cmd/internal/gc: drop unused Reslice field from Node

Dead code.

This field is left over from Go 1.4, when we elided the fake write
barrier in this case. Today, it's unused (always false).
The upcoming append/slice changes handle this case again,
but without needing this field.

Change-Id: Ic6f160b64efdc1bbed02097ee03050f8cd0ab1b8
Reviewed-on: https://go-review.googlesource.com/9789Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/gc/syntax.go

@@ -48,7 +48,6 @@ type Node struct {
 	Assigned    bool   // is the variable ever assigned to
 	Captured    bool   // is the variable captured by a closure
 	Byval       bool   // is the variable captured by value or by reference
-	Reslice     bool   // this is a reslice x = x[0:y] or x = append(x, ...)
 	Likely      int8   // likeliness of if statement
 	Hasbreak    bool   // has break statement
 	Needzero    bool   // if it contains pointers, needs to be zeroed on function entry

src/cmd/internal/gc/typecheck.go

@@ -3360,29 +3360,6 @@ func typecheckas(n *Node) {
 	if n.Left.Typecheck == 0 {
 		typecheck(&n.Left, Erv|Easgn)
 	}
-
-	// Recognize slices being updated in place, for better code generation later.
-	// Don't rewrite if using race detector, to avoid needing to teach race detector
-	// about this optimization.
-	if n.Left != nil && n.Left.Op != OINDEXMAP && n.Right != nil && flag_race == 0 {
-		switch n.Right.Op {
-		// For x = x[0:y], x can be updated in place, without touching pointer.
-		// TODO(rsc): Reenable once it is actually updated in place without touching the pointer.
-		case OSLICE, OSLICE3, OSLICESTR:
-			if false && samesafeexpr(n.Left, n.Right.Left) && (n.Right.Right.Left == nil || iszero(n.Right.Right.Left)) {
-				n.Right.Reslice = true
-			}
-
-			// For x = append(x, ...), x can be updated in place when there is capacity,
-		// without touching the pointer; otherwise the emitted code to growslice
-		// can take care of updating the pointer, and only in that case.
-		// TODO(rsc): Reenable once the emitted code does update the pointer.
-		case OAPPEND:
-			if false && n.Right.List != nil && samesafeexpr(n.Left, n.Right.List.N) {
-				n.Right.Reslice = true
-			}
-		}
-	}
 }
 
 func checkassignto(src *Type, dst *Node) {

src/cmd/internal/gc/walk.go

@@ -2185,26 +2185,6 @@ func needwritebarrier(l *Node, r *Node) bool {
 		return false
 	}
 
-	// No write barrier for reslice: x = x[0:y] or x = append(x, ...).
-	// Both are compiled to modify x directly.
-	// In the case of append, a write barrier may still be needed
-	// if the underlying array grows, but the append code can
-	// generate the write barrier directly in that case.
-	// (It does not yet, but the cost of the write barrier will be
-	// small compared to the cost of the allocation.)
-	if r.Reslice {
-		switch r.Op {
-		case OSLICE, OSLICE3, OSLICESTR, OAPPEND:
-			break
-
-		default:
-			Dump("bad reslice-l", l)
-			Dump("bad reslice-r", r)
-		}
-
-		return false
-	}
-
 	// Otherwise, be conservative and use write barrier.
 	return true
 }