cmd/compile: resurrect clobberdead mode

This CL resurrects the clobberdead debugging mode (CL 23924).
When -clobberdead flag is set (TODO: make it GOEXPERIMENT?), the
compiler inserts code that clobbers all dead stack slots that
contains pointers.

Mark windows syscall functions cgo_unsafe_args, as the code
actually does that, by taking the address of one argument and
passing it to cgocall.

Change-Id: Ie09a015f4bd14ae6053cc707866e30ae509b9d6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/301791
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/test/codegen/clobberdead.go b/test/codegen/clobberdead.go
new file mode 100644
index 0000000000..f8d964cba6
--- /dev/null
+++ b/test/codegen/clobberdead.go
@@ -0,0 +1,32 @@
+// asmcheck -gcflags=-clobberdead
+
+// +build amd64
+
+// 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 codegen
+
+type T [2]*int // contain pointer, not SSA-able (so locals are not registerized)
+
+var p1, p2, p3 T
+
+func F() {
+	// 3735936685 is 0xdeaddead
+	// clobber x, y at entry. not clobber z (stack object).
+	// amd64:`MOVL\t\$3735936685, ""\.x`, `MOVL\t\$3735936685, ""\.y`, -`MOVL\t\$3735936685, ""\.z`
+	x, y, z := p1, p2, p3
+	addrTaken(&z)
+	// x is dead at the call (the value of x is loaded before the CALL), y is not
+	// amd64:`MOVL\t\$3735936685, ""\.x`, -`MOVL\t\$3735936685, ""\.y`
+	use(x)
+	// amd64:`MOVL\t\$3735936685, ""\.x`, `MOVL\t\$3735936685, ""\.y`
+	use(y)
+}
+
+//go:noinline
+func use(T) {}
+
+//go:noinline
+func addrTaken(*T) {}