runtime: improve CALLFN macro for ppc64x

The previous CALLFN macro was copying a single byte at a
time which is extremely inefficient on ppc64x. This changes
the macro so it copies 8 bytes at a time.

benchmark in reflect:
name                      old time/op    new time/op    delta
Call-8                       177ns ± 0%     165ns ± 0%    -6.78%  (p=1.000 n=1+1)
CallArgCopy/size=128-8       194ns ± 0%     140ns ± 0%   -27.84%  (p=1.000 n=1+1)
CallArgCopy/size=256-8       253ns ± 0%     159ns ± 0%   -37.15%  (p=1.000 n=1+1)
CallArgCopy/size=1024-8      612ns ± 0%     222ns ± 0%   -63.73%  (p=1.000 n=1+1)
CallArgCopy/size=4096-8     2.14µs ± 0%    0.53µs ± 0%   -75.01%  (p=1.000 n=1+1)
CallArgCopy/size=65536-8    33.0µs ± 0%     7.3µs ± 0%   -77.72%  (p=1.000 n=1+1)

Change-Id: I71f6ee788264e61bb072264d21b77b83592c9dca
Reviewed-on: https://go-review.googlesource.com/134635
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Michael Munday <mike.munday@ibm.com>

src/runtime/asm_ppc64x.s

@@ -390,15 +390,36 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-24;		\
 	/* copy arguments to stack */		\
 	MOVD	arg+16(FP), R3;			\
 	MOVWZ	argsize+24(FP), R4;			\
-	MOVD	R1, R5;				\
-	ADD	$(FIXED_FRAME-1), R5;			\
-	SUB	$1, R3;				\
-	ADD	R5, R4;				\
-	CMP	R5, R4;				\
-	BEQ	4(PC);				\
-	MOVBZU	1(R3), R6;			\
-	MOVBZU	R6, 1(R5);			\
-	BR	-4(PC);				\
+	MOVD    R1, R5;				\
+	CMP	R4, $8;				\
+	BLT	tailsetup;			\
+	/* copy 8 at a time if possible */	\
+	ADD	$(FIXED_FRAME-8), R5;			\
+	SUB	$8, R3;				\
+top: \
+	MOVDU	8(R3), R7;			\
+	MOVDU	R7, 8(R5);			\
+	SUB	$8, R4;				\
+	CMP	R4, $8;				\
+	BGE	top;				\
+	/* handle remaining bytes */	\
+	CMP	$0, R4;			\
+	BEQ	callfn;			\
+	ADD	$7, R3;			\
+	ADD	$7, R5;			\
+	BR	tail;			\
+tailsetup: \
+	CMP	$0, R4;			\
+	BEQ	callfn;			\
+	ADD     $(FIXED_FRAME-1), R5;	\
+	SUB     $1, R3;			\
+tail: \
+	MOVBU	1(R3), R6;		\
+	MOVBU	R6, 1(R5);		\
+	SUB	$1, R4;			\
+	CMP	$0, R4;			\
+	BGT	tail;			\
+callfn: \
 	/* call function */			\
 	MOVD	f+8(FP), R11;			\
 	MOVD	(R11), R12;			\