SEH Dynamic Unwinding with auto correction

To achieve this objective, I have made some modifications.

I have added next fields to the TYieldObject class

InnerSEHCount:DWORD;
InnerSEHOffsets:array[0..$F] of DWORD;

And of course the code has been changed a little.

function TYieldObject.MoveNext: boolean;
asm
push ebp;
push ebx;
push edi;
push esi;
push eax;
xor edx,edx;
mov eax.TYieldObject.IsYield,dl;
push offset @a1
{
Is it first call
}
mov ecx,eax.TYieldObject.BESP;
cmp ecx,edx;
jnz @NotFirstCall;
mov eax.TYieldObject.BESP,esp;
jmp @JustBeforeTheJump;
@NotFirstCall:
cmp eax.TYieldObject.StackFrameSize,edx;
jz @RestoreRegisters;
{
is need any correction
}
mov edx,esp;
sub edx,ecx;
jz @RestoreStackFrame;
{
Correction
}
{
Correct ebp
}
add [eax.TYieldObject.REBP],edx;
{
Is any SEH frames
}
mov ecx,eax.TYieldObject.InnerSEHCount;
jecxz @ChangeBESP;
{
correct SEH frames
}
mov ebx,eax.TYieldObject.REBP;
lea esi,eax.TYieldObject.StackFrame;
add esi,eax.TYieldObject.StackFrameSize;
dec ecx;
mov edi,esi;
sub edi,DWORD PTR eax.TYieldObject.InnerSEHOffsets+4*ecx;
mov [edi+$08],ebx;
@SEHCorrection:
dec ecx;
jl @ChangeBESP
mov edi,esi;
sub edi,DWORD PTR eax.TYieldObject.InnerSEHOffsets+4*ecx;
mov [edi+$08],ebx;
add [edi],edx;
jmp @SEHCorrection;
{
Change BESP
}
@ChangeBESP:
mov eax.TYieldObject.BESP,esp;
{
Restore stack frame
}
@RestoreStackFrame:
mov ecx,eax.TYieldObject.StackFrameSize;
sub esp,ecx;
mov edi,esp;
lea esi,eax.TYieldObject.StackFrame;
rep movsb;
{
Connect Inner SEH frame. Are any inner SEH?
}
mov ecx,eax.TYieldObject.InnerSEHCount;
jecxz @RestoreRegisters;
{
Connect Inner SEH frame
}
xor ecx,ecx;
mov edi,eax.TYieldObject.BESP;
sub edi,DWORD PTR eax.TYieldObject.InnerSEHOffsets+4*ecx;
mov fs:[ecx],edi;
{
Restore Registers
}
@RestoreRegisters:
mov ebx,eax.TYieldObject.REBX;
mov ecx,eax.TYieldObject.RECX;
mov edx,eax.TYieldObject.REDX;
mov esi,eax.TYieldObject.RESI;
mov edi,eax.TYieldObject.REDI;
mov ebp,eax.TYieldObject.REBP;

@JustBeforeTheJump:
push [eax.TYieldObject.NextItemEntryPoint];
mov eax,eax.TYieldObject.REAX;
ret;
@a1:;
pop eax;
pop esi;
pop edi;
pop ebx;
pop ebp;
mov al,eax.TYieldObject.IsYield;
end;

procedure TYieldObject.Yield(const Value);
asm
mov eax.TYieldObject.REBP,ebp;
mov eax.TYieldObject.REAX,eax;
mov eax.TYieldObject.REBX,ebx;
mov eax.TYieldObject.RECX,ecx;
mov eax.TYieldObject.REDX,edx; // This is the Ref to const param
mov eax.TYieldObject.RESI,ESI;
mov eax.TYieldObject.REDI,EDI;
pop ecx;
mov eax.TYieldObject.NextItemEntryPoint,ecx;
//We must do it first for valid const reference
push eax;
mov ecx,[eax];
CALL DWORD PTR [ecx+VMTOFFSET TYieldObject.SaveYieldedValue];
pop eax;
{
Unwind SEH
// There is no need ebx,esi,edi to retain
}
xor ebx,ebx;
mov ecx,fs:[ebx];
@SEHUnwind:
jecxz @JustAfterSEHUnwind;
cmp ecx,eax.TYieldObject.BESP;
jnl @JustAfterSEHUnwind
mov esi,eax.TYieldObject.BESP;
sub esi,ecx;
mov DWORD PTR eax.TYieldObject.InnerSEHOffsets+4*ebx,esi;
inc ebx;
mov ecx,[ecx];
jmp @SEHUnwind;
@JustAfterSEHUnwind:
mov eax.TYieldObject.InnerSEHCount,ebx;
{
Connect Outer SEH frame.
If no local SEH frames next two commands are redundant
}
xor ebx,ebx;
mov fs:[ebx],ecx;
{
Save local stack frame
}
mov ecx,eax.TYieldObject.BESP;
sub ecx,esp;
mov eax.TYieldObject.StackFrameSize,ecx;
jz @AfterSaveStack;
lea esi,[esp];
lea edi,[eax.TYieldObject.StackFrame];
rep movsb;
mov esp,eax.TYieldObject.BESP;
@AfterSaveStack:
mov eax.TYieldObject.IsYield,1;
end;

So for now you may use try/finally and try/except construction in procedure that uses yielding.
Next article will be about additional improvements.
Now I am thinking about separate stack.