runtime: In Frames.Next, delay file/line lookup until just before return

That way we will never have to look up the file/line for the frame
that's next to be returned when the user stops calling Next.

For the benchmark from #32093:

name      old time/op  new time/op  delta
Helper-4   948ns ± 1%   836ns ± 3%  -11.89%  (p=0.000 n=9+9)

(#32093 was fixed with a more specific, and better, fix, but this
fix is much more general.)

Change-Id: I89e796f80c9706706d8d8b30eb14be3a8a442846
Reviewed-on: https://go-review.googlesource.com/c/go/+/178077
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/symtab.go

@@ -52,6 +52,11 @@ type Frame struct {
 	// if not known. If Func is not nil then Entry ==
 	// Func.Entry().
 	Entry uintptr
+
+	// The runtime's internal view of the function. This field
+	// is set (funcInfo.valid() returns true) only for Go functions,
+	// not for C functions.
+	funcInfo funcInfo
 }
 
 // CallersFrames takes a slice of PC values returned by Callers and
@@ -95,7 +100,6 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 			pc--
 		}
 		name := funcname(funcInfo)
-		file, line := funcline1(funcInfo, pc, false)
 		if inldata := funcdata(funcInfo, _FUNCDATA_InlTree); inldata != nil {
 			inltree := (*[1 << 20]inlinedCall)(inldata)
 			ix := pcdatavalue(funcInfo, _PCDATA_InlTreeIndex, pc, nil)
@@ -111,9 +115,9 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 			PC:       pc,
 			Func:     f,
 			Function: name,
-			File:     file,
-			Line:     int(line),
 			Entry:    entry,
+			funcInfo: funcInfo,
+			// Note: File,Line set below
 		})
 	}
 
@@ -121,6 +125,7 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 	// Avoid allocation in the common case, which is 1 or 2 frames.
 	switch len(ci.frames) {
 	case 0: // In the rare case when there are no frames at all, we return Frame{}.
+		return
 	case 1:
 		frame = ci.frames[0]
 		ci.frames = ci.frameStore[:0]
@@ -133,6 +138,13 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 		ci.frames = ci.frames[1:]
 	}
 	more = len(ci.frames) > 0
+	if frame.funcInfo.valid() {
+		// Compute file/line just before we need to return it,
+		// as it can be expensive. This avoids computing file/line
+		// for the Frame we find but don't return. See issue 32093.
+		file, line := funcline1(frame.funcInfo, frame.PC, false)
+		frame.File, frame.Line = file, int(line)
+	}
 	return
 }
 
@@ -157,6 +169,8 @@ func expandCgoFrames(pc uintptr) []Frame {
 			File:     gostring(arg.file),
 			Line:     int(arg.lineno),
 			Entry:    arg.entry,
+			// funcInfo is zero, which implies !funcInfo.valid().
+			// That ensures that we use the File/Line info given here.
 		})
 		if arg.more == 0 {
 			break