runtime: speed up eqstring

eqstring does not need to check the length of the strings.

6g

benchmark                              old ns/op     new ns/op     delta
BenchmarkCompareStringEqual            7.03          6.14          -12.66%
BenchmarkCompareStringIdentical        3.36          3.04          -9.52%

5g

benchmark                                 old ns/op     new ns/op     delta
BenchmarkCompareStringEqual               238           232           -2.52%
BenchmarkCompareStringIdentical           90.8          80.7          -11.12%

The equivalent PPC changes are in a separate commit
because I don't have the hardware to test them.

Change-Id: I292874324b9bbd9d24f57a390cfff8b550cdd53c
Reviewed-on: https://go-review.googlesource.com/3955Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/gc/walk.c

@@ -1273,6 +1273,7 @@ walkexpr(Node **np, NodeList **init)
 				conv(n->right, types[TSTRING]));
 
 			// quick check of len before full compare for == or !=
+			// eqstring assumes that the lengths are equal
 			if(n->etype == OEQ) {
 				// len(left) == len(right) && eqstring(left, right)
 				r = nod(OANDAND, nod(OEQ, nod(OLEN, n->left, N), nod(OLEN, n->right, N)), r);

src/runtime/asm_386.s

@@ -1298,26 +1298,22 @@ eq:
 	RET
 
 // eqstring tests whether two strings are equal.
+// The compiler guarantees that strings passed
+// to eqstring have equal length.
 // See runtime_test.go:eqstring_generic for
 // equivalent Go code.
 TEXT runtime·eqstring(SB),NOSPLIT,$0-17
-	MOVL	s1len+4(FP), AX
-	MOVL	s2len+12(FP), BX
-	CMPL	AX, BX
-	JNE	different
 	MOVL	s1str+0(FP), SI
 	MOVL	s2str+8(FP), DI
 	CMPL	SI, DI
 	JEQ	same
+	MOVL	s1len+4(FP), BX
 	CALL	runtime·memeqbody(SB)
 	MOVB	AX, v+16(FP)
 	RET
 same:
 	MOVB	$1, v+16(FP)
 	RET
-different:
-	MOVB	$0, v+16(FP)
-	RET
 
 TEXT bytes·Equal(SB),NOSPLIT,$0-25
 	MOVL	a_len+4(FP), BX

src/runtime/asm_amd64.s

@@ -1262,26 +1262,22 @@ eq:
 	RET
 
 // eqstring tests whether two strings are equal.
+// The compiler guarantees that strings passed
+// to eqstring have equal length.
 // See runtime_test.go:eqstring_generic for
 // equivalent Go code.
 TEXT runtime·eqstring(SB),NOSPLIT,$0-33
-	MOVQ	s1len+8(FP), AX
-	MOVQ	s2len+24(FP), BX
-	CMPQ	AX, BX
-	JNE	noteq
 	MOVQ	s1str+0(FP), SI
 	MOVQ	s2str+16(FP), DI
 	CMPQ	SI, DI
 	JEQ	eq
+	MOVQ	s1len+8(FP), BX
 	CALL	runtime·memeqbody(SB)
 	MOVB	AX, v+32(FP)
 	RET
 eq:
 	MOVB	$1, v+32(FP)
 	RET
-noteq:
-	MOVB	$0, v+32(FP)
-	RET
 
 // a in SI
 // b in DI

src/runtime/asm_amd64p32.s

@@ -704,26 +704,22 @@ eq:
 	RET
 
 // eqstring tests whether two strings are equal.
+// The compiler guarantees that strings passed
+// to eqstring have equal length.
 // See runtime_test.go:eqstring_generic for
 // equivalent Go code.
 TEXT runtime·eqstring(SB),NOSPLIT,$0-17
-	MOVL	s1len+4(FP), AX
-	MOVL	s2len+12(FP), BX
-	CMPL	AX, BX
-	JNE	different
 	MOVL	s1str+0(FP), SI
 	MOVL	s2str+8(FP), DI
 	CMPL	SI, DI
 	JEQ	same
+	MOVL	s1len+4(FP), BX
 	CALL	runtime·memeqbody(SB)
 	MOVB	AX, v+16(FP)
 	RET
 same:
 	MOVB	$1, v+16(FP)
 	RET
-different:
-	MOVB	$0, v+16(FP)
-	RET
 
 // a in SI
 // b in DI

src/runtime/asm_arm.s

@@ -806,21 +806,18 @@ eq:
 	RET
 
 // eqstring tests whether two strings are equal.
+// The compiler guarantees that strings passed
+// to eqstring have equal length.
 // See runtime_test.go:eqstring_generic for
 // equivalent Go code.
 TEXT runtime·eqstring(SB),NOSPLIT,$-4-17
-	MOVW	s1len+4(FP), R0
-	MOVW	s2len+12(FP), R1
-	MOVW	$0, R7
-	CMP	R0, R1
-	MOVB.NE R7, v+16(FP)
-	RET.NE
 	MOVW	s1str+0(FP), R2
 	MOVW	s2str+8(FP), R3
 	MOVW	$1, R8
 	MOVB	R8, v+16(FP)
 	CMP	R2, R3
 	RET.EQ
+	MOVW	s1len+4(FP), R0
 	ADD	R2, R0, R6
 loop:
 	CMP	R2, R6
@@ -829,7 +826,8 @@ loop:
 	MOVBU.P	1(R3), R5
 	CMP	R4, R5
 	BEQ	loop
-	MOVB	R7, v+16(FP)
+	MOVW	$0, R8
+	MOVB	R8, v+16(FP)
 	RET
 
 // void setg_gcc(G*); set g called from gcc.