I implemented RMS, a (pretty basic) SuperCollider UGen that calculates the root means square of an incoming signal. It is available at the sc-plugins directory.

It is a pretty rudimentary and straight forward implementation of a broadly used feature which I guess could well be part of the standard SC3 UGens collection.

Here is an example use case where it modulates delay times in a feedback patch and is used to analyse the input signal for a limiter:

// adapted from a Pd patch by Dario Sanfilippo
(
Ndef(\fb).addSpec(\modScale,  [-20, 20, \lin, 1]);
Ndef(\fb).addSpec(\gain,      [0, 2, \lin, 0]);
Ndef(\fb).addSpec(\amp,       [0, 2, \lin, 0]);
Ndef(\fb).addSpec(\limit,     [0, 1, \lin, 0]);
Ndef(\fb).addSpec(\lookahead, [0, 1, \lin, 0]);
Ndef(\fb).addSpec(\lpf, [0.01, 4, \exp, 0]);

Ndef(\fb, {
    // var in = SoundIn.ar([0, 1]) * \gain.kr(0);
    var in = SoundIn.ar(0) * \gain.kr(0);

    var rms = RMS.ar(in, \lpf.kr(0.01));
    // alternatives
    // var rms = Amplitude.ar(in, 2, 2);
    // var rms = EnvFollow.ar(in, 0.2);
    var snd = DelayC.ar(in, 1, 0.5 + LeakDC.ar(rms, 0.9998) * \modScale.kr(0));

    //////////////////
    // AGC2.1 ( a limiter)
    var rms500 = RMS.ar(snd, 500);
    var rms1   = RMS.ar(snd, 1) * 2;
    var analytics = max(rms500 - rms1, 0) + rms1;

    snd = (DelayC.ar(snd, 0.02, \lookahead.kr(0) * 0.02) / max(analytics / \limit.kr(0), 1)).clip2(1, 1);


    // the standard SC limiter
    // snd = Limiter.ar(snd, \limit.kr(0), \lookahead.kr(0)).sum * \amp.kr(0)

    snd * \amp.kr(0.1)

})
)

Ndef(\fb).edit;