint64Val(v.Value)
 			return []api.Location{{PC: addr}}, nil
 		case reflect.Func:
-			pc, err := d.process.FunctionEntryToFirstLine(uint64(v.Base))
+			_, _, fn := d.process.PCToLine(uint64(v.Base))
+			pc, err := d.process.FirstPCAfterPrologue(fn, false)
 			if err != nil {
 				return nil, err
 			}

service/test/integration_test.go

@@ -252,6 +252,7 @@ func testnext(testcases []nextTest, initialLocation string, t *testing.T) {
 
 func TestNextGeneral(t *testing.T) {
 	testcases := []nextTest{
+		{17, 19},
 		{19, 20},
 		{20, 23},
 		{23, 24},
@@ -272,6 +273,7 @@ func TestNextGeneral(t *testing.T) {
 
 func TestNextFunctionReturn(t *testing.T) {
 	testcases := []nextTest{
+		{13, 14},
 		{14, 15},
 		{15, 35},
 	}
@@ -592,9 +594,10 @@ func findLocationHelper(t *testing.T, c service.Client, loc string, shouldErr bo
 
 func TestClientServer_FindLocations(t *testing.T) {
 	withTestClient("locationsprog", t, func(c service.Client) {
-		someFunctionCallAddr := findLocationHelper(t, c, "locationsprog.go:27", false, 1, 0)[0]
-		findLocationHelper(t, c, "anotherFunction:1", false, 1, someFunctionCallAddr)
-		findLocationHelper(t, c, "main.anotherFunction:1", false, 1, someFunctionCallAddr)
+		someFunctionCallAddr := findLocationHelper(t, c, "locationsprog.go:26", false, 1, 0)[0]
+		someFunctionLine1 := findLocationHelper(t, c, "locationsprog.go:27", false, 1, 0)[0]
+		findLocationHelper(t, c, "anotherFunction:1", false, 1, someFunctionLine1)
+		findLocationHelper(t, c, "main.anotherFunction:1", false, 1, someFunctionLine1)
 		findLocationHelper(t, c, "anotherFunction", false, 1, someFunctionCallAddr)
 		findLocationHelper(t, c, "main.anotherFunction", false, 1, someFunctionCallAddr)
 		findLocationHelper(t, c, fmt.Sprintf("*0x%x", someFunctionCallAddr), false, 1, someFunctionCallAddr)
@@ -603,8 +606,8 @@ func TestClientServer_FindLocations(t *testing.T) {
 		findLocationHelper(t, c, "String", true, 0, 0)
 		findLocationHelper(t, c, "main.String", true, 0, 0)
 
-		someTypeStringFuncAddr := findLocationHelper(t, c, "locationsprog.go:15", false, 1, 0)[0]
-		otherTypeStringFuncAddr := findLocationHelper(t, c, "locationsprog.go:19", false, 1, 0)[0]
+		someTypeStringFuncAddr := findLocationHelper(t, c, "locationsprog.go:14", false, 1, 0)[0]
+		otherTypeStringFuncAddr := findLocationHelper(t, c, "locationsprog.go:18", false, 1, 0)[0]
 		findLocationHelper(t, c, "SomeType.String", false, 1, someTypeStringFuncAddr)
 		findLocationHelper(t, c, "(*SomeType).String", false, 1, someTypeStringFuncAddr)
 		findLocationHelper(t, c, "main.SomeType.String", false, 1, someTypeStringFuncAddr)
@@ -638,7 +641,7 @@ func TestClientServer_FindLocations(t *testing.T) {
 	})
 
 	withTestClient("testnextdefer", t, func(c service.Client) {
-		firstMainLine := findLocationHelper(t, c, "testnextdefer.go:8", false, 1, 0)[0]
+		firstMainLine := findLocationHelper(t, c, "testnextdefer.go:5", false, 1, 0)[0]
 		findLocationHelper(t, c, "main.main", false, 1, firstMainLine)
 	})
 
@@ -691,7 +694,7 @@ func TestClientServer_FindLocationsAddr(t *testing.T) {
 		<-c.Continue()
 
 		afunction := findLocationHelper(t, c, "main.afunction", false, 1, 0)[0]
-		anonfunc := findLocationHelper(t, c, "locationsprog2.go:25", false, 1, 0)[0]
+		anonfunc := findLocationHelper(t, c, "main.main.func1", false, 1, 0)[0]
 
 		findLocationHelper(t, c, "*fn1", false, 1, afunction)
 		findLocationHelper(t, c, "*fn3", false, 1, anonfunc)
@@ -1025,3 +1028,45 @@ func TestClientServer_CondBreakpoint(t *testing.T) {
 		}
 	})
 }
+
+func TestSkipPrologue(t *testing.T) {
+	withTestClient("locationsprog2", t, func(c service.Client) {
+		<-c.Continue()
+
+		afunction := findLocationHelper(t, c, "main.afunction", false, 1, 0)[0]
+		findLocationHelper(t, c, "*fn1", false, 1, afunction)
+		findLocationHelper(t, c, "locationsprog2.go:8", false, 1, afunction)
+
+		afunction0 := findLocationHelper(t, c, "main.afunction:0", false, 1, 0)[0]
+
+		if afunction == afunction0 {
+			t.Fatal("Skip prologue failed")
+		}
+	})
+}
+
+func TestSkipPrologue2(t *testing.T) {
+	withTestClient("callme", t, func(c service.Client) {
+		callme := findLocationHelper(t, c, "main.callme", false, 1, 0)[0]
+		callmeZ := findLocationHelper(t, c, "main.callme:0", false, 1, 0)[0]
+		findLocationHelper(t, c, "callme.go:5", false, 1, callme)
+		if callme == callmeZ {
+			t.Fatal("Skip prologue failed")
+		}
+
+		callme2 := findLocationHelper(t, c, "main.callme2", false, 1, 0)[0]
+		callme2Z := findLocationHelper(t, c, "main.callme2:0", false, 1, 0)[0]
+		findLocationHelper(t, c, "callme.go:12", false, 1, callme2)
+		if callme2 == callme2Z {
+			t.Fatal("Skip prologue failed")
+		}
+
+		callme3 := findLocationHelper(t, c, "main.callme3", false, 1, 0)[0]
+		callme3Z := findLocationHelper(t, c, "main.callme3:0", false, 1, 0)[0]
+		// callme3 does not have local variables therefore the first line of the function is immediately after the prologue
+		findLocationHelper(t, c, "callme.go:18", false, 1, callme3Z)
+		if callme3 == callme3Z {
+			t.Fatal("Skip prologue failed")
+		}
+	})
+}