CRC computation is a performance bottleneck

[pavel-kirienko]

Changing the CRC implementation to a table-based one improves the CRC computation time for a 21-frame transfer from 32us to 12us on a 180 MHz Cortex-M4 system. The rest of the serialization pipeline is comparatively cheap to execute -- the CRC is one of the worst offenders that is easy to fix. The cost is <512 bytes of ROM.

--- a/lib/libcanard/canard.c
+++ b/lib/libcanard/canard.c
@@ -73,22 +73,33 @@ typedef uint16_t TransferCRC;
 #define CRC_RESIDUE 0x0000U
 #define CRC_SIZE_BYTES 2U
 
+static const uint16_t CRCTable[256] = {
+    0x0000U, 0x1021U, 0x2042U, 0x3063U, 0x4084U, 0x50A5U, 0x60C6U, 0x70E7U, 0x8108U, 0x9129U, 0xA14AU, 0xB16BU, 0xC18CU,
+    0xD1ADU, 0xE1CEU, 0xF1EFU, 0x1231U, 0x0210U, 0x3273U, 0x2252U, 0x52B5U, 0x4294U, 0x72F7U, 0x62D6U, 0x9339U, 0x8318U,
+    0xB37BU, 0xA35AU, 0xD3BDU, 0xC39CU, 0xF3FFU, 0xE3DEU, 0x2462U, 0x3443U, 0x0420U, 0x1401U, 0x64E6U, 0x74C7U, 0x44A4U,
+    0x5485U, 0xA56AU, 0xB54BU, 0x8528U, 0x9509U, 0xE5EEU, 0xF5CFU, 0xC5ACU, 0xD58DU, 0x3653U, 0x2672U, 0x1611U, 0x0630U,
+    0x76D7U, 0x66F6U, 0x5695U, 0x46B4U, 0xB75BU, 0xA77AU, 0x9719U, 0x8738U, 0xF7DFU, 0xE7FEU, 0xD79DU, 0xC7BCU, 0x48C4U,
+    0x58E5U, 0x6886U, 0x78A7U, 0x0840U, 0x1861U, 0x2802U, 0x3823U, 0xC9CCU, 0xD9EDU, 0xE98EU, 0xF9AFU, 0x8948U, 0x9969U,
+    0xA90AU, 0xB92BU, 0x5AF5U, 0x4AD4U, 0x7AB7U, 0x6A96U, 0x1A71U, 0x0A50U, 0x3A33U, 0x2A12U, 0xDBFDU, 0xCBDCU, 0xFBBFU,
+    0xEB9EU, 0x9B79U, 0x8B58U, 0xBB3BU, 0xAB1AU, 0x6CA6U, 0x7C87U, 0x4CE4U, 0x5CC5U, 0x2C22U, 0x3C03U, 0x0C60U, 0x1C41U,
+    0xEDAEU, 0xFD8FU, 0xCDECU, 0xDDCDU, 0xAD2AU, 0xBD0BU, 0x8D68U, 0x9D49U, 0x7E97U, 0x6EB6U, 0x5ED5U, 0x4EF4U, 0x3E13U,
+    0x2E32U, 0x1E51U, 0x0E70U, 0xFF9FU, 0xEFBEU, 0xDFDDU, 0xCFFCU, 0xBF1BU, 0xAF3AU, 0x9F59U, 0x8F78U, 0x9188U, 0x81A9U,
+    0xB1CAU, 0xA1EBU, 0xD10CU, 0xC12DU, 0xF14EU, 0xE16FU, 0x1080U, 0x00A1U, 0x30C2U, 0x20E3U, 0x5004U, 0x4025U, 0x7046U,
+    0x6067U, 0x83B9U, 0x9398U, 0xA3FBU, 0xB3DAU, 0xC33DU, 0xD31CU, 0xE37FU, 0xF35EU, 0x02B1U, 0x1290U, 0x22F3U, 0x32D2U,
+    0x4235U, 0x5214U, 0x6277U, 0x7256U, 0xB5EAU, 0xA5CBU, 0x95A8U, 0x8589U, 0xF56EU, 0xE54FU, 0xD52CU, 0xC50DU, 0x34E2U,
+    0x24C3U, 0x14A0U, 0x0481U, 0x7466U, 0x6447U, 0x5424U, 0x4405U, 0xA7DBU, 0xB7FAU, 0x8799U, 0x97B8U, 0xE75FU, 0xF77EU,
+    0xC71DU, 0xD73CU, 0x26D3U, 0x36F2U, 0x0691U, 0x16B0U, 0x6657U, 0x7676U, 0x4615U, 0x5634U, 0xD94CU, 0xC96DU, 0xF90EU,
+    0xE92FU, 0x99C8U, 0x89E9U, 0xB98AU, 0xA9ABU, 0x5844U, 0x4865U, 0x7806U, 0x6827U, 0x18C0U, 0x08E1U, 0x3882U, 0x28A3U,
+    0xCB7DU, 0xDB5CU, 0xEB3FU, 0xFB1EU, 0x8BF9U, 0x9BD8U, 0xABBBU, 0xBB9AU, 0x4A75U, 0x5A54U, 0x6A37U, 0x7A16U, 0x0AF1U,
+    0x1AD0U, 0x2AB3U, 0x3A92U, 0xFD2EU, 0xED0FU, 0xDD6CU, 0xCD4DU, 0xBDAAU, 0xAD8BU, 0x9DE8U, 0x8DC9U, 0x7C26U, 0x6C07U,
+    0x5C64U, 0x4C45U, 0x3CA2U, 0x2C83U, 0x1CE0U, 0x0CC1U, 0xEF1FU, 0xFF3EU, 0xCF5DU, 0xDF7CU, 0xAF9BU, 0xBFBAU, 0x8FD9U,
+    0x9FF8U, 0x6E17U, 0x7E36U, 0x4E55U, 0x5E74U, 0x2E93U, 0x3EB2U, 0x0ED1U, 0x1EF0U,
+};
+
 CANARD_PRIVATE TransferCRC crcAddByte(const TransferCRC crc, const uint8_t byte)
 {
-    static const TransferCRC Top  = 0x8000U;
-    static const TransferCRC Poly = 0x1021U;
-    TransferCRC              out  = crc ^ (uint16_t) ((uint16_t) (byte) << BITS_PER_BYTE);
-    // Consider adding a compilation option that replaces this with a CRC table. Adds 512 bytes of ROM.
-    // Do not fold this into a loop because a size-optimizing compiler won't unroll it degrading the performance.
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    out = (uint16_t) ((uint16_t) (out << 1U) ^ (((out & Top) != 0U) ? Poly : 0U));
-    return out;
+    return (uint16_t) ((uint16_t) (crc << BITS_PER_BYTE) ^
+                       CRCTable[(uint16_t) ((crc >> BITS_PER_BYTE) ^ byte) & BYTE_MAX]);
 }
 
 CANARD_PRIVATE TransferCRC crcAdd(const TransferCRC crc, const size_t size, const void* const data)

[coderkalyan]

Addressed in V2.