[UDP] replace UDP offsets by struct UdpHeader

[ArnaudD-FR]

Replace UDP array indexes by struct UdpHeader

Description

• less error prone

• decrease code size to flash (save some bytes, see below)

• move ethernet_header(), ethernet_payload(), ip_header(),
  ip_payload().... basic functions to new header EtherUtils.h as those function are required from many places. I didn't want to make them "public" by moving them to EtherCard.h, that's why there is a new header

• adapt htons to void htons(uint16_t &d, const uint16_t v). I've not been able to reduce ASM size of uint16_t htons(const uint16_t v), so I've created a new one accepting the returned value as reference and reduce the code size for each call by 6 bytes.

Rework of htons

for example (building getDHCPandDNS, using CC_VERSION = 5.4.0 (avr-gcc)):

using uint16_t htons(const uint16_t v)

void EtherCard::udpTransmit (uint16_t datalen) {
    IpHeader &iph = ip_header();
    iph.totalLen = htons(sizeof(IpHeader) + sizeof(UdpHeader) + datalen);
    fill_ip_hdr_checksum(iph);
   ...
}

generate the following code:

00000960 <_ZN9EtherCard11udpTransmitEj>:
    0f 93           push    r16
    1f 93           push    r17
    cf 93           push    r28
    df 93           push    r29
    ec 01           movw    r28, r24
    0b ec           ldi r16, 0xCB   ; 203
    12 e0           ldi r17, 0x02   ; 2
    4c 96           adiw    r24, 0x1c   ; 28
    98 27           eor r25, r24
    89 27           eor r24, r25
    98 27           eor r25, r24
    f8 01           movw    r30, r16
    91 8b           std Z+17, r25   ; 0x11
    80 8b           std Z+16, r24   ; 0x10
    89 ed           ldi r24, 0xD9   ; 217
    92 e0           ldi r25, 0x02   ; 2
    0e 94 98 02     call    0x530   ; 0x530 <_ZL20fill_ip_hdr_checksumR8IpHeader.lto_priv.53>

using void htons(uint16_t &d, const uint16_t v)

void EtherCard::udpTransmit (uint16_t datalen) {
    IpHeader &iph = ip_header();
    htons(iph.totalLen, sizeof(IpHeader) + sizeof(UdpHeader) + datalen);
    fill_ip_hdr_checksum(iph);
   ...
}

generate the following code:

<_ZN9EtherCard11udpTransmitEj>:
   0f 93           push    r16
   1f 93           push    r17
   cf 93           push    r28
   df 93           push    r29
   ec 01           movw    r28, r24
   4c 96           adiw    r24, 0x1c   ; 28
   0b ec           ldi r16, 0xCB   ; 203
   12 e0           ldi r17, 0x02   ; 2
   f8 01           movw    r30, r16
   90 8b           std Z+16, r25   ; 0x10
   81 8b           std Z+17, r24   ; 0x11
   89 ed           ldi r24, 0xD9   ; 217
   92 e0           ldi r25, 0x02   ; 2
   0e 94 98 02     call    0x530   ; 0x530 <_ZL20fill_ip_hdr_checksumR8IpHeader.lto_priv.53>

as we can see the following 3 lines doing the bytes swap have disappeared for the same result:

    98 27           eor r25, r24
    89 27           eor r24, r25
    98 27           eor r25, r24

Code size evolution depending on examples

Differences are between revisions 7f42021 and 5ca0418

Negative value: save size
Positive value: increase size

test	old program size	old data size	new program size	new data size	program size delta	data size delta
backSoon	8100	939	8050	939	-50	0
getDHCPandDNS	10326	1246	10302	1246	-24	0
getStaticIP	9674	827	9624	827	-50	0
getViaDNS	10168	820	10144	820	-24	0
multipacket	7730	1279	7682	1279	-48	0
nanether	8020	1093	7982	1093	-38	0
noipClient	14666	1355	14644	1357	-22	2
notifyMyAndroid	11204	1656	11180	1656	-24	0
ntpClient	8984	865	8932	865	-52	0
persistence	10838	823	10814	823	-24	0
pings	10054	1146	10034	1146	-20	0
rbbb_server	8522	881	8470	881	-52	0
SSDP	9220	1153	9138	1153	-82	0
stashTest	7152	539	7152	539	0	0
testDHCP	7808	1101	7788	1101	-20	0
testDHCPOptions	7948	1113	7928	1113	-20	0
thingspeak	14744	1380	14722	1380	-22	0
twitter	11086	1386	11062	1386	-24	0
udpClientSendOnly	8426	1156	8386	1156	-40	0
udpListener	8376	964	8302	964	-74	0
webClient	10314	1164	10290	1164	-24	0
xively	14806	1058	14782	1058	-24	0

[nuno-silva]

This struct should also be __attribute__((__packed__)), right?

[ArnaudD-FR]

No need for this attribute, this struct is perfectly aligned even for 32 bits microcontrollers