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

This CL enables the latent support for string->[]byte conversions
added go.dev/cl/520259.

One catch is that we need to make sure []byte("") evaluates to a
non-nil slice, even if "" is (nil, 0). This CL addresses that by
adding a "ptr != nil" check for OSTR2BYTESTMP, unless the NonNil flag
is set.

The existing uses of OSTR2BYTESTMP (which aren't concerned about
[]byte("") evaluating to nil) are updated to set this flag.

Fixes #2205.

Change-Id: I35a9cb16c164cd86156b7560915aba5108d8b523
Reviewed-on: https://go-review.googlesource.com/c/go/+/520395
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: mdempsky

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

@@ -345,16 +345,11 @@ 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.
-		//
-		// 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)
+		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)
 		}
 	}
 }

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

@@ -2659,6 +2659,14 @@ 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

@@ -815,8 +815,14 @@ 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 n.X.Op() == ir.OSTR2BYTES {
-			n.X.(*ir.ConvExpr).SetOp(ir.OSTR2BYTESTMP)
+		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
+			}
 		}
 
 		t := o.markTemp()

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

@@ -736,6 +736,7 @@ 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$"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$" "zero-copy string->\[\]byte conversion"
 	_ = x
 }
 

test/escape2n.go

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