Improved memset

Sourced from https://gitea.planet-casio.com/Lephenixnoir/gint/src/branch/dev/src/std/memset.s

libfxcg/misc/memset.S

@@ -0,0 +1,70 @@
+.global _memset
+.text
+
+_memset:
+	tst	r6, r6
+	bt	.zero
+
+	/* We will fill from the end */
+	mov	r4, r3
+	add	r6, r4
+
+	/* When setting less than 64 bytes, use the naive method */
+	mov	#64, r0
+	cmp/ge	r6, r0
+	bt	_naive_memset
+
+	mov	#3, r2
+
+	/* Make a 4-byte filler */
+	mov	r5, r0
+	shll8	r5
+	or	r5, r0
+	mov	r0, r5
+	shll16	r5
+	or	r5, r0
+
+_memset_align:
+	/* 4-align the destination */
+	mov.b	r0, @-r4
+	tst	r2, r4
+	bf/s	_memset_align
+	dt	r6
+
+	mov	#40, r2
+
+.aligned4_32:
+	add	#-32, r4
+	add	#-32, r6
+	mov.l	r0, @(28,r4)
+	mov.l	r0, @(24,r4)
+	mov.l	r0, @(20,r4)
+	mov.l	r0, @(16,r4)
+	mov.l	r0, @(12,r4)
+	mov.l	r0, @(8,r4)
+	mov.l	r0, @(4,r4)
+	cmp/ge	r6, r2
+	bf/s	.aligned4_32
+	mov.l	r0, @r4
+
+	mov	#8, r2
+
+.aligned4_4:
+	mov.l	r0, @-r4
+	cmp/ge	r6, r2
+	bf/s	.aligned4_4
+	add	#-4, r6
+
+_naive_memset:
+	/* Tight loop copy one byte */
+	dt	r6
+	bf/s	_naive_memset
+	mov.b	r5, @-r4
+
+.end:
+	rts
+	mov	r3, r0
+
+.zero:
+	rts
+	mov	r4, r0