[dev.ssa] cmd/compile: fix @ rewrite rules

The @ directive used to read the target block after some value
structure had already changed.  I don't think it was ever really
a bug, but it's confusing.

It might fail like this:

(Foo x y) -> @v.Args[0].Block (Bar y (Baz ...))

v.Op = Bar
v.Args[0] = y
v.Args[1] = v.Args[0].Block.NewValue(Baz, ...)

That new value is allocated in the block of y, not the
block of x.

Anyway, read the destination block first so this
potential bug can't happen.

Change-Id: Ie41d2fc349b35cefaa319fa9327808bcb781b4e2
Reviewed-on: https://go-review.googlesource.com/19900
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>

src/cmd/compile/internal/ssa/TODO

@@ -7,8 +7,6 @@ Coverage
 Correctness
 -----------
 - Debugging info (check & fix as much as we can)
-- @ directive in rewrites might read overwritten data.  Save @loc
-  in variable before modifying v.
 
 Optimizations (better compiled code)
 ------------------------------------

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -259,7 +259,7 @@ func genRules(arch arch) {
 			if t[1] == "nil" {
 				fmt.Fprintf(w, "b.Control = nil\n")
 			} else {
-				fmt.Fprintf(w, "b.Control = %s\n", genResult0(w, arch, t[1], new(int), false, "b"))
+				fmt.Fprintf(w, "b.Control = %s\n", genResult0(w, arch, t[1], new(int), false, false))
 			}
 			if len(newsuccs) < len(succs) {
 				fmt.Fprintf(w, "b.Succs = b.Succs[:%d]\n", len(newsuccs))
@@ -415,16 +415,17 @@ func genMatch0(w io.Writer, arch arch, match, v string, m map[string]string, top
 }
 
 func genResult(w io.Writer, arch arch, result string) {
-	loc := "b"
+	move := false
 	if result[0] == '@' {
 		// parse @block directive
 		s := strings.SplitN(result[1:], " ", 2)
-		loc = s[0]
+		fmt.Fprintf(w, "b = %s\n", s[0])
 		result = s[1]
+		move = true
 	}
-	genResult0(w, arch, result, new(int), true, loc)
+	genResult0(w, arch, result, new(int), true, move)
 }
-func genResult0(w io.Writer, arch arch, result string, alloc *int, top bool, loc string) string {
+func genResult0(w io.Writer, arch arch, result string, alloc *int, top, move bool) string {
 	if result[0] != '(' {
 		// variable
 		if top {
@@ -469,7 +470,7 @@ func genResult0(w io.Writer, arch arch, result string, alloc *int, top bool, loc
 		}
 	}
 	var v string
-	if top && loc == "b" {
+	if top && !move {
 		v = "v"
 		fmt.Fprintf(w, "v.reset(%s)\n", opName(s[0], arch))
 		if typeOverride {
@@ -481,8 +482,8 @@ func genResult0(w io.Writer, arch arch, result string, alloc *int, top bool, loc
 		}
 		v = fmt.Sprintf("v%d", *alloc)
 		*alloc++
-		fmt.Fprintf(w, "%s := %s.NewValue0(v.Line, %s, %s)\n", v, loc, opName(s[0], arch), opType)
-		if top {
+		fmt.Fprintf(w, "%s := b.NewValue0(v.Line, %s, %s)\n", v, opName(s[0], arch), opType)
+		if move {
 			// Rewrite original into a copy
 			fmt.Fprintf(w, "v.reset(OpCopy)\n")
 			fmt.Fprintf(w, "v.AddArg(%s)\n", v)
@@ -501,7 +502,7 @@ func genResult0(w io.Writer, arch arch, result string, alloc *int, top bool, loc
 			fmt.Fprintf(w, "%s.Aux = %s\n", v, x)
 		} else {
 			// regular argument (sexpr or variable)
-			x := genResult0(w, arch, a, alloc, false, loc)
+			x := genResult0(w, arch, a, alloc, false, move)
 			fmt.Fprintf(w, "%s.AddArg(%s)\n", v, x)
 		}
 	}

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -5339,7 +5339,8 @@ func rewriteValueAMD64_OpAMD64MOVBQSX(v *Value, config *Config) bool {
 		sym := v.Args[0].Aux
 		ptr := v.Args[0].Args[0]
 		mem := v.Args[0].Args[1]
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpAMD64MOVBQSXload, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpAMD64MOVBQSXload, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
 		v0.AuxInt = off
@@ -5381,7 +5382,8 @@ func rewriteValueAMD64_OpAMD64MOVBQZX(v *Value, config *Config) bool {
 		sym := v.Args[0].Aux
 		ptr := v.Args[0].Args[0]
 		mem := v.Args[0].Args[1]
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpAMD64MOVBQZXload, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpAMD64MOVBQZXload, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
 		v0.AuxInt = off
@@ -5920,7 +5922,8 @@ func rewriteValueAMD64_OpAMD64MOVLQSX(v *Value, config *Config) bool {
 		sym := v.Args[0].Aux
 		ptr := v.Args[0].Args[0]
 		mem := v.Args[0].Args[1]
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpAMD64MOVLQSXload, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpAMD64MOVLQSXload, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
 		v0.AuxInt = off
@@ -5962,7 +5965,8 @@ func rewriteValueAMD64_OpAMD64MOVLQZX(v *Value, config *Config) bool {
 		sym := v.Args[0].Aux
 		ptr := v.Args[0].Args[0]
 		mem := v.Args[0].Args[1]
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpAMD64MOVLQZXload, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpAMD64MOVLQZXload, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
 		v0.AuxInt = off
@@ -7419,7 +7423,8 @@ func rewriteValueAMD64_OpAMD64MOVWQSX(v *Value, config *Config) bool {
 		sym := v.Args[0].Aux
 		ptr := v.Args[0].Args[0]
 		mem := v.Args[0].Args[1]
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpAMD64MOVWQSXload, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpAMD64MOVWQSXload, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
 		v0.AuxInt = off
@@ -7461,7 +7466,8 @@ func rewriteValueAMD64_OpAMD64MOVWQZX(v *Value, config *Config) bool {
 		sym := v.Args[0].Aux
 		ptr := v.Args[0].Args[0]
 		mem := v.Args[0].Args[1]
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpAMD64MOVWQZXload, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpAMD64MOVWQZXload, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
 		v0.AuxInt = off

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

@@ -7102,10 +7102,13 @@ func rewriteValuegeneric_OpStructSelect(v *Value, config *Config) bool {
 		if !(!config.fe.CanSSA(t)) {
 			break
 		}
-		v0 := v.Args[0].Block.NewValue0(v.Line, OpLoad, v.Type)
+		b = v.Args[0].Block
+		v0 := b.NewValue0(v.Line, OpLoad, v.Type)
 		v.reset(OpCopy)
 		v.AddArg(v0)
-		v1 := v.Args[0].Block.NewValue0(v.Line, OpOffPtr, v.Type.PtrTo())
+		v1 := b.NewValue0(v.Line, OpOffPtr, v.Type.PtrTo())
+		v.reset(OpCopy)
+		v.AddArg(v1)
 		v1.AuxInt = t.FieldOff(i)
 		v1.AddArg(ptr)
 		v0.AddArg(v1)