cmd/pprof: use DWARF info to lookup unknown PC addresses

Test to follow in a separate CL that arranges for the runtime package to
store non-Go addresses in a CPU profile.

Change-Id: I33ce1d66b77340b1e62b54505fc9b1abcec108a9
Reviewed-on: https://go-review.googlesource.com/21055
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

1 files changed, 88 insertions, 7 deletions

diff --git a/src/cmd/pprof/pprof.go b/src/cmd/pprof/pprof.go
index 2b20f1da77..1c55d05d5d 100644
--- a/src/cmd/pprof/pprof.go
+++ b/src/cmd/pprof/pprof.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"debug/dwarf"
 	"debug/gosym"
 	"flag"
 	"fmt"
@@ -172,6 +173,9 @@ type file struct {
 	sym  []objfile.Sym
 	file *objfile.File
 	pcln *gosym.Table
+
+	triedDwarf bool
+	dwarf      *dwarf.Data
 }
 
 func (f *file) Name() string {
@@ -197,17 +201,94 @@ func (f *file) SourceLine(addr uint64) ([]plugin.Frame, error) {
 		f.pcln = pcln
 	}
 	file, line, fn := f.pcln.PCToLine(addr)
-	if fn == nil {
-		return nil, fmt.Errorf("no line information for PC=%#x", addr)
+	if fn != nil {
+		frame := []plugin.Frame{
+			{
+				Func: fn.Name,
+				File: file,
+				Line: line,
+			},
+		}
+		return frame, nil
+	}
+
+	frames := f.dwarfSourceLine(addr)
+	if frames != nil {
+		return frames, nil
+	}
+
+	return nil, fmt.Errorf("no line information for PC=%#x", addr)
+}
+
+// dwarfSourceLine tries to get file/line information using DWARF.
+// This is for C functions that appear in the profile.
+// Returns nil if there is no information available.
+func (f *file) dwarfSourceLine(addr uint64) []plugin.Frame {
+	if f.dwarf == nil && !f.triedDwarf {
+		// Ignore any error--we don't care exactly why there
+		// is no DWARF info.
+		f.dwarf, _ = f.file.DWARF()
+		f.triedDwarf = true
+	}
+
+	if f.dwarf != nil {
+		r := f.dwarf.Reader()
+		unit, err := r.SeekPC(addr)
+		if err == nil {
+			if frames := f.dwarfSourceLineEntry(r, unit, addr); frames != nil {
+				return frames
+			}
+		}
+	}
+
+	return nil
+}
+
+// dwarfSourceLineEntry tries to get file/line information from a
+// DWARF compilation unit. Returns nil if it doesn't find anything.
+func (f *file) dwarfSourceLineEntry(r *dwarf.Reader, entry *dwarf.Entry, addr uint64) []plugin.Frame {
+	lines, err := f.dwarf.LineReader(entry)
+	if err != nil {
+		return nil
+	}
+	var lentry dwarf.LineEntry
+	if err := lines.SeekPC(addr, &lentry); err != nil {
+		return nil
 	}
-	frame := []plugin.Frame{
+
+	// Try to find the function name.
+	name := ""
+FindName:
+	for entry, err := r.Next(); entry != nil && err == nil; entry, err = r.Next() {
+		if entry.Tag == dwarf.TagSubprogram {
+			ranges, err := f.dwarf.Ranges(entry)
+			if err != nil {
+				return nil
+			}
+			for _, pcs := range ranges {
+				if pcs[0] <= addr && addr < pcs[1] {
+					var ok bool
+					// TODO: AT_linkage_name, AT_MIPS_linkage_name.
+					name, ok = entry.Val(dwarf.AttrName).(string)
+					if ok {
+						break FindName
+					}
+				}
+			}
+		}
+	}
+
+	// TODO: Report inlined functions.
+
+	frames := []plugin.Frame{
 		{
-			Func: fn.Name,
-			File: file,
-			Line: line,
+			Func: name,
+			File: lentry.File.Name,
+			Line: lentry.Line,
 		},
 	}
-	return frame, nil
+
+	return frames
 }
 
 func (f *file) Symbols(r *regexp.Regexp, addr uint64) ([]*plugin.Sym, error) {