From 87f89f610072c3f4add1370b876b95d6793a8af8 Mon Sep 17 00:00:00 2001 From: Matthijs Kooijman Date: Tue, 22 Apr 2014 22:24:43 +0200 Subject: [PATCH] Optimize SoftwareSerial::recv Similar to SoftwareSerial::write, this rewrites the loop to only touch the MSB and then shift those bits up, allowing the compiler to generate more efficient code. Unlike the write function however, it is not needed to put all instance variables used into local variables, for some reason the compiler already does this (and doing it manually even makes the code bigger). On the Arduino Uno using gcc 4.3 this saves 26 bytes. Using gcc 4.8 this saves 30 bytes. Note that this removes the else clause in the code, making the C code unbalanced, which looks like it breaks timing balance. However, looking at the code generated by the compiler, it turns out that the old code was actually unbalanced, while the new code is properly balanced. --- .../avr/libraries/SoftwareSerial/SoftwareSerial.cpp | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff --git a/hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.cpp b/hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.cpp index 621a2af89..974aa1e31 100644 --- a/hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.cpp +++ b/hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.cpp @@ -242,15 +242,13 @@ void SoftwareSerial::recv() DebugPulse(_DEBUG_PIN2, 1); // Read each of the 8 bits - for (uint8_t i=0x1; i; i <<= 1) + for (uint8_t i=8; i > 0; --i) { tunedDelay(_rx_delay_intrabit); + d >>= 1; DebugPulse(_DEBUG_PIN2, 1); - uint8_t noti = ~i; if (rx_pin_read()) - d |= i; - else // else clause added to ensure function timing is ~balanced - d &= noti; + d |= 0x80; } // skip the stop bit