runtime: when panicking, skip ahead to previous panic

While looking up the stack for a defer to run, if we come across
a panic frame we can skip ahead (up) to where the previous panic
was looking for a defer to run.

Switch from keeping LR (the caller's pc) to PC (the frame's PC).
Seems easier to reason about.

Fixes #77062

Change-Id: Idb39411ebad8c072c8f65c62a518da848bddbd61
Reviewed-on: https://go-review.googlesource.com/c/go/+/738041
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/runtime/panic.go

@@ -920,7 +920,7 @@ func (p *_panic) start(pc uintptr, sp unsafe.Pointer) {
 	// caller instead, we avoid needing to unwind through an extra
 	// frame. It also somewhat simplifies the terminating condition for
 	// deferreturn.
-	p.lr, p.fp = pc, sp
+	p.pc, p.sp = pc, sp
 	p.nextFrame()
 }
 
@@ -995,7 +995,7 @@ func (p *_panic) nextDefer() (func(), bool) {
 
 // nextFrame finds the next frame that contains deferred calls, if any.
 func (p *_panic) nextFrame() (ok bool) {
-	if p.lr == 0 {
+	if p.pc == 0 {
 		return false
 	}
 
@@ -1007,10 +1007,10 @@ func (p *_panic) nextFrame() (ok bool) {
 		}
 
 		var u unwinder
-		u.initAt(p.lr, uintptr(p.fp), 0, gp, 0)
+		u.initAt(p.pc, uintptr(p.sp), 0, gp, 0)
 		for {
 			if !u.valid() {
-				p.lr = 0
+				p.pc = 0
 				return // ok == false
 			}
 
@@ -1027,10 +1027,25 @@ func (p *_panic) nextFrame() (ok bool) {
 				break // found a frame with open-coded defers
 			}
 
+			if p.link != nil && uintptr(u.frame.sp) == uintptr(p.link.startSP) && uintptr(p.link.sp) > u.frame.sp {
+				// Skip ahead to where the next panic up the stack was last looking
+				// for defers. See issue 77062.
+				//
+				// The startSP condition is to check when we have walked up the stack
+				// to where the next panic up the stack started. If so, the processing
+				// of that panic has run all the defers up to its current scanning
+				// position.
+				//
+				// The final condition is just to make sure that the line below
+				// is actually helpful.
+				u.initAt(p.link.pc, uintptr(p.link.sp), 0, gp, 0)
+				continue
+			}
+
 			u.next()
 		}
 
-		p.lr = u.frame.lr
+		p.pc = u.frame.pc
 		p.sp = unsafe.Pointer(u.frame.sp)
 		p.fp = unsafe.Pointer(u.frame.fp)
 
@@ -1702,3 +1717,73 @@ func isAbortPC(pc uintptr) bool {
 	}
 	return f.funcID == abi.FuncID_abort
 }
+
+// For debugging only.
+//go:noinline
+//go:nosplit
+func dumpPanicDeferState(where string, gp *g) {
+	systemstack(func() {
+		println("DUMPPANICDEFERSTATE", where)
+		p := gp._panic
+		d := gp._defer
+		var u unwinder
+		for u.init(gp, 0); u.valid(); u.next() {
+			// Print frame.
+			println("  frame sp=", hex(u.frame.sp), "fp=", hex(u.frame.fp), "pc=", pcName(u.frame.pc), "+", pcOff(u.frame.pc))
+			// Print panic.
+			for p != nil && uintptr(p.sp) == u.frame.sp {
+				println("    panic", p, "sp=", p.sp, "fp=", p.fp, "arg=", p.arg, "recovered=", p.recovered, "pc=", pcName(p.pc), "+", pcOff(p.pc), "retpc=", pcName(p.retpc), "+", pcOff(p.retpc), "startsp=", p.startSP, "gopanicfp=", p.gopanicFP, "startPC=", hex(p.startPC), pcName(p.startPC), "+", pcOff(p.startPC))
+				p = p.link
+			}
+
+			// Print linked defers.
+			for d != nil && d.sp == u.frame.sp {
+				println("    defer(link)", "heap=", d.heap, "rangefunc=", d.rangefunc, fnName(d.fn))
+				d = d.link
+			}
+
+			// Print open-coded defers.
+			// (A function is all linked or all open-coded, so we don't
+			// need to interleave this loop with the one above.)
+			fd := funcdata(u.frame.fn, abi.FUNCDATA_OpenCodedDeferInfo)
+			if fd != nil {
+				deferBitsOffset, fd := readvarintUnsafe(fd)
+				m := *(*uint8)(unsafe.Pointer(u.frame.varp - uintptr(deferBitsOffset)))
+				slotsOffset, fd := readvarintUnsafe(fd)
+				slots := u.frame.varp - uintptr(slotsOffset)
+				for i := 7; i >= 0; i-- {
+					if m>>i&1 == 0 {
+						continue
+					}
+					fn := *(*func())(unsafe.Pointer(slots + uintptr(i)*goarch.PtrSize))
+					println("    defer(open)", fnName(fn))
+				}
+			}
+
+		}
+		if p != nil {
+			println("  REMAINING PANICS!", p)
+		}
+		if d != nil {
+			println("  REMAINING DEFERS!")
+		}
+	})
+}
+
+func pcName(pc uintptr) string {
+	fn := findfunc(pc)
+	if !fn.valid() {
+		return "<unk>"
+	}
+	return funcname(fn)
+}
+func pcOff(pc uintptr) hex {
+	fn := findfunc(pc)
+	if !fn.valid() {
+		return 0
+	}
+	return hex(pc - fn.entry())
+}
+func fnName(fn func()) string {
+	return pcName(**(**uintptr)(unsafe.Pointer(&fn)))
+}

src/runtime/panic_test.go

@@ -5,8 +5,10 @@
 package runtime_test
 
 import (
+	"slices"
 	"strings"
 	"testing"
+	"time"
 )
 
 // Test that panics print out the underlying value
@@ -48,3 +50,51 @@ func TestPanicWithDirectlyPrintableCustomTypes(t *testing.T) {
 		})
 	}
 }
+
+func TestPanicRecoverSpeed(t *testing.T) {
+	// For issue 77062.
+	t.Skip("This test is too flaky at the moment. But it does normally pass. Suggestions for making it less flaky are welcome.")
+
+	// Recursive function that does defer/recover/repanic.
+	var f func(int)
+	f = func(n int) {
+		if n == 0 {
+			panic("done")
+		}
+		defer func() {
+			err := recover()
+			panic(err)
+		}()
+		f(n - 1)
+	}
+
+	time := func(f func()) time.Duration {
+		var times []time.Duration
+		for range 10 {
+			start := time.Now()
+			f()
+			times = append(times, time.Since(start))
+		}
+		slices.Sort(times)
+		times = times[1 : len(times)-1] // skip high and low, to reduce noise
+		var avg time.Duration
+		for _, v := range times {
+			avg += v / time.Duration(len(times))
+		}
+		return avg
+	}
+
+	a := time(func() {
+		defer func() { recover() }()
+		f(1024)
+	})
+	b := time(func() {
+		defer func() { recover() }()
+		f(2048)
+	})
+	m := b.Seconds() / a.Seconds()
+	t.Logf("a: %v, b: %v, m: %v", a, b, m)
+	if m > 3.5 {
+		t.Errorf("more than 2x time increase: %v", m)
+	}
+}

src/runtime/runtime2.go

@@ -1177,16 +1177,17 @@ type _panic struct {
 	link *_panic // link to earlier panic
 
 	// startPC and startSP track where _panic.start was called.
+	// (These are the SP and PC of the gopanic frame itself.)
 	startPC uintptr
 	startSP unsafe.Pointer
 
 	// The current stack frame that we're running deferred calls for.
+	pc uintptr
 	sp unsafe.Pointer
-	lr uintptr
 	fp unsafe.Pointer
 
 	// retpc stores the PC where the panic should jump back to, if the
-	// function last returned by _panic.next() recovers the panic.
+	// function last returned by _panic.nextDefer() recovers the panic.
 	retpc uintptr
 
 	// Extra state for handling open-coded defers.