Register preservation?

el_bobbo · Messages: 660 Registration: 08.07.2003

At least in VS6 I think you are allowed to trash eax, ebx, ecx, edx, esi, edi, and the mmx regs & float regs. Just preserve esp, ebp (?), and the segment regs. And use emms to not leave the float regs in an undefined state where they can't even be zeroed. If you are compiling with the Fastcall option then you also have to preserve ecx. But I'm doing this from memory and for all I know could have been doing it wrong for years. ;) - Tom

whatsaZ3coupe · Messages: 158 Registration: 08.15.2002

Yes, you definately must restore ebp. It is used for referencing the actual parameter area (invocation record). Stack segment and data segment registers should be restored, but the extra (es, fs, and gs) need not be. The code segment will be restored by the ret. ENTER and LEAVE will handle esp and ebp for you. Some languages have further restrictions, such as Fortran (I know, who uses that, but it's an example I can remember atm) requires EDI and ESI to be preserved.

Kenshiro · Messages: 3,368 Registration: 06.27.2001

While we are on this, does anyone know of any (commonly used) projects that do use the Fastcall option in Visual Studio? I typically do not and so probably have not always kept ecx intact. But I wonder if there are any out there to watch out for. - Tom

Keith · Messages: 54 Registration: 11.26.2002

[Thinking deeply] IIRC Dvd2avi is using __fastcall for each IDCT code [/Thinking deeply]

scott_freedman · Messages: 503 Registration: 04.29.2003

eax, ecx, edx (not ebx!) are considered "general registers", and you can trash them. But trashing any other register could cause side effects.

jbob · Messages: 3,165 Registration: 07.17.2002

I guess the must-push-ebp-esp-esi-edi mantra came from experience with gcc-compatible code, as gcc requires those registers to be preserved (). Indeed for inline assembly, VC6 only needs ebp and esp to be preserved (). However, I've not been able to find anything concrete, but seeing as most core win32 functions preserve ebp, ebx, esi and edi () I guess it's good form to do the same when not using inline assembly. It's a tad disappointing that there's nothing I can find in VC's documentation saying "if you jump to an arbitrary piece of linked code, VC expects these registers to be preserved upon returning." Ah well. -h

wicked · Messages: 754 Registration: 12.14.2001

EBX is sometimes used as index, as in [esi+4*ebx], and I know that the Delphi compiler requires ebx to be restored. I don't know if VC6 requires it to be restored, but if you encounter strange effects, then try restoring it...

BMWFAN · Messages: 676 Registration: 03.03.2001

I tend to write 'hybrid' code, where the variables and initialization is done in C++, and the heavy processing is done in _asm{}; blocks. My experience is - never touch esp (interrupts can happen at any time). EBP is used to access local variables(don't use it unless you are really desperate for a register). If you have a lot of local variables (I think more than about 256 bytes worth, not counting passed arguments) then EBX becomes a secondary local variable pointer. (If ebx is used this way, I get a warning from the compiler if I change it).