cmd/compile: catch and report nowritebarrier violations later

Prior to this CL, the SSA backend reported violations
of the //go:nowritebarrier annotation immediately.
This necessitated emitting errors during SSA compilation,
which is not compatible with a concurrent backend.

Instead, check for such violations later.
We already save the data required to do a late check
for violations of the //go:nowritebarrierrec annotation.
Use the same data, and check //go:nowritebarrier at the same time.

One downside to doing this is that now only a single
violation will be reported per function.
Given that this is for the runtime only,
and violations are rare, this seems an acceptable cost.

While we are here, remove several 'nerrors != 0' checks
that are rendered pointless.

Updates #15756
Fixes #19250 (as much as it ever can be)

Change-Id: Ia44c4ad5b6fd6f804d9f88d9571cec8d23665cb3
Reviewed-on: https://go-review.googlesource.com/38973
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1223,6 +1223,9 @@ func checknowritebarrierrec() {
 	visitBottomUp(xtop, func(list []*Node, recursive bool) {
 		// Functions with write barriers have depth 0.
 		for _, n := range list {
+			if n.Func.WBPos.IsKnown() && n.Func.Pragma&Nowritebarrier != 0 {
+				yyerrorl(n.Func.WBPos, "write barrier prohibited")
+			}
 			if n.Func.WBPos.IsKnown() && n.Func.Pragma&Yeswritebarrierrec == 0 {
 				c.best[n] = nowritebarrierrecCall{target: nil, depth: 0, lineno: n.Func.WBPos}
 			}

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

@@ -269,19 +269,12 @@ func compile(fn *Node) {
 	if instrumenting {
 		instrument(fn)
 	}
-	if nerrors != 0 {
-		return
-	}
 
 	// From this point, there should be no uses of Curfn. Enforce that.
 	Curfn = nil
 
 	// Build an SSA backend function.
 	ssafn := buildssa(fn)
-	if nerrors != 0 {
-		return
-	}
-
 	pp := newProgs(fn)
 	genssa(ssafn, pp)
 	fieldtrack(pp.Text.From.Sym, fn.Func.FieldTrack)

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

@@ -122,9 +122,6 @@ func buildssa(fn *Node) *ssa.Func {
 	if fn.Func.Pragma&Nosplit != 0 {
 		s.f.NoSplit = true
 	}
-	if fn.Func.Pragma&Nowritebarrier != 0 {
-		s.f.NoWB = true
-	}
 	defer func() {
 		if s.f.WBPos.IsKnown() {
 			fn.Func.WBPos = s.f.WBPos
@@ -197,10 +194,6 @@ func buildssa(fn *Node) *ssa.Func {
 		s.popLine()
 	}
 
-	if nerrors > 0 {
-		return nil
-	}
-
 	s.insertPhis()
 
 	// Don't carry reference this around longer than necessary
@@ -208,10 +201,6 @@ func buildssa(fn *Node) *ssa.Func {
 
 	// Main call to ssa package to compile function
 	ssa.Compile(s.f)
-	if nerrors > 0 {
-		return nil
-	}
-
 	return s.f
 }
 
@@ -4884,11 +4873,6 @@ func (e *ssafn) Fatalf(pos src.XPos, msg string, args ...interface{}) {
 	Fatalf(msg, args...)
 }
 
-// Error reports a compiler error but keep going.
-func (e *ssafn) Error(pos src.XPos, msg string, args ...interface{}) {
-	yyerrorl(pos, msg, args...)
-}
-
 // Warnl reports a "warning", which is usually flag-triggered
 // logging output for the benefit of tests.
 func (e *ssafn) Warnl(pos src.XPos, fmt_ string, args ...interface{}) {

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

@@ -80,9 +80,6 @@ type Logger interface {
 	// Fatal reports a compiler error and exits.
 	Fatalf(pos src.XPos, msg string, args ...interface{})
 
-	// Error reports a compiler error but keep going.
-	Error(pos src.XPos, msg string, args ...interface{})
-
 	// Warnl writes compiler messages in the form expected by "errorcheck" tests
 	Warnl(pos src.XPos, fmt_ string, args ...interface{})
 

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

@@ -124,7 +124,6 @@ func (d DummyFrontend) Logf(msg string, args ...interface{}) { d.t.Logf(msg, arg
 func (d DummyFrontend) Log() bool                            { return true }
 
 func (d DummyFrontend) Fatalf(_ src.XPos, msg string, args ...interface{}) { d.t.Fatalf(msg, args...) }
-func (d DummyFrontend) Error(_ src.XPos, msg string, args ...interface{})  { d.t.Errorf(msg, args...) }
 func (d DummyFrontend) Warnl(_ src.XPos, msg string, args ...interface{})  { d.t.Logf(msg, args...) }
 func (d DummyFrontend) Debug_checknil() bool                               { return false }
 func (d DummyFrontend) Debug_wb() bool                                     { return false }

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

@@ -37,7 +37,6 @@ type Func struct {
 	scheduled bool // Values in Blocks are in final order
 	NoSplit   bool // true if function is marked as nosplit.  Used by schedule check pass.
 
-	NoWB  bool     // write barrier is not allowed
 	WBPos src.XPos // line number of first write barrier
 
 	// when register allocation is done, maps value ids to locations

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

@@ -208,9 +208,6 @@ func writebarrier(f *Func) {
 				memElse.Aux = w.Aux
 			}
 
-			if f.NoWB {
-				f.fe.Error(pos, "write barrier prohibited")
-			}
 			if !f.WBPos.IsKnown() {
 				f.WBPos = pos
 			}