cmd/compile: do nil check before calling duff functions, on arm64 and amd64

On these platforms, we set up a frame pointer record below
the current stack pointer, so when we're in duffcopy or duffzero,
we get a reasonable traceback. See #73753.

But because this frame pointer record is below SP, it is vulnerable.
Anything that adds a new stack frame to the stack might clobber it.
Which actually happens in #73748 on amd64. I have not yet come across
a repro on arm64, but might as well be safe here.

The only real situation this could happen is when duffzero or duffcopy
is passed a nil pointer. So we can just avoid the problem by doing the
nil check outside duffzero/duffcopy. That way we never add a frame
below duffzero/duffcopy. (Most other ways to get a new frame below the
current one, like async preempt or debugger-generated calls, don't
apply to duffzero/duffcopy because they are runtime functions; we're
not allowed to preempt there.)

Longer term, we should stop putting stuff below SP. #73753 will
include that as part of its remit. But that's not for 1.25, so we'll
do the simple thing for 1.25 for this issue.

Fixes #73748

Change-Id: I913c49ee46dcaee8fb439415a4531f7b59d0f612
Reviewed-on: https://go-review.googlesource.com/c/go/+/676916
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/_gen/AMD64Ops.go

@@ -897,8 +897,8 @@ func init() {
 				inputs:   []regMask{buildReg("DI")},
 				clobbers: buildReg("DI"),
 			},
-			faultOnNilArg0: true,
-			unsafePoint:    true, // FP maintenance around DUFFCOPY can be clobbered by interrupts
+			//faultOnNilArg0: true, // Note: removed for 73748. TODO: reenable at some point
+			unsafePoint: true, // FP maintenance around DUFFCOPY can be clobbered by interrupts
 		},
 
 		// arg0 = address of memory to zero
@@ -935,10 +935,10 @@ func init() {
 				inputs:   []regMask{buildReg("DI"), buildReg("SI")},
 				clobbers: buildReg("DI SI X0"), // uses X0 as a temporary
 			},
-			clobberFlags:   true,
-			faultOnNilArg0: true,
-			faultOnNilArg1: true,
-			unsafePoint:    true, // FP maintenance around DUFFCOPY can be clobbered by interrupts
+			clobberFlags: true,
+			//faultOnNilArg0: true, // Note: removed for 73748. TODO: reenable at some point
+			//faultOnNilArg1: true,
+			unsafePoint: true, // FP maintenance around DUFFCOPY can be clobbered by interrupts
 		},
 
 		// arg0 = destination pointer

src/cmd/compile/internal/ssa/_gen/ARM64Ops.go

@@ -554,8 +554,8 @@ func init() {
 				inputs:   []regMask{buildReg("R20")},
 				clobbers: buildReg("R16 R17 R20 R30"),
 			},
-			faultOnNilArg0: true,
-			unsafePoint:    true, // FP maintenance around DUFFZERO can be clobbered by interrupts
+			//faultOnNilArg0: true, // Note: removed for 73748. TODO: reenable at some point
+			unsafePoint: true, // FP maintenance around DUFFZERO can be clobbered by interrupts
 		},
 
 		// large zeroing
@@ -595,9 +595,9 @@ func init() {
 				inputs:   []regMask{buildReg("R21"), buildReg("R20")},
 				clobbers: buildReg("R16 R17 R20 R21 R26 R30"),
 			},
-			faultOnNilArg0: true,
-			faultOnNilArg1: true,
-			unsafePoint:    true, // FP maintenance around DUFFCOPY can be clobbered by interrupts
+			//faultOnNilArg0: true, // Note: removed for 73748. TODO: reenable at some point
+			//faultOnNilArg1: true,
+			unsafePoint: true, // FP maintenance around DUFFCOPY can be clobbered by interrupts
 		},
 
 		// large move

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

@@ -0,0 +1,32 @@
+// 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
+
+import (
+	"context"
+	"io"
+	"runtime/trace"
+)
+
+type T struct {
+	a [16]int
+}
+
+//go:noinline
+func f(x *T) {
+	*x = T{}
+}
+
+func main() {
+	trace.Start(io.Discard)
+	defer func() {
+		recover()
+		trace.Log(context.Background(), "a", "b")
+
+	}()
+	f(nil)
+}

test/fixedbugs/issue73748a.go

@@ -0,0 +1,32 @@
+// 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
+
+import (
+	"context"
+	"io"
+	"runtime/trace"
+)
+
+type T struct {
+	a [16]int
+}
+
+//go:noinline
+func f(x, y *T) {
+	*x = *y
+}
+
+func main() {
+	trace.Start(io.Discard)
+	defer func() {
+		recover()
+		trace.Log(context.Background(), "a", "b")
+
+	}()
+	f(nil, nil)
+}