C Implementation of nasm code - c

Question

I'd like to start converting a little nasm project {synth.asm, synth_core.nh} to c to learn a little bit more about that little soft-synthesizer.
Problem is my asm knowledge is very very rusty, I'm wondering where to start off. I thought maybe one decompiler could help me out but I haven't found anything open-source able to convert these simple nasm listings to c.
Another alternative would be doing the conversion asm->c manually but I'm struggling to understand one of the most simplest functions :(
ie:
;distortion_machine
;---------------------------
;float a
;float b
;---------------------------
;ebp: distort definition
;edi: stackptr
;ecx: length
section distcode code align=1
distortion_machine:
pusha
add ecx, ecx
.sampleloop:
fld dword [edi]
fld dword [ebp+0]
fpatan
fmul dword [ebp+4]
fstp dword [edi]
scasd
loop .sampleloop
popa
add esi, byte 8
ret
broken attempt:
void distortion_machine(???) { // pusha; saving all registers
int ecx = ecx+ecx; // add ecx, ecx; this doesn't make sense
while(???) { // .sampleloop; what's the condition?
float a = [edi]; // fld dword [edi]; docs says edi is stackptr, what's the meaning?
float b = [ebp+0]; // fld dword [ebp+0]; docs says ebp is distort definition, is that an input parameter?
float c = atan(a,b); // fpatan;
float d = c*[ebp+4]; // fmul dword [ebp+4];
// scasd; what's doing this instruction?
}
return ???;
// popa; restoring all registers
// add esi, byte 8;
}
I guess the above nasm listing is a very simple loop distorting a simple audio buffer but I don't understand which ones are the inputs and which ones are the outputs, I don't even understand the loop conditions :')
Any help with the above routine and how to progress with this little educational project would be really appreciated.

Answer 1

There's a bit of guesswork here:
;distortion_machine
;---------------------------
;float a << input is 2 arrays of floats, a and b, successive on stack
;float b
;---------------------------
;ebp: distort definition << 2 floats that control distortion
;edi: stackptr << what it says
;ecx: length << of each input array (a and b)
section distcode code align=1
distortion_machine:
pusha ; << save all registers
add ecx, ecx ; << 2 arrays, so double for element count of both
.sampleloop:
fld dword [edi] ; << Load next float from stack
fld dword [ebp+0] ; << Load first float of distortion control
fpatan ; << Distort with partial atan.
fmul dword [ebp+4] ; << Scale by multiplying with second distortion float
fstp dword [edi] ; << Store back to same location
scasd ; << Funky way to incremement stack pointer
loop .sampleloop ; << decrement ecx and jump if not zero
popa ; << restore registers
add esi, byte 8 ; << See call site. si purpose here isn't stated
ret
It's a real guess, but esi may be a separate argument stack pointer, and the addresses of a and b have been pushed there. This code ignores them by making assumptions about the data stack layout, but it still needs to remove those pointers from the arg stack.
Approximate C:
struct distortion_control {
float level;
float scale;
};
// Input: float vectors a and b stored consecutively in buf.
void distort(struct distortion_control *c, float *buf, unsigned buf_size) {
buf_size *= 2;
do { // Note both this and the assembly misbehave if buf_size==0
*buf = atan2f(*buf, c->level) * c->scale;
++buf;
} while (--buf_size);
}
In a C re-implementation, you'd probably want to be more explicit and fix the zero-size buffer bug. It wouldn't cost much:
void distort(struct distortion_control *c, float *a, float *b, unsigned size) {
for (unsigned n = size; n; --n, ++a) *a = atan2f(*a, c->level) * c->scale;
for (unsigned n = size; n; --n, ++b) *b = atan2f(*b, c->level) * c->scale;
}