fix(vm): avoid NaNs in trisaw oscillator

CHANGELOG.md

@@ -40,7 +40,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - VSTi queries the host sample rate more robustly. Cubase previously reported
   the sample rate as 0 Hz, leading to persistent error message about the sample
   rate not being 44100 Hz. ([#222][i222])
-- Occasional NaNs in the Trisaw oscillator when the color was 0 in the Go VM.
+- Occasional NaNs in the Trisaw oscillator when color was = 0 or color = 128
 - The tracker thought that "sync" unit pops the value from stack, even if the VM
   did not, resulting it claiming errors in patches that worked once compiled.
 

vm/compiler/templates/amd64-386/sources.asm

@@ -269,7 +269,10 @@ su_oscillat_pulse_up:
 {{- if .HasCall "su_oscillat_trisaw"}}
 {{.Func "su_oscillat_trisaw"}}
     fucomi  st1                             ; // c      p
-    jnc     short su_oscillat_trisaw_up
+    ; we do jnbe instead of jnc to avoid NaN. phase cannot be < 0 or >= 1, so the important cases are
+    ; c = 0 => the branch is never taken, because c > p can never happen, and thus results in .../(1-c)    
+    ; c = 1 => the branch is always taken, because p < c and thus always gives .../c
+    jnbe    short su_oscillat_trisaw_up     
     fld1                                    ; // 1      c       p
     fsubr   st2, st0                        ; // 1      c       1-p
     fsubrp  st1, st0                        ; // 1-c    1-p

vm/go_synth.go

@@ -481,10 +481,10 @@ func (s *GoSynth) Render(buffer sointu.AudioBuffer, maxtime int) (samples int, r
 						}
 						omega += float64(unit.ports[6]) // add frequency modulation
 						var amplitude float32
-						*statevar += float32(omega)
+						phase := float64(*statevar) + omega
 						if flags&0x80 == 0x80 { // if this is a sample oscillator
-							phase := *statevar
+							*statevar = float32(phase)
-							phase += params[2]
+							phase += float64(params[2])
 							sampleno := operandsAtTransform[3] // reuse color as the sample number
 							sampleoffset := s.bytecode.SampleOffsets[sampleno]
 							sampleindex := int(phase*84.28074964676522 + 0.5)
@@ -497,22 +497,25 @@ func (s *GoSynth) Render(buffer sointu.AudioBuffer, maxtime int) (samples int, r
 							sampleindex += int(sampleoffset.Start)
 							amplitude = float32(int16(binary.LittleEndian.Uint16(su_sample_table[sampleindex*2:]))) / 32767.0
 						} else {
-							*statevar -= float32(int(*statevar+1) - 1)
+							// at this point, the native synth actually uses 80-bit precision, so emulate that as closely as possible by using 64-bit math here
-							phase := *statevar
+							phase += 1
-							phase += params[2]
+							phase -= float64(int(phase))
-							phase -= float32(int(phase+1) - 1)
+							*statevar = float32(phase)
-							color := params[3]
+							phase += float64(params[2])
+							phase += 1
+							phase -= float64(int(phase)) // this should guaranteee that phase is [0,1), so that the Trisaw should not nan even if color = 1
+							color := float64(params[3])
 							switch {
 							case flags&0x40 == 0x40: // Sine
 								if phase < color {
-									amplitude = float32(math.Sin(2 * math.Pi * float64(phase/color)))
+									amplitude = float32(math.Sin(2 * math.Pi * phase / color))
 								}
 							case flags&0x20 == 0x20: // Trisaw
-								if phase >= color {
+								if phase >= color { // since phase cannot be 1, if color = 1, then this condition never fires
 									phase = 1 - phase
 									color = 1 - color
 								}
-								amplitude = phase/color*2 - 1
+								amplitude = float32(phase/color*2 - 1)
 							case flags&0x10 == 0x10: // Pulse
 								if phase >= color {
 									amplitude = -1