feat(vm/compiler): export su_power function in case user needs it

CHANGELOG.md

@@ -21,6 +21,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Dbgain unit, which allows defining the gain in decibels (-40 dB to +40dB)
 - `+` and `-` keys add/subtract values in order editor and pattern editor
   ([#65][i65])
+- The function `su_power` is exported so people can reuse it in the main code;
+  however, as it assumes the parameter passed in st0 on the x87 stack and
+  similarly returns it value in st0 on the x87 stack, to my knowledge there is
+  no calling convention that would correspond this behaviour, so you need to
+  define a header for it yourself and take care of putting the float value on
+  x87 stack.
 
 ### Fixed
 - When recording notes from VSTI, no track was created for instruments that had

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

@@ -114,6 +114,7 @@ su_op_advance_finish:
 ;   Output:     st0     :   2^x
 ;-------------------------------------------------------------------------------
 {{- if not (.HasCall "su_nonlinear_map")}}{{.SectText "su_power"}}{{end}}
+{{.Export "su_power" 0}}
 su_power:
     fld1          ; 1 x
     fld st1       ; x 1 x

vm/compiler/x86_macros.go

@@ -353,6 +353,16 @@ func (p *X86Macros) FmtStack() string {
 	return b.String()
 }
 
+func (p *X86Macros) Export(name string, numParams int) string {
+	if !p.Amd64 && p.OS == "windows" {
+		return fmt.Sprintf("global _%[1]v@%[2]v\n_%[1]v@%[2]v:", name, numParams*4)
+	}
+	if p.OS == "darwin" {
+		return fmt.Sprintf("global _%[1]v\n_%[1]v:", name)
+	}
+	return fmt.Sprintf("global %[1]v\n%[1]v:", name)
+}
+
 func (p *X86Macros) ExportFunc(name string, params ...string) string {
 	numRegisters := 0 // in 32-bit systems, we use stdcall: everything in stack
 	switch {
@@ -371,13 +381,7 @@ func (p *X86Macros) ExportFunc(name string, params ...string) string {
 		reverseParams[len(params)-1-i] = param
 	}
 	p.Stacklocs = append(reverseParams, "retaddr_"+name) // in 32-bit, we use stdcall and parameters are in the stack
-	if !p.Amd64 && p.OS == "windows" {
-		return fmt.Sprintf("%[1]v\nglobal _%[2]v@%[3]v\n_%[2]v@%[3]v:", p.SectText(name), name, len(params)*4)
-	}
-	if p.OS == "darwin" {
-		return fmt.Sprintf("%[1]v\nglobal _%[2]v\n_%[2]v:", p.SectText(name), name)
-	}
-	return fmt.Sprintf("%[1]v\nglobal %[2]v\n%[2]v:", p.SectText(name), name)
+	return fmt.Sprintf("%[1]v\n%[2]v", p.SectText(name), p.Export(name, len(params)))
 }
 
 func (p *X86Macros) Input(unit string, port string) (string, error) {