20 Oct
2014
20 Oct
'14
5:15 p.m.
On 2014-10-17 19.00, ros-dev-request(a)reactos.org wrote:
Fastcall passes 64 bit arguments on the stack.
Aaagh.. I didn't know that.. Thanks for pointing it out.
IMO that's incredibly stupid, as a 32-bit CPU (or x64 running 32bit)
uses the register combination EAX:EDX for 64 bit integers, and
EAX:EDX is used for 64 bit return values (again in 32bit mode).
The compiler designers need to smarten up!
Best Regards
Love
And yes, this is about an ntdll/ntoskrnl export, so we
must be binary
compatible with what Windows does.
But the implementation that is normally used is an inline/intrinsic
function anyway. This is just about the export (or rather, importing
it).
On 2014-10-17 00:24, Love Nystrom wrote:
>@Timo, @Thomas, @Ged
>
>Byte order swappers should always be fastcall for perfomance reasons.
>They have no need for the benefits of the cdecl call convention.
>Using cdecl in this case would make the binary code pitifully slow.
>
>Think about it for a bit.. Some pseudocode show what I mean:
>
>..CDECL...
>
> push hi
> push lo
> call Swapper
> mov dsthi, edx
> mov dstlo, eax
> add esp, 4
> ...
>
>// UINT64 __cdecl
>Swapper:
> push ebp
> mov ebp, esp
> mov eax, ebp+8 // lo
> mov eax, ebp+12 // hi
> bswap
> xchg eax, edx
> bswap
> pop ebp
> ret
>
>..FASTCALL...
>
> mov edx, hi
> mov eax, lo
> call Swapper
> mov dsthi, edx
> mov dstlo, eax
> ...
>
>// UINT64 __declspec(naked) __fastcall
>Swapper:
> bswap
> xchg eax, edx
> bswap
> ret
>
>Sadly the compiler designers were not (yet) clever enough
>to make the fastcall regs EAX, EDX, ECX, in that exact order,
>but even as it stands today..
>
>Swapper:
> mov eax, ecx
> bswap
> xchg eax, edx
> bswap
> ret
>
>
>(If you actually link against a binary swapper compiled out of your
>control with
>cdecl convention, the argument falls of course, as you must comply with
>the binary.)
>
>Keep up the good work..
>Best Regards
>// Love