Diff: arch/checksum.c

Revision:

10:09b0951b1899

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/arch/checksum.c	Tue Sep 10 15:14:42 2013 +0300
@@ -0,0 +1,126 @@
+/* Copyright (C) 2013 - Adam Green (https://github.com/adamgreen)
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+#if defined(TOOLCHAIN_GCC) && defined(__thumb2__)
+
+
+/* This is a hand written Thumb-2 assembly language version of the
+   algorithm 3 version of lwip_standard_chksum in lwIP's inet_chksum.c.  It
+   performs the checksumming 32-bits at a time and even unrolls the loop to
+   perform two of these 32-bit adds per loop iteration.
+   
+   Returns:
+        16-bit 1's complement summation (not inversed).
+        
+   NOTE: This function does return a uint16_t from the assembly language code
+         but is marked as void so that GCC doesn't issue warning because it
+         doesn't know about this low level return.
+*/
+__attribute__((naked)) void /*uint16_t*/ thumb2_checksum(const void* pData, int length)
+{
+    __asm (
+        ".syntax unified\n"
+        ".thumb\n"
+
+        // Push non-volatile registers we use on stack.  Push link register too to
+        // keep stack 8-byte aligned and allow single pop to restore and return.
+        "    push        {r4, lr}\n"
+        // Initialize sum, r2, to 0.
+        "    movs    r2, #0\n"
+        // Remember whether pData was at odd address in r3.  This is used later to
+        // know if it needs to swap the result since the summation will be done at
+        // an offset of 1, rather than 0.
+        "    ands    r3, r0, #1\n"
+        // Need to 2-byte align?  If not skip ahead.
+        "    beq     1$\n"
+        // We can return if there are no bytes to sum.
+        "    cbz     r1, 9$\n"
+
+        // 2-byte align.
+        // Place the first data byte in odd summation location since it needs to be
+        // swapped later.  It's ok to overwrite r2 here as it only had a value of 0
+        // up until now.  Advance r0 pointer and decrement r1 length as we go.
+        "    ldrb    r2, [r0], #1\n"
+        "    lsls    r2, r2, #8\n"
+        "    subs    r1, r1, #1\n"
+
+        // Need to 4-byte align?  If not skip ahead.
+        "1$:\n"
+        "    ands    r4, r0, #3\n"
+        "    beq     2$\n"
+        // Have more than 1 byte left to align?  If not skip ahead to take care of
+        // trailing byte.
+        "    cmp     r1, #2\n"
+        "    blt     7$\n"
+
+        // 4-byte align.
+        "    ldrh    r4, [r0], #2\n"
+        "    adds    r2, r2, r4\n"
+        "    subs    r1, r1, #2\n"
+
+        // Main summing loop which sums up data 2 words at a time.
+        // Make sure that we have more than 7 bytes left to sum.
+        "2$:\n"
+        "    cmp     r1, #8\n"
+        "    blt     3$\n"
+        // Sum next two words.  Applying previous upper 16-bit carry to
+        // lower 16-bits.
+        "    ldr     r4, [r0], #4\n"
+        "    adds    r2, r4\n"
+        "    adc     r2, r2, #0\n"
+        "    ldr     r4, [r0], #4\n"
+        "    adds    r2, r4\n"
+        "    adc     r2, r2, #0\n"
+        "    subs    r1, r1, #8\n"
+        "    b       2$\n"
+
+        // Sum up any remaining half-words.
+        "3$:\n"
+        // Make sure that we have more than 1 byte left to sum.
+        "    cmp     r1, #2\n"
+        "    blt     7$\n"
+        // Sum up next half word, continue to apply carry.
+        "    ldrh    r4, [r0], #2\n"
+        "    adds    r2, r4\n"
+        "    adc     r2, r2, #0\n"
+        "    subs    r1, r1, #2\n"
+        "    b       3$\n"
+
+        // Handle trailing byte, if it exists
+        "7$:\n"
+        "    cbz     r1, 8$\n"
+        "    ldrb    r4, [r0]\n"
+        "    adds    r2, r4\n"
+        "    adc     r2, r2, #0\n"
+
+        // Fold 32-bit checksum into 16-bit checksum.
+        "8$:\n"
+        "    ubfx    r4, r2, #16, #16\n"
+        "    ubfx    r2, r2, #0, #16\n"
+        "    adds    r2, r4\n"
+        "    ubfx    r4, r2, #16, #16\n"
+        "    ubfx    r2, r2, #0, #16\n"
+        "    adds    r2, r4\n"
+
+        // Swap bytes if started at odd address
+        "    cbz     r3, 9$\n"
+        "    rev16   r2, r2\n"
+
+        // Return final sum.
+        "9$: mov     r0, r2\n"
+        "    pop     {r4, pc}\n"
+    );
+}
+
+#endif