fix uninitialized cutting power warning in M3-M5.cpp

[alx3dev]

Description

When compiling Marlin for Creality v4.2.7 board and LASER_FEATURE is enabled, there is warning saying that u may be used uninitialized:

Marlin/src/gcode/control/M3-M5.cpp: In function 'GcodeSuite::M3_M4(bool)::<lambda()>':
Marlin/src/gcode/control/M3-M5.cpp:103:41: warning: 'u' may be used uninitialized in this function [-Wmaybe-uninitialized]
  103 |     cutter.menuPower = cutter.unitPower = u;

This should be independent of board, and can be suppressed with adding 0.0 value to u variable.

change line 95:
float u; to float u = 0.0f

Requirements

Require LASER_FEATURE enabled, should happen on all boards, but checked on STM32F1.

Benefits

Remove warning that say u may be run uninitialized in function.

Configurations

configuration.zip

[thinkyhead]

It might be that the intent was to not modify cutter.unitPower outside of M3 S/M4 S when cutter.cutter_mode != CUTTER_MODE_STANDARD so I went ahead and reverted that part of the commit 55daab0a from last April. If @descipher is still around then maybe he can confirm that this is the correct fix.

Another equivalent is…

auto get_s_power = [] {
  float u = cutter.unitPower;
  if (parser.seenval('S')) {
    const float v = parser.value_float();
    u = TERN(LASER_POWER_TRAP, v, cutter.power_to_range(v));
  }
  else if (cutter.cutter_mode == CUTTER_MODE_STANDARD)
    u = cutter.cpwr_to_upwr(SPEED_POWER_STARTUP);

  cutter.menuPower = cutter.unitPower = u;

  // PWM not implied, power converted to OCR from unit definition and on/off if not PWM.
  cutter.power = TERN(SPINDLE_LASER_USE_PWM, cutter.upower_to_ocr(u), u > 0 ? 255 : 0);
};

… but this results in cutter.menuPower always being set equal to cutter.unitPower, and I'm not sure that is always appropriate.

[descipher]

I will review it and post what I see.

[descipher]

@thinkyhead, Thanks for the ping on the change, you are correct the intent was to keep the STANDARD_MODE as backward compatible for older use cases. The one exception is when SYNC mode is enabled which allows a user to override it immediately appliying the PWM value within the planner and not waiting for the M3 move to finnish. The menu code should update the active display value, however its very slow and clunky with the 8 bit implementation. The menuPower is a quirky thing where the previous code needed some separation in order to display the chosen power units e.g. 15000 RPM, 100% etc. could be changed to work better but those legacy display functions are not very co-operative in some cases.