Use custom LWIP checksum for ~13% faster checksums (#2172)

Use -O2 only on the LWIP checksum routine, resulting in a speedup of
around 13% (checksumming only, not entire LWIP stack) for 72 add'l bytes
of flash.

cores/rp2040/sdkoverride/inet_chksum.cpp

+// Taken from LWIP's inet_chksum.c just to set the -O2 flag
+
+/*
+    Copyright (c) 2001-2004 Swedish Institute of Computer Science.
+    All rights reserved.
+
+    Redistribution and use in source and binary forms, with or without modification,
+    are permitted provided that the following conditions are met:
+
+    1. Redistributions of source code must retain the above copyright notice,
+      this list of conditions and the following disclaimer.
+    2. Redistributions in binary form must reproduce the above copyright notice,
+      this list of conditions and the following disclaimer in the documentation
+      and/or other materials provided with the distribution.
+    3. The name of the author may not be used to endorse or promote products
+      derived from this software without specific prior written permission.
+
+    THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
+    WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+    MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
+    SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+    EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
+    OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+    INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+    CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+    IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
+    OF SUCH DAMAGE.
+
+    This file is part of the lwIP TCP/IP stack.
+
+    Author: Adam Dunkels <adam@sics.se>
+
+*/
+
+
+
+#include "lwip/opt.h"
+
+#include "lwip/inet_chksum.h"
+#include "lwip/def.h"
+#include "lwip/ip_addr.h"
+
+#pragma GCC optimize ("O2")
+/**
+    An optimized checksum routine. Basically, it uses loop-unrolling on
+    the checksum loop, treating the head and tail bytes specially, whereas
+    the inner loop acts on 8 bytes at a time.
+
+    @arg start of buffer to be checksummed. May be an odd byte address.
+    @len number of bytes in the buffer to be checksummed.
+    @return host order (!) lwip checksum (non-inverted Internet sum)
+
+    by Curt McDowell, Broadcom Corp. December 8th, 2005
+*/
+extern "C" u16_t lwip_standard_chksum(const void *dataptr, int len) {
+    const u8_t *pb = (const u8_t *)dataptr;
+    const u16_t *ps;
+    u16_t t = 0;
+    const u32_t *pl;
+    u32_t sum = 0, tmp;
+    /* starts at odd byte address? */
+    int odd = ((mem_ptr_t)pb & 1);
+
+    if (odd && len > 0) {
+        ((u8_t *)&t)[1] = *pb++;
+        len--;
+    }
+
+    ps = (const u16_t *)(const void *)pb;
+
+    if (((mem_ptr_t)ps & 3) && len > 1) {
+        sum += *ps++;
+        len -= 2;
+    }
+
+    pl = (const u32_t *)(const void *)ps;
+
+    while (len > 7)  {
+        tmp = sum + *pl++;          /* ping */
+        if (tmp < sum) {
+            tmp++;                    /* add back carry */
+        }
+
+        sum = tmp + *pl++;          /* pong */
+        if (sum < tmp) {
+            sum++;                    /* add back carry */
+        }
+
+        len -= 8;
+    }
+
+    /* make room in upper bits */
+    sum = FOLD_U32T(sum);
+
+    ps = (const u16_t *)pl;
+
+    /* 16-bit aligned word remaining? */
+    while (len > 1) {
+        sum += *ps++;
+        len -= 2;
+    }
+
+    /* dangling tail byte remaining? */
+    if (len > 0) {                /* include odd byte */
+        ((u8_t *)&t)[0] = *(const u8_t *)ps;
+    }
+
+    sum += t;                     /* add end bytes */
+
+    /*  Fold 32-bit sum to 16 bits
+        calling this twice is probably faster than if statements... */
+    sum = FOLD_U32T(sum);
+    sum = FOLD_U32T(sum);
+
+    if (odd) {
+        sum = SWAP_BYTES_IN_WORD(sum);
+    }
+
+    return (u16_t)sum;
+}
+

include/lwipopts.h

@@ -43,7 +43,7 @@ extern unsigned long __lwip_rand(void);
 #define MEMP_STATS                  0
 #define LINK_STATS                  0
 // #define ETH_PAD_SIZE                2
-#define LWIP_CHKSUM_ALGORITHM       3
+#define LWIP_CHKSUM_ALGORITHM       0
 #define LWIP_DHCP                   1
 #define LWIP_IPV4                   1
 #define LWIP_TCP                    1

tools/libpico/lwipopts.h

@@ -43,7 +43,7 @@ extern unsigned long __lwip_rand(void);
 #define MEMP_STATS                  0
 #define LINK_STATS                  0
 // #define ETH_PAD_SIZE                2
-#define LWIP_CHKSUM_ALGORITHM       3
+#define LWIP_CHKSUM_ALGORITHM       0
 #define LWIP_DHCP                   1
 #define LWIP_IPV4                   1
 #define LWIP_TCP                    1