Floating-point bug Pico 2W with sin and acos

[vgroenhuis]

Following code on Pico 2W (programmed with Arduino IDE) prints totally wrong output for sin(). Expected value is sin(pi) = 0 (or a very small value). Funny thing is that if you comment out the acos(few) statement, the output is OK again. Let's dig deeper into this, any help welcome!

void bug(float pie) {
  float few = sin(pie);
  Serial.println(few, 20); // Pico 2W: output: 0.92649161815643310546
  acos(few);
  return;
}

void setup() {
  Serial.begin(115200);
  while(!Serial);
  bug(M_PI);
}
void loop() {}

[earlephilhower]

This was a known bug in the SDK floating point acceleration that was fixed in the pico-sdk develop branch a little bit back. It's really sinister because it depends on the alignment of a pointer table so if you add or remove a call it can wither work or fail like you're seeing.

Upgrade to 4.6.0 to get the new version. When I run with the release I get the expected ~0 result on a Pico2:

-0.00000008742277657348

[maxgerhardt]

References:

• Incorrect results of sin() and cos() for some input values in develop branch? raspberrypi/pico-sdk#2406
• Fix alignment of lookup table in vfp float implmenetation raspberrypi/pico-sdk#2429

Just update your Arduino-Pico core version to get the fixed SDK that doesn't have that bug, @vgroenhuis.

[vgroenhuis]

Thanks a lot Earle and Max for your explanations!
I just updated to 4.6.0 and can confirm that the bug is gone.
Have a nice day! Vincent