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>

diff --git a/src/cmd/compile/internal/escape/escape.go b/src/cmd/compile/internal/escape/escape.go
index 5f5dab31f7d95e53cf9ccbdd1d60c75361000c19..2882f9fda3e1e580ae91e878f0c26e568d187454 100644 (file)
--- a/src/cmd/compile/internal/escape/escape.go
+++ b/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)
                }
        }
 }

diff --git a/src/cmd/compile/internal/ssagen/ssa.go b/src/cmd/compile/internal/ssagen/ssa.go
index c3a47ac1d02a006ee871645a4a072fd56b1b877e..25e93b531d55ebe649192633404fc57a2d13c758 100644 (file)
--- a/src/cmd/compile/internal/ssagen/ssa.go
+++ b/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:

diff --git a/src/cmd/compile/internal/walk/order.go b/src/cmd/compile/internal/walk/order.go
index c38477f33e05f9c97ed4626035454f85cda6a22f..3e3bda15e7bd04a26cb747ecefb71618ef68fc4e 100644 (file)
--- a/src/cmd/compile/internal/walk/order.go
+++ b/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()

diff --git a/src/cmd/compile/internal/walk/switch.go b/src/cmd/compile/internal/walk/switch.go
index f59ae33f51afc38dba986669e00c4411090e20ab..3af457b8c002e2badd391df8a928b970a12d2df5 100644 (file)
--- a/src/cmd/compile/internal/walk/switch.go
+++ b/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.

diff --git a/test/escape2.go b/test/escape2.go
index 99f85914a32ed7caa1d3addaf444ff45e4b63ddf..e3e5904cde525bc9b4c2412a1c52b5764e23f837 100644 (file)
--- a/test/escape2.go
+++ b/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
 }
 

diff --git a/test/escape2n.go b/test/escape2n.go
index 350be65202c00b14b24ce9ad258842554a1d9268..57cc1a01639eb410656caaba1e6bc092c4b59d0e 100644 (file)
--- a/test/escape2n.go
+++ b/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
 }