cmd/compile,runtime: change unsafe.Slice((*T)(nil), 0) to return []T(nil)

This CL removes the unconditional OCHECKNIL check added in
walkUnsafeSlice by instead passing it as a pointer to
runtime.unsafeslice, and hiding the check behind a `len == 0` check.

While here, this CL also implements checkptr functionality for
unsafe.Slice and disallows use of unsafe.Slice with //go:notinheap
types.

Updates #46742.

Change-Id: I743a445ac124304a4d7322a7fe089c4a21b9a655
Reviewed-on: https://go-review.googlesource.com/c/go/+/331070
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 9 insertions, 17 deletions

diff --git a/src/cmd/compile/internal/walk/builtin.go b/src/cmd/compile/internal/walk/builtin.go
index 62eb4298f4..1f08e4d312 100644
--- a/src/cmd/compile/internal/walk/builtin.go
+++ b/src/cmd/compile/internal/walk/builtin.go
@@ -654,36 +654,28 @@ func walkRecover(nn *ir.CallExpr, init *ir.Nodes) ir.Node {
 }
 
 func walkUnsafeSlice(n *ir.BinaryExpr, init *ir.Nodes) ir.Node {
+	ptr := safeExpr(n.X, init)
 	len := safeExpr(n.Y, init)
 
 	fnname := "unsafeslice64"
-	argtype := types.Types[types.TINT64]
+	lenType := types.Types[types.TINT64]
 
 	// Type checking guarantees that TIDEAL len/cap are positive and fit in an int.
 	// The case of len or cap overflow when converting TUINT or TUINTPTR to TINT
 	// will be handled by the negative range checks in unsafeslice during runtime.
-	if len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size() {
+	if ir.ShouldCheckPtr(ir.CurFunc, 1) {
+		fnname = "unsafeslicecheckptr"
+		// for simplicity, unsafeslicecheckptr always uses int64
+	} else if len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size() {
 		fnname = "unsafeslice"
-		argtype = types.Types[types.TINT]
+		lenType = types.Types[types.TINT]
 	}
 
 	t := n.Type()
 
-	// Call runtime.unsafeslice[64] to check that the length argument is
-	// non-negative and smaller than the max length allowed for the
-	// element type.
+	// Call runtime.unsafeslice{,64,checkptr} to check ptr and len.
 	fn := typecheck.LookupRuntime(fnname)
-	init.Append(mkcall1(fn, nil, init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(len, argtype)))
-
-	ptr := walkExpr(n.X, init)
-
-	c := ir.NewUnaryExpr(n.Pos(), ir.OCHECKNIL, ptr)
-	c.SetTypecheck(1)
-	init.Append(c)
-
-	// TODO(mdempsky): checkptr instrumentation. Maybe merge into length
-	// check above, along with nil check? Need to be careful about
-	// notinheap pointers though: can't pass them as unsafe.Pointer.
+	init.Append(mkcall1(fn, nil, init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(ptr, types.Types[types.TUNSAFEPTR]), typecheck.Conv(len, lenType)))
 
 	h := ir.NewSliceHeaderExpr(n.Pos(), t,
 		typecheck.Conv(ptr, types.Types[types.TUNSAFEPTR]),