Fix Overflow in Fader.cpp

[Umcaruje]

Fixes #3420

I just implemented @zonkmachine's idea. I don't have the superdebug code set up so testing is appreciated.

[zonkmachine]

:-O

(gdb) 
(gdb) bt full
#0  0x00007ffff47d51ac in feraiseexcept (__excepts=4) at ../sysdeps/x86/fpu/bits/fenv.h:135
No locals.
#1  __log10f (x=0) at w_log10f.c:32
No locals.
#2  0x00000000005eee07 in ampToDbfs (amp=0) at /home/zonkmachine/builds/lmms/lmms/include/lmms_math.h:273
No locals.
#3  0x00000000005f067f in Fader::paintDBFSLevels (this=0x1b80360, ev=0x7fffffff8e60, painter=...)
    at /home/zonkmachine/builds/lmms/lmms/src/gui/widgets/Fader.cpp:382
        peak_L = 0
        persistentLeftPeakDBFS = -nan(0x7f8e74)
        width = 11
        center = 12
...

I didn't get this before but ampToDbfs (amp=0) is because m_fPeakValue_L and m_fPeakValue_R is initialised to 0.0f , here and here... More qMax?
I think this probably depends on #3381 for testing apart form the debug code, as envelope floating point errors will shut down LMMS immediately and after this you will get the Fader issue. However, I'm not yet confident with the envelope testing/fix but I'm still surprised by this result. I'll have to poke the envelope some more and get back to you on this.

[Umcaruje]

Does this still happen on the Empty template only? or in general?

[zonkmachine]

In general. This crash was new so I haven't looked into it that much.
No crash on just loading default.mpt or Empty.mpt .
Open mixer and click 'add channel', boom! It's the initialisation to zero that's fed to ampToDbfs().
I think either we initialise it to something over 0.0f , use the safe version that hands back -inf for 0.0f, or use the same qMax fix. Go with the qMax fix. I think it's most reasonable to go with the same fix.
tested, works... :-)

float const leftSpan = ampToDbfs( qMax<float>( 0.0001, m_fPeakValue_L ) ) - minDB;

[Umcaruje]

What happens if you change all the instances to SafeAmpToDBFS? do you get the crash?

[Umcaruje]

If you don't have time I can do it in the PR too

[zonkmachine]

What happens if you change all the instances to SafeAmpToDBFS? do you get the crash?

The function hands back -inf which creates a crash in the line after.

Program received signal SIGFPE, Arithmetic exception.
0x00000000005f06b2 in Fader::paintDBFSLevels (this=0x1b866d0, ev=0x7fffffffb470, painter=...)
    at /home/zonkmachine/builds/lmms/lmms/src/gui/widgets/Fader.cpp:383
383		int peak_L = height * leftSpan * fullSpanReciprocal;

[zonkmachine]

If you don't have time I can do it in the PR too

👍

[Umcaruje]

Let's see, I'll make all the values initalize at something other than 0

[zonkmachine]

Let's see, I'll make all the values initalize at something other than 0

I don't think there's any real point doing this other than to show that the value can't be 0 or lower. But it wont really work as a fix since we got negative values, probably from an overflowing int., in the original thread so you'll likely end up having to use qMax anyway.

[Umcaruje]

k then, qmax it is 😄

[Umcaruje]

@zonkmachine done. I didn't add the code to the min and max values, they're constants defined in the FXMixerView