cmd/compile: enable PAUTO capture variables on arch != 6

Fixes #9865.

Change-Id: I8ce5b1708ed938910c59899706e470271c2e7e9d
Reviewed-on: https://go-review.googlesource.com/11699
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -382,12 +382,9 @@ func transformclosure(xfunc *Node) {
 			cv.Xoffset = offset
 			offset += cv.Type.Width
 
-			if v.Name.Byval && v.Type.Width <= int64(2*Widthptr) && Thearch.Thechar == '6' {
-				//  If it is a small variable captured by value, downgrade it to PAUTO.
-				// This optimization is currently enabled only for amd64, see:
-				// https://github.com/golang/go/issues/9865
+			if v.Name.Byval && v.Type.Width <= int64(2*Widthptr) {
+				// If it is a small variable captured by value, downgrade it to PAUTO.
 				v.Class = PAUTO
-
 				v.Ullman = 1
 				xfunc.Func.Dcl = list(xfunc.Func.Dcl, v)
 				body = list(body, Nod(OAS, v, cv))

src/cmd/compile/internal/x86/gsubr.go

@@ -556,7 +556,7 @@ var resvd = []int{
 
 	x86.REG_AX, // for divide
 	x86.REG_CX, // for shift
-	x86.REG_DX, // for divide
+	x86.REG_DX, // for divide, context
 	x86.REG_SP, // for stack
 }
 
@@ -909,10 +909,13 @@ func gmove(f *gc.Node, t *gc.Node) {
 			gins(x86.AMOVL, &flo, &tlo)
 			gins(x86.AMOVL, &fhi, &thi)
 		} else {
+			// Implementation of conversion-free x = y for int64 or uint64 x.
+			// This is generated by the code that copies small values out of closures,
+			// and that code has DX live, so avoid DX and use CX instead.
 			var r1 gc.Node
 			gc.Nodreg(&r1, gc.Types[gc.TUINT32], x86.REG_AX)
 			var r2 gc.Node
-			gc.Nodreg(&r2, gc.Types[gc.TUINT32], x86.REG_DX)
+			gc.Nodreg(&r2, gc.Types[gc.TUINT32], x86.REG_CX)
 			gins(x86.AMOVL, &flo, &r1)
 			gins(x86.AMOVL, &fhi, &r2)
 			gins(x86.AMOVL, &r1, &tlo)