cmd/compile: get rid of "volatile" in SSA

A value is "volatile" if it is a pointer to the argument region
on stack which will be clobbered by function call. This is used
to make sure the value is safe when inserting write barrier calls.
The writebarrier pass can tell whether a value is such a pointer.
Therefore no need to mark it when building SSA and thread this
information through.

Passes "toolstash -cmp" on std.

Updates #17583.

Change-Id: Idc5fc0d710152b94b3c504ce8db55ea9ff5b5195
Reviewed-on: https://go-review.googlesource.com/36835
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/gen/genericOps.go

@@ -300,7 +300,6 @@ var genericOps = []opData{
 	// Expand to runtime calls. Write barrier will be removed if write on stack.
 	{name: "StoreWB", argLength: 3, typ: "Mem", aux: "Int64"},          // Store arg1 to arg0. arg2=memory, auxint=size.  Returns memory.
 	{name: "MoveWB", argLength: 3, typ: "Mem", aux: "SymSizeAndAlign"}, // arg0=destptr, arg1=srcptr, arg2=mem, auxint=size+alignment, aux=symbol-of-type (for typedmemmove).  Returns memory.
-	{name: "MoveWBVolatile", argLength: 3, typ: "Mem", aux: "SymSizeAndAlign"}, // arg0=destptr, arg1=srcptr, arg2=mem, auxint=size+alignment, aux=symbol-of-type (for typedmemmove).  Returns memory. Src is volatile, i.e. needs to move to a temp space before calling typedmemmove.
 	{name: "ZeroWB", argLength: 2, typ: "Mem", aux: "SymSizeAndAlign"}, // arg0=destptr, arg1=mem, auxint=size+alignment, aux=symbol-of-type. Returns memory.
 
 	// Function calls. Arguments to the call have already been written to the stack.

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

@@ -1818,7 +1818,6 @@ const (
 	OpZero
 	OpStoreWB
 	OpMoveWB
-	OpMoveWBVolatile
 	OpZeroWB
 	OpClosureCall
 	OpStaticCall
@@ -21623,12 +21622,6 @@ var opcodeTable = [...]opInfo{
 		argLen:  3,
 		generic: true,
 	},
-	{
-		name:    "MoveWBVolatile",
-		auxType: auxSymSizeAndAlign,
-		argLen:  3,
-		generic: true,
-	},
 	{
 		name:    "ZeroWB",
 		auxType: auxSymSizeAndAlign,

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

@@ -35,12 +35,12 @@ func writebarrier(f *Func) {
 		hasStore := false
 		for _, v := range b.Values {
 			switch v.Op {
-			case OpStoreWB, OpMoveWB, OpMoveWBVolatile, OpZeroWB:
+			case OpStoreWB, OpMoveWB, OpZeroWB:
 				if IsStackAddr(v.Args[0]) {
 					switch v.Op {
 					case OpStoreWB:
 						v.Op = OpStore
-					case OpMoveWB, OpMoveWBVolatile:
+					case OpMoveWB:
 						v.Op = OpMove
 						v.Aux = nil
 					case OpZeroWB:
@@ -103,7 +103,7 @@ func writebarrier(f *Func) {
 		values := b.Values
 		for i := len(values) - 1; i >= 0; i-- {
 			w := values[i]
-			if w.Op == OpStoreWB || w.Op == OpMoveWB || w.Op == OpMoveWBVolatile || w.Op == OpZeroWB {
+			if w.Op == OpStoreWB || w.Op == OpMoveWB || w.Op == OpZeroWB {
 				if last == nil {
 					last = w
 					end = i + 1
@@ -159,7 +159,7 @@ func writebarrier(f *Func) {
 			var val *Value
 			ptr := w.Args[0]
 			siz := w.AuxInt
-			typ := w.Aux // only non-nil for MoveWB, MoveWBVolatile, ZeroWB
+			typ := w.Aux // only non-nil for MoveWB, ZeroWB
 			pos = w.Pos
 
 			var op Op
@@ -169,7 +169,7 @@ func writebarrier(f *Func) {
 				op = OpStore
 				fn = writebarrierptr
 				val = w.Args[1]
-			case OpMoveWB, OpMoveWBVolatile:
+			case OpMoveWB:
 				op = OpMove
 				fn = typedmemmove
 				val = w.Args[1]
@@ -179,7 +179,8 @@ func writebarrier(f *Func) {
 			}
 
 			// then block: emit write barrier call
-			memThen = wbcall(pos, bThen, fn, typ, ptr, val, memThen, sp, sb, w.Op == OpMoveWBVolatile)
+			volatile := w.Op == OpMoveWB && isVolatile(val)
+			memThen = wbcall(pos, bThen, fn, typ, ptr, val, memThen, sp, sb, volatile)
 
 			// else block: normal store
 			if op == OpZero {
@@ -223,7 +224,7 @@ func writebarrier(f *Func) {
 
 		// if we have more stores in this block, do this block again
 		for _, w := range b.Values {
-			if w.Op == OpStoreWB || w.Op == OpMoveWB || w.Op == OpMoveWBVolatile || w.Op == OpZeroWB {
+			if w.Op == OpStoreWB || w.Op == OpMoveWB || w.Op == OpZeroWB {
 				goto again
 			}
 		}
@@ -303,3 +304,12 @@ func IsStackAddr(v *Value) bool {
 	}
 	return false
 }
+
+// isVolatile returns whether v is a pointer to argument region on stack which
+// will be clobbered by a function call.
+func isVolatile(v *Value) bool {
+	for v.Op == OpOffPtr || v.Op == OpAddPtr || v.Op == OpPtrIndex || v.Op == OpCopy {
+		v = v.Args[0]
+	}
+	return v.Op == OpSP
+}