runtime: fix regression in BenchmarkCompareBytes* for ppc64x

Between go1.7 and go1.8, a performance regression was introduced in some of the
BenchmarkCompareBytes benchmarks.

Go1.7 vs Go1.8:
BenchmarkCompareBytesToNil-8               7.44          8.44          +13.44%
BenchmarkCompareBytesIdentical-8           6.96          11.5          +65.23%
BenchmarkCompareBytesBigIdentical-8        6.65          47112         +708351.13%

This change fixes the problem by optimizing the case where the byte slices being
compared are equal:

Go1.9 vs current:
BenchmarkCompareBytesToNil-8               7.35          7.00          -4.76%
BenchmarkCompareBytesIdentical-8           11.4          6.81          -40.26%
BenchmarkCompareBytesBigIdentical-8        48396         9.26          -99.98%

runtime.cmpstring can benefit from the same approach and is also changed.

Change-Id: I3cb25f59d8b940a83a2cf687eea764cfeff90688
Reviewed-on: https://go-review.googlesource.com/59650
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>

src/runtime/asm_ppc64x.s

@@ -1258,28 +1258,65 @@ small_string:
 
 TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40
 	MOVD	s1_base+0(FP), R5
-	MOVD	s1_len+8(FP), R3
 	MOVD	s2_base+16(FP), R6
+	MOVD	s1_len+8(FP), R3
+	CMP	R5,R6,CR7
 	MOVD	s2_len+24(FP), R4
 	MOVD	$ret+32(FP), R7
+	CMP	R3,R4,CR6
+	BEQ	CR7,equal
+
+notequal:
 #ifdef	GOARCH_ppc64le
 	BR	cmpbodyLE<>(SB)
 #else
 	BR      cmpbodyBE<>(SB)
 #endif
 
+equal:
+	BEQ	CR6,done
+	MOVD	$1, R8
+	BGT	CR6,greater
+	NEG	R8
+
+greater:
+	MOVD	R8, (R7)
+	RET
+
+done:
+	MOVD	$0, (R7)
+	RET
+
 TEXT bytes·Compare(SB),NOSPLIT|NOFRAME,$0-56
 	MOVD	s1+0(FP), R5
-	MOVD	s1+8(FP), R3
 	MOVD	s2+24(FP), R6
+	MOVD	s1+8(FP), R3
+	CMP	R5,R6,CR7
 	MOVD	s2+32(FP), R4
 	MOVD	$ret+48(FP), R7
+	CMP	R3,R4,CR6
+	BEQ	CR7,equal
+
 #ifdef	GOARCH_ppc64le
 	BR	cmpbodyLE<>(SB)
 #else
 	BR      cmpbodyBE<>(SB)
 #endif
 
+equal:
+	BEQ	CR6,done
+	MOVD	$1, R8
+	BGT	CR6,greater
+	NEG	R8
+
+greater:
+	MOVD	R8, (R7)
+	RET
+
+done:
+	MOVD	$0, (R7)
+	RET
+
 TEXT runtime·return0(SB), NOSPLIT, $0
 	MOVW	$0, R3
 	RET