cmd/compile, runtime: use deferreturn as target PC for recover from deferrangefunc

The existing code for recover from deferrangefunc was broken in
several ways.

1. the code following a deferrangefunc call did not check the return
value for an out-of-band value indicating "return now" (i.e., recover
was called)

2. the returned value was delivered using a bespoke ABI that happened
to match on register-ABI platforms, but not on older stack-based
ABI.

3. the returned value was the wrong width (1 word versus 2) and
type/value(integer 1, not a pointer to anything) for deferrangefunc's
any-typed return value (in practice, the OOB value check could catch
this, but still, it's sketchy).

This -- using the deferreturn lookup method already in place for
open-coded defers -- turned out to be a much-less-ugly way of
obtaining the desired transfer of control for recover().

TODO: we also could do this for regular defer, and delete some code.

Fixes #71675

Change-Id: If7d7ea789ad4320821aab3b443759a7d71647ff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/650476
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/ssa/func.go

@@ -41,11 +41,12 @@ type Func struct {
 	ABISelf        *abi.ABIConfig // ABI for function being compiled
 	ABIDefault     *abi.ABIConfig // ABI for rtcall and other no-parsed-signature/pragma functions.
 
-	scheduled   bool  // Values in Blocks are in final order
-	laidout     bool  // Blocks are ordered
-	NoSplit     bool  // true if function is marked as nosplit.  Used by schedule check pass.
-	dumpFileSeq uint8 // the sequence numbers of dump file. (%s_%02d__%s.dump", funcname, dumpFileSeq, phaseName)
-	IsPgoHot    bool
+	scheduled         bool  // Values in Blocks are in final order
+	laidout           bool  // Blocks are ordered
+	NoSplit           bool  // true if function is marked as nosplit.  Used by schedule check pass.
+	dumpFileSeq       uint8 // the sequence numbers of dump file. (%s_%02d__%s.dump", funcname, dumpFileSeq, phaseName)
+	IsPgoHot          bool
+	HasDeferRangeFunc bool // if true, needs a deferreturn so deferrangefunc can use it for recover() return PC
 
 	// when register allocation is done, maps value ids to locations
 	RegAlloc []Location

src/cmd/compile/internal/ssagen/ssa.go

@@ -4433,6 +4433,9 @@ func (s *state) call(n *ir.CallExpr, k callKind, returnResultAddr bool, deferExt
 					callABI = s.f.ABI1
 				}
 			}
+			if fn := n.Fun.Sym().Name; n.Fun.Sym().Pkg == ir.Pkgs.Runtime && fn == "deferrangefunc" {
+				s.f.HasDeferRangeFunc = true
+			}
 			break
 		}
 		closure = s.expr(fn)
@@ -6568,10 +6571,13 @@ func genssa(f *ssa.Func, pp *objw.Progs) {
 		// nop (which will never execute) after the call.
 		Arch.Ginsnop(s.pp)
 	}
-	if openDeferInfo != nil {
+	if openDeferInfo != nil || f.HasDeferRangeFunc {
 		// When doing open-coded defers, generate a disconnected call to
 		// deferreturn and a return. This will be used to during panic
 		// recovery to unwind the stack and return back to the runtime.
+		//
+		// deferrangefunc needs to be sure that at least one of these exists;
+		// if all returns are dead-code eliminated, there might not be.
 		s.pp.NextLive = s.livenessMap.DeferReturn
 		p := s.pp.Prog(obj.ACALL)
 		p.To.Type = obj.TYPE_MEM

src/runtime/panic.go

@@ -391,10 +391,15 @@ func deferrangefunc() any {
 		throw("defer on system stack")
 	}
 
+	fn := findfunc(sys.GetCallerPC())
+	if fn.deferreturn == 0 {
+		throw("no deferreturn")
+	}
+
 	d := newdefer()
 	d.link = gp._defer
 	gp._defer = d
-	d.pc = sys.GetCallerPC()
+	d.pc = fn.entry() + uintptr(fn.deferreturn)
 	// We must not be preempted between calling GetCallerSP and
 	// storing it to d.sp because GetCallerSP's result is a
 	// uintptr stack pointer.
@@ -1258,6 +1263,8 @@ func recovery(gp *g) {
 		// only gets us to the caller's fp.
 		gp.sched.bp = sp - goarch.PtrSize
 	}
+	// The value in ret is delivered IN A REGISTER, even if there is a
+	// stack ABI.
 	gp.sched.ret = 1
 	gogo(&gp.sched)
 }

test/fixedbugs/issue71675.go

@@ -0,0 +1,99 @@
+// run
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+package main
+
+//go:noinline
+func i() {
+	for range yieldInts {
+		defer func() {
+			println("I")
+			recover()
+		}()
+	}
+	// This panic causes dead code elimination of the return block.
+	// The compiler should nonetheless emit a deferreturn.
+	panic("i panic")
+}
+
+//go:noinline
+func h() {
+	defer func() {
+		println("H first")
+	}()
+	for range yieldInts {
+		defer func() {
+			println("H second")
+		}()
+	}
+	defer func() {
+		println("H third")
+	}()
+	for range yieldIntsPanic {
+		defer func() {
+			println("h recover:called")
+			recover()
+		}()
+	}
+}
+
+//go:noinline
+func yieldInts(yield func(int) bool) {
+	if !yield(0) {
+		return
+	}
+}
+
+//go:noinline
+func g() {
+	defer func() {
+		println("G first")
+	}()
+	for range yieldIntsPanic {
+		defer func() {
+			println("g recover:called")
+			recover()
+		}()
+	}
+}
+
+//go:noinline
+func yieldIntsPanic(yield func(int) bool) {
+	if !yield(0) {
+		return
+	}
+	panic("yield stop")
+}
+
+//go:noinline
+func next(i int) int {
+	if i == 0 {
+		panic("next stop")
+	}
+	return i + 1
+}
+
+//go:noinline
+func f() {
+	defer func() {
+		println("F first")
+	}()
+	for i := 0; i < 1; i = next(i) {
+		defer func() {
+			println("f recover:called")
+			recover()
+		}()
+	}
+}
+func main() {
+	f()
+	println("f returned")
+	g()
+	println("g returned")
+	h()
+	println("h returned")
+	i()
+	println("i returned")
+
+}

test/fixedbugs/issue71675.out

@@ -0,0 +1,13 @@
+f recover:called
+F first
+f returned
+g recover:called
+G first
+g returned
+h recover:called
+H third
+H second
+H first
+h returned
+I
+i returned