[release-branch.go1.16] cmd/compile: avoid adding LECall to the entry block when has opendefers

The openDeferRecord always insert vardef/varlive pairs into the entry block, it may destroy the mem chain when LECall's args are writing into the same block. So create a new block before that happens.

Fixes #49412

Change-Id: Ibda6c4a45d960dd412a641f5e02276f663c80785
Reviewed-on: https://go-review.googlesource.com/c/go/+/361410
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
(cherry picked from commit 4f083c7dcf)
Reviewed-on: https://go-review.googlesource.com/c/go/+/362055
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -4709,6 +4709,17 @@ func (s *state) call(n *Node, k callKind, returnResultAddr bool) *ssa.Value {
 			}
 		}
 
+		// Split the entry block if there are open defers, because later calls to
+		// openDeferSave may cause a mismatch between the mem for an OpDereference
+		// and the call site which uses it. See #49282.
+		if s.curBlock.ID == s.f.Entry.ID && s.hasOpenDefers {
+			b := s.endBlock()
+			b.Kind = ssa.BlockPlain
+			curb := s.f.NewBlock(ssa.BlockPlain)
+			b.AddEdgeTo(curb)
+			s.startBlock(curb)
+		}
+
 		// Write args.
 		t := n.Left.Type
 		args := n.Rlist.Slice()

test/fixedbugs/issue49282.go

@@ -0,0 +1,44 @@
+// compile
+
+// Copyright 2021 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 p
+
+//go:noinline
+func g(d uintptr, a, m []int, s struct {
+	a, b, c, d, e int
+}, u uint) {
+	_ = a
+	_ = m
+	_ = s
+	func() {
+		for i := 0; i < 5; i++ {
+			_ = a
+			_ = m
+			_, _ = s, s
+		}
+	}()
+}
+
+var One float64 = 1.0
+
+func f(d uintptr) {
+	var a, m []int
+	var s struct {
+		a, b, c, d, e int
+	}
+
+	g(d, a, m, s, uint(One)) // Uint of not-a-constant inserts a conditional, necessary to bug
+
+	defer func() uint {
+		return 0
+	}()
+}
+
+var d uintptr
+
+func h() {
+	f(d)
+}