cabana: optimize get_raw_value() function for CAN signal extraction (#35137)

optimize get_raw_value() for CAN signal extraction

tools/cabana/dbc/dbc.cc

@@ -180,25 +180,36 @@ bool cabana::Signal::operator==(const cabana::Signal &other) const {
 // helper functions
 
 double get_raw_value(const uint8_t *data, size_t data_size, const cabana::Signal &sig) {
-  int64_t val = 0;
+  const int msb_byte = sig.msb / 8;
+  if (msb_byte >= (int)data_size) return 0;
 
-  int i = sig.msb / 8;
-  int bits = sig.size;
-  while (i >= 0 && i < data_size && bits > 0) {
-    int lsb = (int)(sig.lsb / 8) == i ? sig.lsb : i * 8;
-    int msb = (int)(sig.msb / 8) == i ? sig.msb : (i + 1) * 8 - 1;
-    int size = msb - lsb + 1;
+  const int lsb_byte = sig.lsb / 8;
+  uint64_t val = 0;
 
-    uint64_t d = (data[i] >> (lsb - (i * 8))) & ((1ULL << size) - 1);
-    val |= d << (bits - size);
-
-    bits -= size;
-    i = sig.is_little_endian ? i - 1 : i + 1;
+  // Fast path: signal fits in a single byte
+  if (msb_byte == lsb_byte) {
+    val = (data[msb_byte] >> (sig.lsb & 7)) & ((1ULL << sig.size) - 1);
+  } else {
+    // Multi-byte case: signal spans across multiple bytes
+    int bits = sig.size;
+    int i = msb_byte;
+    const int step = sig.is_little_endian ? -1 : 1;
+    while (i >= 0 && i < (int)data_size && bits > 0) {
+      const int msb = (i == msb_byte) ? sig.msb & 7 : 7;
+      const int lsb = (i == lsb_byte) ? sig.lsb & 7 : 0;
+      const int nbits = msb - lsb + 1;
+      val = (val << nbits) | ((data[i] >> lsb) & ((1ULL << nbits) - 1));
+      bits -= nbits;
+      i += step;
+    }
   }
-  if (sig.is_signed) {
-    val -= ((val >> (sig.size - 1)) & 0x1) ? (1ULL << sig.size) : 0;
+
+  // Sign extension (if needed)
+  if (sig.is_signed && (val & (1ULL << (sig.size - 1)))) {
+    val |= ~((1ULL << sig.size) - 1);
   }
-  return val * sig.factor + sig.offset;
+
+  return static_cast<int64_t>(val) * sig.factor + sig.offset;
 }
 
 void updateMsbLsb(cabana::Signal &s) {