runtime: expand inlining iteratively in CallersFrames

Currently CallersFrames expands each PC to a slice of Frames and then
iteratively returns those Frames. However, this makes it very
difficult to avoid heap allocation: either the Frames slice will be
heap allocated, or, if it uses internal scratch space for small slices
(as it currently does), the Frames object itself has to be heap
allocated.

Fix this, at least in the common case, by expanding each PC
iteratively. We introduce a new pcExpander type that's responsible for
expanding a single PC. This maintains state from one Frame to the next
in the same PC. Frames then becomes a wrapper around this responsible
for feeding it the next PC when the pcExpander runs out of frames for
the current PC.

This makes it possible to stack-allocate a Frames object, which will
make it possible to use this API for PC expansion from within the
runtime itself.

Change-Id: I993463945ab574557cf1d6bedbe79ce7e9cbbdcd
Reviewed-on: https://go-review.googlesource.com/40434
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Lazar <lazard@golang.org>

src/runtime/symtab.go

@@ -13,6 +13,7 @@ import (
 // Frames may be used to get function/file/line information for a
 // slice of PC values returned by Callers.
 type Frames struct {
+	// callers is a slice of PCs that have not yet been expanded.
 	callers []uintptr
 
 	// If previous caller in iteration was a panic, then
@@ -20,16 +21,12 @@ type Frames struct {
 	// instead of the return address of the call.
 	wasPanic bool
 
-	// Frames to return for subsequent calls to the Next method.
-	// Used for non-Go or inlined frames.
-	framesNext []Frame
+	// expander expands the current PC into a sequence of Frames.
+	expander pcExpander
 
-	// This buffer is used when expanding PCs into multiple frames.
-	// Initially it points to the scratch space.
-	frames []Frame
-
-	// Scratch space to avoid allocation.
-	scratch [4]Frame
+	// skip > 0 indicates that skip frames in the first expansion
+	// should be skipped over and callers[1] should also be skipped.
+	skip int
 }
 
 // Frame is the information returned by Frames for each call frame.
@@ -59,138 +56,194 @@ type Frame struct {
 // Do not change the slice until you are done with the Frames.
 func CallersFrames(callers []uintptr) *Frames {
 	ci := &Frames{}
-	ci.frames = ci.scratch[:0]
+	ci.init(callers)
+	return ci
+}
+
+func (ci *Frames) init(callers []uintptr) {
 	if len(callers) >= 1 {
 		pc := callers[0]
 		s := pc - skipPC
 		if s >= 0 && s < sizeofSkipFunction {
 			// Ignore skip frame callers[0] since this means the caller trimmed the PC slice.
 			ci.callers = callers[1:]
-			return ci
+			return
 		}
 	}
 	if len(callers) >= 2 {
 		pc := callers[1]
 		s := pc - skipPC
-		if s >= 0 && s < sizeofSkipFunction {
-			// Expand callers[0] and skip s logical frames at this PC.
-			ci.frames = ci.expandPC(ci.frames[:0], callers[0])
-			ci.framesNext = ci.frames[int(s):]
-			ci.callers = callers[2:]
-			return ci
+		if s > 0 && s < sizeofSkipFunction {
+			// Skip the first s inlined frames when we expand the first PC.
+			ci.skip = int(s)
 		}
 	}
 	ci.callers = callers
-	return ci
 }
 
 // Next returns frame information for the next caller.
 // If more is false, there are no more callers (the Frame value is valid).
 func (ci *Frames) Next() (frame Frame, more bool) {
-	if len(ci.framesNext) > 0 {
-		// We have saved up frames to return.
-		f := ci.framesNext[0]
-		ci.framesNext = ci.framesNext[1:]
-		return f, len(ci.framesNext) > 0 || len(ci.callers) > 0
+	if !ci.expander.more {
+		// Expand the next PC.
+		if len(ci.callers) == 0 {
+			ci.wasPanic = false
+			return Frame{}, false
+		}
+		ci.expander.init(ci.callers[0], ci.wasPanic)
+		ci.callers = ci.callers[1:]
+		ci.wasPanic = ci.expander.funcInfo.valid() && ci.expander.funcInfo.entry == sigpanicPC
+		if ci.skip > 0 {
+			for ; ci.skip > 0; ci.skip-- {
+				ci.expander.next()
+			}
+			ci.skip = 0
+			// Drop skipPleaseUseCallersFrames.
+			ci.callers = ci.callers[1:]
+		}
+		if !ci.expander.more {
+			// No symbolic information for this PC.
+			// However, we return at least one frame for
+			// every PC, so return an invalid frame.
+			return Frame{}, len(ci.callers) > 0
+		}
 	}
 
-	if len(ci.callers) == 0 {
-		ci.wasPanic = false
-		return Frame{}, false
-	}
-	pc := ci.callers[0]
-	ci.callers = ci.callers[1:]
-	more = len(ci.callers) > 0
+	frame = ci.expander.next()
+	return frame, ci.expander.more || len(ci.callers) > 0
+}
 
-	ci.frames = ci.expandPC(ci.frames[:0], pc)
-	if len(ci.frames) == 0 {
-		// Expansion failed, so there's no useful symbolic information.
-		return Frame{}, more
-	}
+// A pcExpander expands a single PC into a sequence of Frames.
+type pcExpander struct {
+	// more indicates that the next call to next will return a
+	// valid frame.
+	more bool
+
+	// pc is the pc being expanded.
+	pc uintptr
+
+	// frames is a pre-expanded set of Frames to return from the
+	// iterator. If this is set, then this is everything that will
+	// be returned from the iterator.
+	frames []Frame
+
+	// funcInfo is the funcInfo of the function containing pc.
+	funcInfo funcInfo
 
-	ci.framesNext = ci.frames[1:]
-	return ci.frames[0], more || len(ci.framesNext) > 0
+	// inlTree is the inlining tree of the function containing pc.
+	inlTree *[1 << 20]inlinedCall
+
+	// file and line are the file name and line number of the next
+	// frame.
+	file string
+	line int32
+
+	// inlIndex is the inlining index of the next frame, or -1 if
+	// the next frame is an outermost frame.
+	inlIndex int32
 }
 
-// expandPC appends the frames corresponding to pc to frames
-// and returns the new slice.
-func (ci *Frames) expandPC(frames []Frame, pc uintptr) []Frame {
-	f := FuncForPC(pc)
-	if f == nil {
-		ci.wasPanic = false
+// init initializes this pcExpander to expand pc. It sets ex.more if
+// pc expands to any Frames.
+//
+// A pcExpander can be reused by calling init again.
+//
+// If pc was a "call" to sigpanic, panicCall should be true. In this
+// case, pc is treated as the address of a faulting instruction
+// instead of the return address of a call.
+func (ex *pcExpander) init(pc uintptr, panicCall bool) {
+	ex.more = false
+
+	ex.funcInfo = findfunc(pc)
+	if !ex.funcInfo.valid() {
 		if cgoSymbolizer != nil {
-			frames = expandCgoFrames(frames, pc)
+			// Pre-expand cgo frames. We could do this
+			// incrementally, too, but there's no way to
+			// avoid allocation in this case anyway.
+			ex.frames = expandCgoFrames(pc)
+			ex.more = len(ex.frames) > 0
 		}
-		return frames
+		return
 	}
 
-	entry := f.Entry()
-	xpc := pc
-	if xpc > entry && !ci.wasPanic {
-		xpc--
+	ex.more = true
+	entry := ex.funcInfo.entry
+	ex.pc = pc
+	if ex.pc > entry && !panicCall {
+		ex.pc--
 	}
-	ci.wasPanic = entry == sigpanicPC
 
-	frames = expandInlinedCalls(frames, xpc, f)
-	return frames
+	// file and line are the innermost position at pc.
+	ex.file, ex.line = funcline1(ex.funcInfo, ex.pc, false)
+
+	// Get inlining tree at pc
+	inldata := funcdata(ex.funcInfo, _FUNCDATA_InlTree)
+	if inldata != nil {
+		ex.inlTree = (*[1 << 20]inlinedCall)(inldata)
+		ex.inlIndex = pcdatavalue(ex.funcInfo, _PCDATA_InlTreeIndex, ex.pc, nil)
+	} else {
+		ex.inlTree = nil
+		ex.inlIndex = -1
+	}
 }
 
-// expandInlinedCalls expands xpc into multiple frames using the inlining
-// info in fn. expandInlinedCalls appends to frames and returns the new
-// slice. The resulting slice has at least one frame for the physical frame
-// that contains xpc (i.e., the function represented by fn).
-func expandInlinedCalls(frames []Frame, xpc uintptr, fn *Func) []Frame {
-	entry := fn.Entry()
+// next returns the next Frame in the expansion of pc and sets ex.more
+// if there are more Frames to follow.
+func (ex *pcExpander) next() Frame {
+	if !ex.more {
+		return Frame{}
+	}
 
-	// file and line are the innermost position at xpc.
-	file, line := fn.FileLine(xpc)
+	if len(ex.frames) > 0 {
+		// Return pre-expended frame.
+		frame := ex.frames[0]
+		ex.frames = ex.frames[1:]
+		ex.more = len(ex.frames) > 0
+		return frame
+	}
 
-	funcInfo := fn.funcInfo()
-	inldata := funcdata(funcInfo, _FUNCDATA_InlTree)
-	if inldata != nil {
-		inltree := (*[1 << 20]inlinedCall)(inldata)
-		ix := pcdatavalue(funcInfo, _PCDATA_InlTreeIndex, xpc, nil)
-		for ix >= 0 {
-			call := inltree[ix]
-			frames = append(frames, Frame{
-				PC:       xpc,
-				Func:     nil, // nil for inlined functions
-				Function: funcnameFromNameoff(funcInfo, call.func_),
-				File:     file,
-				Line:     line,
-				Entry:    entry,
-			})
-			file = funcfile(funcInfo, call.file)
-			line = int(call.line)
-			ix = call.parent
+	if ex.inlIndex >= 0 {
+		// Return inner inlined frame.
+		call := ex.inlTree[ex.inlIndex]
+		frame := Frame{
+			PC:       ex.pc,
+			Func:     nil, // nil for inlined functions
+			Function: funcnameFromNameoff(ex.funcInfo, call.func_),
+			File:     ex.file,
+			Line:     int(ex.line),
+			Entry:    ex.funcInfo.entry,
 		}
+		ex.file = funcfile(ex.funcInfo, call.file)
+		ex.line = call.line
+		ex.inlIndex = call.parent
+		return frame
 	}
 
-	physicalFrame := Frame{
-		PC:       xpc,
-		Func:     fn,
-		Function: fn.Name(),
-		File:     file,
-		Line:     line,
-		Entry:    entry,
+	// No inlining or pre-expanded frames.
+	ex.more = false
+	return Frame{
+		PC:       ex.pc,
+		Func:     ex.funcInfo._Func(),
+		Function: funcname(ex.funcInfo),
+		File:     ex.file,
+		Line:     int(ex.line),
+		Entry:    ex.funcInfo.entry,
 	}
-	frames = append(frames, physicalFrame)
-
-	return frames
 }
 
 // expandCgoFrames expands frame information for pc, known to be
 // a non-Go function, using the cgoSymbolizer hook. expandCgoFrames
-// appends to frames and returns the new slice.
-func expandCgoFrames(frames []Frame, pc uintptr) []Frame {
+// returns nil if pc could not be expanded.
+func expandCgoFrames(pc uintptr) []Frame {
 	arg := cgoSymbolizerArg{pc: pc}
 	callCgoSymbolizer(&arg)
 
 	if arg.file == nil && arg.funcName == nil {
 		// No useful information from symbolizer.
-		return frames
+		return nil
 	}
 
+	var frames []Frame
 	for {
 		frames = append(frames, Frame{
 			PC:       pc,
@@ -487,7 +540,7 @@ func moduledataverify1(datap *moduledata) {
 // FuncForPC returns a *Func describing the function that contains the
 // given program counter address, or else nil.
 func FuncForPC(pc uintptr) *Func {
-	return (*Func)(unsafe.Pointer(findfunc(pc)._func))
+	return findfunc(pc)._Func()
 }
 
 // Name returns the name of the function.
@@ -529,6 +582,10 @@ func (f funcInfo) valid() bool {
 	return f._func != nil
 }
 
+func (f funcInfo) _Func() *Func {
+	return (*Func)(unsafe.Pointer(f._func))
+}
+
 func findfunc(pc uintptr) funcInfo {
 	datap := findmoduledatap(pc)
 	if datap == nil {