Date: Sun, 17 Mar 1996 18:23:58 +1100 From: Bruce Evans <bde@zeta.org.au> To: jdp@polstra.com, terry@lambert.org Cc: freebsd-hackers@FreeBSD.org, nate@sneezy.sri.com Subject: Re: GAS question Message-ID: <199603170723.SAA27501@godzilla.zeta.org.au>
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>> > : "=a" (*eax), "=b" (*ebx), "=c" (*ecx), "=D" (cf)
>> > : "0" (*eax), "1" (*ebx), "2" (*ecx)
>>
>> This is GCC's "extended asm" syntax. The documentation is in the GCC
>> info pages. From the top-level node, follow the menu entry "C
>> Extensions", then "Extended Asm".
>Oh god, this sucks.
Does not.
>A VC++ C variable reference from an inline addembler statement:
>void
>foo( unsigned long locklist)
>{
> unsigned long delta;
> ...
> __asm mov eax, delta
> __asm mov ebx, locklist
> ...
>}
This is very primitive. Instead of letting the compiler decide which
registers to use, you have to do all the register loading yourself.
This wastes your time, and wastes the cpu's time doing unnecessary
moves if the registers are already in suitable places (delta in eax
and locklist in ebx in the above).
>And Assembly for a C callable function (Win95 blue screen from a VXD,
>actually):
>IFSMgr_SYSMODAL_Message PROC C PUBLIC USES EBX EDI ESI \
> pszMessage:DWORD, \
> pszCaption:DWORD, \
> dwFlags:DWORD
> VxDCall Get_Sys_VM_Handle ; handle in ebx
> mov eax, dwFlags
> mov ecx, pszMessage
> mov edi, pszCaption
> VxDCall SHELL_SYSMODAL_Message ; bluescreen with message
> ret
>IFSMgr_SYSMODAL_Message ENDP
Assembly for a C callable function in gas:
foo:
ret
:-)
>A C function without any preamble/postamble, and block inline assembly:
>__declspec(naked) void
>_peneter(save_edx)
>{
> _asm {
> mov eax, esp
> pushfd
> cli
> push eax
> mov edx, save_edx
> ...
> popfd
> ret
> }
>}
The preamble/postamble can't be avoided for extern functions in gcc asm
(it's always needed for pic and profiling code anyway). The above
should be written as an inline functions if the `...' part is short.
>Variable references are plain old variable references. Why is GCC so
>complicated?
It's more powerful. Plain old variable references are used, just not
in the part that looks like it will be processed by the assembler.
Bruce
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