cmd/compile: enforce nowritebarrier in SSA compiler

Make sure we don't generate write barriers in runtime
code that is marked to forbid write barriers.

Implement the optimization that if we're writing a sliced
slice back to the location it came from, we don't need a
write barrier.

Fixes #14784

Change-Id: I04b6a3b2ac303c19817e932a36a3b006de103aaa
Reviewed-on: https://go-review.googlesource.com/20791Reviewed-by: Austin Clements <austin@google.com>

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

@@ -96,6 +96,14 @@ func buildssa(fn *Node) *ssa.Func {
 	if fn.Func.Pragma&CgoUnsafeArgs != 0 {
 		s.cgoUnsafeArgs = true
 	}
+	if fn.Func.Pragma&Nowritebarrier != 0 {
+		s.noWB = true
+	}
+	defer func() {
+		if s.WBLineno != 0 {
+			fn.Func.WBLineno = s.WBLineno
+		}
+	}()
 	// TODO(khr): build config just once at the start of the compiler binary
 
 	ssaExp.log = printssa
@@ -271,6 +279,8 @@ type state struct {
 	returns []*Node
 
 	cgoUnsafeArgs bool
+	noWB          bool
+	WBLineno      int32 // line number of first write barrier. 0=no write barriers
 }
 
 type funcLine struct {
@@ -2780,6 +2790,13 @@ func (s *state) insertWBmove(t *Type, left, right *ssa.Value, line int32) {
 	// } else {
 	//   *left = *right
 	// }
+
+	if s.noWB {
+		s.Fatalf("write barrier prohibited")
+	}
+	if s.WBLineno == 0 {
+		s.WBLineno = left.Line
+	}
 	bThen := s.f.NewBlock(ssa.BlockPlain)
 	bElse := s.f.NewBlock(ssa.BlockPlain)
 	bEnd := s.f.NewBlock(ssa.BlockPlain)
@@ -2823,6 +2840,12 @@ func (s *state) insertWBstore(t *Type, left, right *ssa.Value, line int32) {
 	//   store pointer fields
 	// }
 
+	if s.noWB {
+		s.Fatalf("write barrier prohibited")
+	}
+	if s.WBLineno == 0 {
+		s.WBLineno = left.Line
+	}
 	s.storeTypeScalars(t, left, right)
 
 	bThen := s.f.NewBlock(ssa.BlockPlain)

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

@@ -2146,6 +2146,13 @@ func needwritebarrier(l *Node, r *Node) bool {
 		return false
 	}
 
+	// No write barrier for writing a sliced slice back to its
+	// original location.
+	if (r.Op == OSLICE || r.Op == OSLICE3 || r.Op == OSLICESTR) &&
+		samesafeexpr(r.Left, l) {
+		return false
+	}
+
 	// Otherwise, be conservative and use write barrier.
 	return true
 }

test/writebarrier.go

@@ -168,3 +168,16 @@ func f17(x *T17) {
 	x.f = f17                      // no barrier
 	x.f = func(y *T17) { *y = *x } // ERROR "write barrier"
 }
+
+type T18 struct {
+	a []int
+	s string
+}
+
+func f18(p *T18, x *[]int) {
+	p.a = p.a[:5]    // no barrier
+	p.a = p.a[3:5]   // no barrier
+	p.a = p.a[1:2:3] // no barrier
+	p.s = p.s[8:9]   // no barrier
+	*x = (*x)[3:5]   // no barrier
+}