optimized memset

fixes #109

teensy3/memset.S

+/*
+ * Copyright (c) 2014 Travis Geiselbrecht
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files
+ * (the "Software"), to deal in the Software without restriction,
+ * including without limitation the rights to use, copy, modify, merge,
+ * publish, distribute, sublicense, and/or sell copies of the Software,
+ * and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+ * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+ * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+ * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+//#include <asm.h>
+//#include <arch/arm/cores.h>
+#if defined (__ARM_ARCH_7M__) || defined (__ARM_ARCH_7EM__)
+.global	memset
+.text
+.syntax unified
+.thumb
+.align 2
+
+/* void *memset(void *s, int c, size_t n); */
+	.type	memset, %function
+	.thumb_func
+	memset:
+//FUNCTION(memset)
+    // save the original pointer
+    push    { r0, lr }
+
+    // check for zero length
+    cbz     r2, .L_done
+
+    // short memsets aren't worth optimizing and make sure we have
+    // enough headroom to try to do dwordwise move optimization
+    cmp     r2, #16
+    blt     .L_bytewise
+
+    // see how many bytes we need to move to align to dword boundaries
+    and     r3, r0, #7
+    cbz     r3, .L_prepare_dwordwise
+    rsb     r3, #8
+    subs    r2, r3
+
+.L_bytewise_align:
+    // bytewise to align memset
+    subs    r3, r3, #1
+    strb    r1, [r0], #1
+    bgt     .L_bytewise_align
+
+.L_prepare_dwordwise:
+    // fill a pair of 32 bit registers with the 8 bit value
+    uxtb    r1, r1
+    orr     r1, r1, r1, lsl #8
+    orr     r1, r1, r1, lsl #16
+    mov     r12, r1
+
+    // load the number of dwords left
+    lsrs    r3, r2, #3
+
+.L_dwordwise:
+    // dwordwise memset
+		
+    subs    r3, r3, #1
+    strd    r1, r12, [r0], #8
+    bgt     .L_dwordwise
+
+    // remaining bytes
+    ands     r2, #7
+    beq     .L_done
+
+.L_bytewise:
+    // bytewise memset
+    subs    r2, r2, #1
+    strb    r1, [r0], #1
+    bgt     .L_bytewise
+
+.L_done:
+    // restore the base pointer as return value
+    pop     { r0, pc }
+
+#endif