.:madworm:.'s photos with the keyword: stupidity

3.7µs

8.3µs

By accident I had observed that the runtime gets longer and longer. See here as well. Code excerpt: if(bit <= 7) { // read data-bits 0...7 if( (PINA & _BV(PA0)) ) { soft_uart_rx_byte |= _BV(bit); } else { soft_uart_rx_byte &= ~_BV(bit); } [...] As shown here , the _BV(bit) part gets turned into a loop. As 'bit' is a variable, it can't be replaced by a number at compile time. And the loop gets longer for higher bits.

When dealing with this I realized (finally) that I could remove duplicate code and use this as the replacement: uint8_t bit_value = _BV(bit); if(bit <= 7) { // read data-bits 0...7 if( (PINA & _BV(PA0)) ) { soft_uart_rx_byte |= bit_value; } else { soft_uart_rx_byte &= ~bit_value; } [...] Now the _BV(bit) expression has to be evaluated just once. But that didn't help at all. It seems the compiler had already optimized the previous code.

Still 4.0µs...

8.1µs

Now the pulses of trace #3 are more or less constant ;-) Much later it came to my mind that calculating _BV(bit) every single time is just stupid, so I distributed the bit shifting to each invocation of the ISR: static uint8_t bit_value = 1; if( (bit_value > 0) && (bit_value <= 0x80) ) { // 0x80 = 0b10000000 - read data-bits 0...7 if( (PINA & _BV(PA0)) ) { soft_uart_rx_byte |= bit_value; } else { soft_uart_rx_byte &= ~bit_value; } [...] The compiler is also quite clever in optimizing the first if() statement: 4c4: lds r24, 0x0062 4c8: mov r25, r24 4ca: subi r25, 0x01 ; 1 4cc: sbrc r25, 7 4ce: rjmp .+40 ; 0x4f8 4c4: load 'bit_value' to R24 4c8: move R24 to R25 (to keep R24 safe) 4ca: subtract 1 from R25 <-- this is where the magic happens 4cc: check if BIT7 (MSB) is set in R25, if so SKIP 0x4ce 4ce: jump to 0x4f8 'bit_value' should be a) greater than 0 and b) smaller or equal 128. For an uint8_t that means the following: a) At least one bit must be set b) Either the MSB is set, or any combination of the lower bits Condition a) is almost completely included in b) Now subtract one. b) Now only bits 0-6 will be set, bit 7 will have to stay empty ! a) If 'bit_value' started as '0', it would now be 255. the MSB would be set. The if()-statement is now reduced to a _single_ check if BIT7 is empty.

3.3µs

3.2 µs