Revert "cmd/compile: enable zero-copy string->[]byte conversions"

This reverts CL 520395.

Reason for revert: thanm@ pointed out failure cases.

Change-Id: I3fd60b73118be3652be2c08b77ab39e793b42110
Reviewed-on: https://go-review.googlesource.com/c/go/+/520596
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/escape/escape.go

@@ -345,11 +345,16 @@ func (b *batch) finish(fns []*ir.Func) {
 
 		// If the result of a string->[]byte conversion is never mutated,
 		// then it can simply reuse the string's memory directly.
-		if n, ok := n.(*ir.ConvExpr); ok && n.Op() == ir.OSTR2BYTES && !loc.hasAttr(attrMutates) {
-			if base.Flag.LowerM >= 1 {
-				base.WarnfAt(n.Pos(), "zero-copy string->[]byte conversion")
+		//
+		// TODO(mdempsky): Enable in a subsequent CL. We need to ensure
+		// []byte("") evaluates to []byte{}, not []byte(nil).
+		if false {
+			if n, ok := n.(*ir.ConvExpr); ok && n.Op() == ir.OSTR2BYTES && !loc.hasAttr(attrMutates) {
+				if base.Flag.LowerM >= 1 {
+					base.WarnfAt(n.Pos(), "zero-copy string->[]byte conversion")
+				}
+				n.SetOp(ir.OSTR2BYTESTMP)
 			}
-			n.SetOp(ir.OSTR2BYTESTMP)
 		}
 	}
 }

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

@@ -2659,14 +2659,6 @@ func (s *state) exprCheckPtr(n ir.Node, checkPtrOK bool) *ssa.Value {
 		n := n.(*ir.ConvExpr)
 		str := s.expr(n.X)
 		ptr := s.newValue1(ssa.OpStringPtr, s.f.Config.Types.BytePtr, str)
-		if !n.NonNil() {
-			// We need to ensure []byte("") evaluates to []byte{}, and not []byte(nil).
-			//
-			// TODO(mdempsky): Investigate using "len != 0" instead of "ptr != nil".
-			cond := s.newValue2(ssa.OpNeqPtr, types.Types[types.TBOOL], ptr, s.constNil(ptr.Type))
-			zerobase := s.newValue1A(ssa.OpAddr, ptr.Type, ir.Syms.Zerobase, s.sb)
-			ptr = s.ternary(cond, ptr, zerobase)
-		}
 		len := s.newValue1(ssa.OpStringLen, types.Types[types.TINT], str)
 		return s.newValue3(ssa.OpSliceMake, n.Type(), ptr, len, len)
 	case ir.OCFUNC:

src/cmd/compile/internal/walk/order.go

@@ -815,14 +815,8 @@ func (o *orderState) stmt(n ir.Node) {
 		// Mark []byte(str) range expression to reuse string backing storage.
 		// It is safe because the storage cannot be mutated.
 		n := n.(*ir.RangeStmt)
-		if x, ok := n.X.(*ir.ConvExpr); ok {
-			switch x.Op() {
-			case ir.OSTR2BYTES:
-				x.SetOp(ir.OSTR2BYTESTMP)
-				fallthrough
-			case ir.OSTR2BYTESTMP:
-				x.MarkNonNil() // "range []byte(nil)" is fine
-			}
+		if n.X.Op() == ir.OSTR2BYTES {
+			n.X.(*ir.ConvExpr).SetOp(ir.OSTR2BYTESTMP)
 		}
 
 		t := o.markTemp()

src/cmd/compile/internal/walk/switch.go

@@ -736,7 +736,6 @@ func stringSearch(expr ir.Node, cc []exprClause, out *ir.Nodes) {
 	// Convert expr to a []int8
 	slice := ir.NewConvExpr(base.Pos, ir.OSTR2BYTESTMP, types.NewSlice(types.Types[types.TINT8]), expr)
 	slice.SetTypecheck(1) // legacy typechecker doesn't handle this op
-	slice.MarkNonNil()
 	// Load the byte we're splitting on.
 	load := ir.NewIndexExpr(base.Pos, slice, ir.NewInt(base.Pos, int64(bestIdx)))
 	// Compare with the value we're splitting on.

test/escape2.go

@@ -1729,7 +1729,7 @@ func intstring2() {
 
 func stringtoslicebyte0() {
 	s := "foo"
-	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$" "zero-copy string->\[\]byte conversion"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$"
 	_ = x
 }
 

test/escape2n.go

@@ -1729,7 +1729,7 @@ func intstring2() {
 
 func stringtoslicebyte0() {
 	s := "foo"
-	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$" "zero-copy string->\[\]byte conversion"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$"
 	_ = x
 }