cmd/compile/internal/x86: avoid CX in a couple of places in the int64 code

I want to use CX as a scratch register in position independent code and these
uses are easy to remove.

Change-Id: I9e3cb470d7f0000d85786c30bd769d9ec86d532a
Reviewed-on: https://go-review.googlesource.com/16382Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/x86/cgen64.go

@@ -113,19 +113,18 @@ func cgen64(n *gc.Node, res *gc.Node) {
 		gins(x86.ASUBL, &lo2, &ax)
 		gins(x86.ASBBL, &hi2, &dx)
 
-		// let's call the next two EX and FX.
 	case gc.OMUL:
-		var ex gc.Node
+		// let's call the next three EX, FX and GX
+		var ex, fx, gx gc.Node
 		gc.Regalloc(&ex, gc.Types[gc.TPTR32], nil)
-
-		var fx gc.Node
 		gc.Regalloc(&fx, gc.Types[gc.TPTR32], nil)
+		gc.Regalloc(&gx, gc.Types[gc.TPTR32], nil)
 
-		// load args into DX:AX and EX:CX.
+		// load args into DX:AX and EX:GX.
 		gins(x86.AMOVL, &lo1, &ax)
 
 		gins(x86.AMOVL, &hi1, &dx)
-		gins(x86.AMOVL, &lo2, &cx)
+		gins(x86.AMOVL, &lo2, &gx)
 		gins(x86.AMOVL, &hi2, &ex)
 
 		// if DX and EX are zero, use 32 x 32 -> 64 unsigned multiply.
@@ -133,25 +132,26 @@ func cgen64(n *gc.Node, res *gc.Node) {
 
 		gins(x86.AORL, &ex, &fx)
 		p1 := gc.Gbranch(x86.AJNE, nil, 0)
-		gins(x86.AMULL, &cx, nil) // implicit &ax
+		gins(x86.AMULL, &gx, nil) // implicit &ax
 		p2 := gc.Gbranch(obj.AJMP, nil, 0)
 		gc.Patch(p1, gc.Pc)
 
 		// full 64x64 -> 64, from 32x32 -> 64.
-		gins(x86.AIMULL, &cx, &dx)
+		gins(x86.AIMULL, &gx, &dx)
 
 		gins(x86.AMOVL, &ax, &fx)
 		gins(x86.AIMULL, &ex, &fx)
 		gins(x86.AADDL, &dx, &fx)
-		gins(x86.AMOVL, &cx, &dx)
+		gins(x86.AMOVL, &gx, &dx)
 		gins(x86.AMULL, &dx, nil) // implicit &ax
 		gins(x86.AADDL, &fx, &dx)
 		gc.Patch(p2, gc.Pc)
 
 		gc.Regfree(&ex)
 		gc.Regfree(&fx)
+		gc.Regfree(&gx)
 
-		// We only rotate by a constant c in [0,64).
+	// We only rotate by a constant c in [0,64).
 	// if c >= 32:
 	//	lo, hi = hi, lo
 	//	c -= 32

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

@@ -952,15 +952,13 @@ func gmove(f *gc.Node, t *gc.Node) {
 		} 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.
+			// and that code has DX live, so avoid DX and just use AX twice.
 			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_CX)
 			gins(x86.AMOVL, &flo, &r1)
-			gins(x86.AMOVL, &fhi, &r2)
 			gins(x86.AMOVL, &r1, &tlo)
-			gins(x86.AMOVL, &r2, &thi)
+			gins(x86.AMOVL, &fhi, &r1)
+			gins(x86.AMOVL, &r1, &thi)
 		}
 
 		splitclean()