cmd/compile: avoid using destination pointer base type in memmove optimization

The type of the source and destination of a memmove call isn't
always accurate. It will always be a pointer (or an unsafe.Pointer), but
the base type might not be accurate. This comes about because multiple
copies of a pointer with different base types are coalesced into a single value.

In the failing example, the IData selector of the input argument is a
*[32]byte in one branch of the type switch, and a *[]byte in the other branch.
During the expand_calls pass both IDatas become just copies of the input
register. Those copies are deduped and an arbitrary one wins (in this case,
*[]byte is the unfortunate winner).

Generally an op v can rely on v.Type during rewrite rules. But relying
on v.Args[i].Type is discouraged.

Fixes #55122

Change-Id: I348fd9accf2058a87cd191eec01d39cda612f120
Reviewed-on: https://go-review.googlesource.com/c/go/+/431496
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -2112,39 +2112,42 @@
 
 // Inline small or disjoint runtime.memmove calls with constant length.
 // See the comment in op Move in genericOps.go for discussion of the type.
-
+//
+// Note that we've lost any knowledge of the type and alignment requirements
+// of the source and destination. We only know the size, and that the type
+// contains no pointers.
+// The type of the move is not necessarily v.Args[0].Type().Elem()!
+// See issue 55122 for details.
+//
 // Because expand calls runs after prove, constants useful to this pattern may not appear.
 // Both versions need to exist; the memory and register variants.
 //
 // Match post-expansion calls, memory version.
 (SelectN [0] call:(StaticCall {sym} s1:(Store _ (Const(64|32) [sz]) s2:(Store  _ src s3:(Store {t} _ dst mem)))))
 	&& sz >= 0
 	&& isSameCall(sym, "runtime.memmove")
-	&& t.IsPtr() // avoids TUNSAFEPTR, see issue 30061
 	&& s1.Uses == 1 && s2.Uses == 1 && s3.Uses == 1
 	&& isInlinableMemmove(dst, src, int64(sz), config)
 	&& clobber(s1, s2, s3, call)
-	=> (Move {t.Elem()} [int64(sz)] dst src mem)
+	=> (Move {types.Types[types.TUINT8]} [int64(sz)] dst src mem)
 
 // Match post-expansion calls, register version.
 (SelectN [0] call:(StaticCall {sym} dst src (Const(64|32) [sz]) mem))
 	&& sz >= 0
 	&& call.Uses == 1 // this will exclude all calls with results
 	&& isSameCall(sym, "runtime.memmove")
-	&& dst.Type.IsPtr() // avoids TUNSAFEPTR, see issue 30061
 	&& isInlinableMemmove(dst, src, int64(sz), config)
 	&& clobber(call)
-	=> (Move {dst.Type.Elem()} [int64(sz)] dst src mem)
+	=> (Move {types.Types[types.TUINT8]} [int64(sz)] dst src mem)
 
 // Match pre-expansion calls.
 (SelectN [0] call:(StaticLECall {sym} dst src (Const(64|32) [sz]) mem))
 	&& sz >= 0
 	&& call.Uses == 1 // this will exclude all calls with results
 	&& isSameCall(sym, "runtime.memmove")
-	&& dst.Type.IsPtr() // avoids TUNSAFEPTR, see issue 30061
 	&& isInlinableMemmove(dst, src, int64(sz), config)
 	&& clobber(call)
-	=> (Move {dst.Type.Elem()} [int64(sz)] dst src mem)
+	=> (Move {types.Types[types.TUINT8]} [int64(sz)] dst src mem)
 
 // De-virtualize late-expanded interface calls into late-expanded static calls.
 // Note that (ITab (IMake)) doesn't get rewritten until after the first opt pass,

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

@@ -0,0 +1,42 @@
+// run
+
+// Copyright 2022 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
+
+func main() {
+	for i := 0; i < 10000; i++ {
+		h(i)
+		sink = make([]byte, 1024) // generate some garbage
+	}
+}
+
+func h(iter int) {
+	var x [32]byte
+	for i := 0; i < 32; i++ {
+		x[i] = 99
+	}
+	g(&x)
+	if x == ([32]byte{}) {
+		return
+	}
+	for i := 0; i < 32; i++ {
+		println(x[i])
+	}
+	panic(iter)
+}
+
+//go:noinline
+func g(x interface{}) {
+	switch e := x.(type) {
+	case *[32]byte:
+		var c [32]byte
+		*e = c
+	case *[]byte:
+		*e = nil
+	}
+}
+
+var sink []byte

test/fixedbugs/issue55122.go

@@ -0,0 +1,43 @@
+// run
+
+// Copyright 2022 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
+
+func main() {
+	for i := 0; i < 10000; i++ {
+		h(i)
+		sink = make([]byte, 1024) // generate some garbage
+	}
+}
+
+func h(iter int) {
+	var x [32]byte
+	for i := 0; i < 32; i++ {
+		x[i] = 99
+	}
+	g(&x)
+	if x == ([32]byte{}) {
+		return
+	}
+	for i := 0; i < 32; i++ {
+		println(x[i])
+	}
+	panic(iter)
+}
+
+//go:noinline
+func g(x interface{}) {
+	switch e := x.(type) {
+	case *[32]byte:
+		var c [32]byte
+		*e = c
+	case *[3]*byte:
+		var c [3]*byte
+		*e = c
+	}
+}
+
+var sink []byte