Do not recompile the sketch if there are no changes and libraries have not changed

[cmaglie]

Describe the request

If a sketch is recompiled twice in a row, the Arduino-preprocessed sketch Sketch.ino.cpp is recompiled and re-linked even if not necessary.

Describe the current behavior

If you do the same compile twice in a row:

~/Arduino/Blink$ LANG=en arduino-cli compile -b arduino:avr:uno --clean
Sketch uses 1636 bytes (5%) of program storage space. Maximum is 32256 bytes.
Global variables use 122 bytes (5%) of dynamic memory, leaving 1926 bytes for local variables. Maximum is 2048 bytes.

Used library Version Path                                                                       
Wire         1.0     /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/libraries/Wire

Used platform Version Path                                                        
arduino:avr   1.8.6   /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6

~/Arduino/Blink$ LANG=en arduino-cli compile -b arduino:avr:uno -v
FQBN: arduino:avr:uno
Using board 'uno' from platform in folder: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6
Using core 'arduino' from platform in folder: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6

Detecting libraries used...
[...]
Generating function prototypes...
[...]
Compiling sketch...
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-g++ -c -g -Os -w -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto -mmcu=atmega328p -DF_CPU=16000000L -DARDUINO=10607 -DARDUINO_AVR_UNO -DARDUINO_ARCH_AVR -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/variants/standard -I/home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/libraries/Wire/src /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp -o /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp.o
Compiling libraries...
Compiling library "Wire"
Using previously compiled file: /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/libraries/Wire/utility/twi.c.o
Using previously compiled file: /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/libraries/Wire/Wire.cpp.o
Compiling core...
Using precompiled core: /tmp/arduino/cores/arduino_avr_uno_12e4cfbdc0590d50ab9cd20e50a4c3c5/core.a
Linking everything together...
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-gcc -w -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections -mmcu=atmega328p -o /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.elf /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/sketch/Blink.ino.cpp.o /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/libraries/Wire/Wire.cpp.o /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/libraries/Wire/utility/twi.c.o /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/../../cores/arduino_avr_uno_12e4cfbdc0590d50ab9cd20e50a4c3c5/core.a -L/tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3 -lm
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy -O ihex -j .eeprom --set-section-flags=.eeprom=alloc,load --no-change-warnings --change-section-lma .eeprom=0 /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.elf /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.eep
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-objcopy -O ihex -R .eeprom /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.elf /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.hex
Multiple libraries were found for "Wire.h"
  Used: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/libraries/Wire
  Not used: /home/megabug/Workspace/Arduino_Threads
Using library Wire at version 1.0 in folder: /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/libraries/Wire 
/home/megabug/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin/avr-size -A /tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.elf
Sketch uses 1636 bytes (5%) of program storage space. Maximum is 32256 bytes.
Global variables use 122 bytes (5%) of dynamic memory, leaving 1926 bytes for local variables. Maximum is 2048 bytes.

Used library Version Path                                                                       
Wire         1.0     /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6/libraries/Wire

Used platform Version Path                                                        
arduino:avr   1.8.6   /home/megabug/.arduino15/packages/arduino/hardware/avr/1.8.6

🐛 The sketch file Blink.ino.cpp is recompiled even if there are no changes.

The sketch and linking phase could be completely skipped in this particular case.

Arduino CLI version

nightly

Operating system

N/A

Operating system version

N/A

Additional context

Additional requests

• Sketch re-compiled when unnecessary #1996
• Sketch re-compiled when unnecessary #1996 (comment)
• Sketch re-compiled when unnecessary #1996 (comment)
• Avoid unnecessary compiling when clicking "Upload" button #2015
• Retry upload arduino-ide#270
• Avoid recompiling when uploading multiple times without making changes arduino-ide#1789
• "Upload" also does a build/verify arduino-ide#2103
• Avoid recompiling if the code has not changed arduino-ide#2140
• Can we please have the compiled code SHA stamped so that is doesn't have to recompile after EVERY upload attempt? arduino-ide#2732
• Sketch gets recompiled for every verification and upload Arduino#2335
• Switch in IDE to skip compiling Arduino#6625
• feature request: upload without compile Arduino#8149
• https://forum.arduino.cc/t/recompiling-on-upload-without-code-change/897394
• https://forum.arduino.cc/t/arduino-ide-compiles-every-time-and-is-slow/1044606
• https://forum.arduino.cc/t/ide-2-0-2-why-recompile-every-time/1064264
• https://forum.arduino.cc/t/upload-without-compile/1175639
• https://forum.arduino.cc/t/still-no-upload-without-recompile/1300242

Issue checklist

• I searched for previous requests in the issue tracker
• I verified the feature was still missing when using the nightly build
• My request contains all necessary details