string: use gint's optimized memcmp (DONE)

CMakeLists.txt

@@ -172,6 +172,7 @@ if(sh-generic IN_LIST TARGET_FOLDERS)
     src/libc/setjmp/target/sh-generic/setjmp.S
     src/libc/setjmp/target/sh-generic/longjmp.S
     src/libc/string/target/sh-generic/memchr.S
+    src/libc/string/target/sh-generic/memcmp.S
     src/libc/string/target/sh-generic/memcpy.S
     src/libc/string/target/sh-generic/memset.S
     src/libc/string/target/sh-generic/strlen.S

src/libc/string/target/sh-generic/memcmp.S

@@ -0,0 +1,117 @@
+#include <bits/asm/cpucap.h>
+
+.global _memcmp
+.text
+
+_memcmp:
+	tst	r6, r6
+	bt	.zero
+
+	/* When comparing less than 64 bytes, use the naive method */
+	mov	#64, r0
+	cmp/ge	r6, r0
+	bt	_naive_memcmp
+
+	mov	#4, r2
+	mov	#3, r3
+
+_memcmp_align_rhs:
+	/* 4-align the right-hand side */
+	mov.b	@r4+, r0
+	mov.b	@r5+, r1
+	cmp/eq	r0, r1
+	bf/s	.end
+	dt	r6
+	tst	r3, r5
+	bf	_memcmp_align_rhs
+
+	/* If left-hand side is 4-aligned, use mov.l */
+	tst	r3, r4
+	bt	.aligned4
+
+	/* If unaligned but SH4, use movua.l */
+	mov.l	.___cpucap, r0
+	mov.l	@r0, r0
+	tst	#__CPUCAP_SH4ALDSP, r0
+	bf	.unaligned4
+
+	/* If left-hand side is 2-aligned, use mov.w and mov.l */
+	mov	r4, r0
+	tst	#1, r0
+	bt	.aligned2
+
+	/* Otherwise use a naive comparison */
+	bra	_naive_memcmp
+	nop
+
+.aligned4:
+	/* Compare 4 bytes at a time until at most 4 bytes are left */
+	mov.l	@r4+, r0
+	mov.l	@r5+, r1
+	cmp/eq	r0, r1
+	bf/s	_fail
+	add	#-4, r6
+	cmp/ge	r6, r2
+	bf	.aligned4
+
+	bra	_naive_memcmp
+	nop
+
+.unaligned4:
+	/* Compare 4 bytes at a time until at most 4 bytes are left. Since
+	   left-hand side is aligned, use movua.l */
+	movua.l	@r4+, r0
+	mov.l	@r5+, r1
+	cmp/eq	r0, r1
+	bf/s	_fail
+	add	#-4, r6
+	cmp/ge	r6, r2
+	bf	.unaligned4
+
+	bra	_naive_memcmp
+	nop
+
+.aligned2:
+	/* Read 4 bytes from r4 in two steps */
+	mov.w	@r4+, r0
+	mov.l	@r5+, r1
+	mov.w	@r4+, r2
+	shll16	r0
+	or	r2, r0
+	cmp/eq	r0, r1
+	bf/s	_fail
+	add	#-4, r6
+	cmp/ge	r6, r2
+	bf	.aligned2
+
+	bra	_naive_memcmp
+	nop
+
+_fail:
+	/* Rewind 4 bytes to compare manually */
+	add	#-4, r4
+	add	#-4, r5
+	add	#4, r6
+
+_naive_memcmp:
+	mov.b	@r4+, r0
+	mov.b	@r5+, r1
+	cmp/eq	r0, r1
+	bf/s	.end
+	dt	r6
+	bf	_naive_memcmp
+
+.end:
+	extu.b	r0, r0
+	extu.b	r1, r1
+	rts
+	sub	r1, r0
+
+.zero:
+	rts
+	mov	#0, r0
+
+.align 4
+
+.___cpucap:
+	.long ___cpucap