cmd/compile: clean up SSA-building code

Now that the write barrier insertion is moved to SSA, the SSA
building code can be simplified.

Updates #17583.

Change-Id: I5cacc034b11aa90b0abe6f8dd97e4e3994e2bc25
Reviewed-on: https://go-review.googlesource.com/36840
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -95,6 +95,9 @@ 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{})
 
@@ -354,6 +357,7 @@ func (c *Config) NewFunc() *Func {
 func (c *Config) Logf(msg string, args ...interface{})                 { c.fe.Logf(msg, args...) }
 func (c *Config) Log() bool                                            { return c.fe.Log() }
 func (c *Config) Fatalf(pos src.XPos, msg string, args ...interface{}) { c.fe.Fatalf(pos, msg, args...) }
+func (c *Config) Error(pos src.XPos, msg string, args ...interface{})  { c.fe.Error(pos, msg, args...) }
 func (c *Config) Warnl(pos src.XPos, msg string, args ...interface{})  { c.fe.Warnl(pos, msg, args...) }
 func (c *Config) Debug_checknil() bool                                 { return c.fe.Debug_checknil() }
 func (c *Config) Debug_wb() bool                                       { return c.fe.Debug_wb() }

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

@@ -96,6 +96,7 @@ 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

@@ -27,6 +27,9 @@ 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
 	RegAlloc []Location
 

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

@@ -19,7 +19,7 @@ func needwb(v *Value) bool {
 	if !t.HasPointer() {
 		return false
 	}
-	if IsStackAddr(v.Args[0]) {
+	if isStackAddr(v.Args[0]) {
 		return false // write on stack doesn't need write barrier
 	}
 	return true
@@ -207,6 +207,12 @@ func writebarrier(f *Func) {
 				memElse = bElse.NewValue3I(pos, op, TypeMem, siz, ptr, val, memElse)
 			}
 
+			if f.NoWB {
+				f.Config.fe.Error(pos, "write barrier prohibited")
+			}
+			if !f.WBPos.IsKnown() {
+				f.WBPos = pos
+			}
 			if f.Config.fe.Debug_wb() {
 				f.Config.Warnl(pos, "write barrier")
 			}
@@ -309,8 +315,8 @@ func round(o int64, r int64) int64 {
 	return (o + r - 1) &^ (r - 1)
 }
 
-// IsStackAddr returns whether v is known to be an address of a stack slot
-func IsStackAddr(v *Value) bool {
+// isStackAddr returns whether v is known to be an address of a stack slot
+func isStackAddr(v *Value) bool {
 	for v.Op == OpOffPtr || v.Op == OpAddPtr || v.Op == OpPtrIndex || v.Op == OpCopy {
 		v = v.Args[0]
 	}