cmd/compile: pre-spill pointers in aggregate-typed register args

There's a problem in liveness, where liveness of any
part of an aggregate keeps the whole aggregate alive,
but the not-live parts don't get spilled.  The GC
can observe those live-but-not-spilled slots, which
can contain junk.

A better fix is to change liveness to work
pointer-by-pointer, but that is also a riskier,
trickier fix.

To avoid this, in the case of

(1) an aggregate input parameter
(2) containing pointers
(3) passed in registers

pre-spill the pointers.

Updates #40724.

Change-Id: I6beb8e0a353b1ae3c68c16072f56698061922c04
Reviewed-on: https://go-review.googlesource.com/c/go/+/307909
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

2 files changed, 101 insertions, 0 deletions

diff --git a/test/abi/part_live.go b/test/abi/part_live.go
new file mode 100644
index 0000000000..592b6b3a07
--- /dev/null
+++ b/test/abi/part_live.go
@@ -0,0 +1,48 @@
+// run
+
+// 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.
+
+// A test for partial liveness / partial spilling / compiler-induced GC failure
+
+package main
+
+import "runtime"
+import "unsafe"
+
+//go:registerparams
+func F(s []int) {
+	for i, x := range s {
+		G(i, x)
+	}
+	GC()
+	G(len(s), cap(s))
+	GC()
+}
+
+//go:noinline
+//go:registerparams
+func G(int, int) {}
+
+//go:registerparams
+func GC() { runtime.GC(); runtime.GC() }
+
+func main() {
+	s := make([]int, 3)
+	escape(s)
+	p := int(uintptr(unsafe.Pointer(&s[2])) + 42) // likely point to unallocated memory
+	poison([3]int{p, p, p})
+	F(s)
+}
+
+//go:noinline
+//go:registerparams
+func poison([3]int) {}
+
+//go:noinline
+//go:registerparams
+func escape(s []int) {
+	g = s
+}
+var g []int
diff --git a/test/abi/part_live_2.go b/test/abi/part_live_2.go
new file mode 100644
index 0000000000..f9d7b01bce
--- /dev/null
+++ b/test/abi/part_live_2.go
@@ -0,0 +1,53 @@
+// run
+
+// 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.
+
+// A test for partial liveness / partial spilling / compiler-induced GC failure
+
+package main
+
+import "runtime"
+import "unsafe"
+
+//go:registerparams
+func F(s []int) {
+	for i, x := range s {
+		G(i, x)
+	}
+	GC()
+	H(&s[0]) // It's possible that this will make the spill redundant, but there's a bug in spill slot allocation.
+	G(len(s), cap(s))
+	GC()
+}
+
+//go:noinline
+//go:registerparams
+func G(int, int) {}
+
+//go:noinline
+//go:registerparams
+func H(*int) {}
+
+//go:registerparams
+func GC() { runtime.GC(); runtime.GC() }
+
+func main() {
+	s := make([]int, 3)
+	escape(s)
+	p := int(uintptr(unsafe.Pointer(&s[2])) + 42) // likely point to unallocated memory
+	poison([3]int{p, p, p})
+	F(s)
+}
+
+//go:noinline
+//go:registerparams
+func poison([3]int) {}
+
+//go:noinline
+//go:registerparams
+func escape(s []int) {
+	g = s
+}
+var g []int