runtime/pprof: adjust reported line numbers to show call sites

This is the same logic used in the standard tracebacks.
The caller pc is the pc after the call, so except in the
fake "call" caused by a panic, back up the pc enough
that the lookup will use the previous instruction.

Fixes #4150.
Fixes #4151.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7317047

src/pkg/runtime/pprof/pprof.go

@@ -318,21 +318,33 @@ func printCountProfile(w io.Writer, debug int, name string, p countProfile) erro
 // for a single stack trace.
 func printStackRecord(w io.Writer, stk []uintptr, allFrames bool) {
 	show := allFrames
-	for _, pc := range stk {
+	wasPanic := false
+	for i, pc := range stk {
 		f := runtime.FuncForPC(pc)
 		if f == nil {
 			show = true
 			fmt.Fprintf(w, "#\t%#x\n", pc)
+			wasPanic = false
 		} else {
-			file, line := f.FileLine(pc)
+			tracepc := pc
+			// Back up to call instruction.
+			if i > 0 && pc > f.Entry() && !wasPanic {
+				if runtime.GOARCH == "386" || runtime.GOARCH == "amd64" {
+					tracepc--
+				} else {
+					tracepc -= 4 // arm, etc
+				}
+			}
+			file, line := f.FileLine(tracepc)
 			name := f.Name()
 			// Hide runtime.goexit and any runtime functions at the beginning.
 			// This is useful mainly for allocation traces.
+			wasPanic = name == "runtime.panic"
 			if name == "runtime.goexit" || !show && strings.HasPrefix(name, "runtime.") {
 				continue
 			}
 			show = true
-			fmt.Fprintf(w, "#\t%#x\t%s+%#x\t%s:%d\n", pc, f.Name(), pc-f.Entry(), file, line)
+			fmt.Fprintf(w, "#\t%#x\t%s+%#x\t%s:%d\n", pc, name, pc-f.Entry(), file, line)
 		}
 	}
 	if !show {