OSDev.org

NickJohnson wrote:In general, inline assembly is a bad idea. It's a compiler-specific extension (as you have seen) and is therefore even less portable than assembly. I would highly recommend using an external assembler (which could also be more easily set to use Intel syntax) and then linking the assembler-generated object files with the compiler-generated ones. If you're using just C, this should be quite straightforward, and even if you're using C++, the name mangling is not that hard to deal with. You should also make sure to read up on whichever C calling convention (most likely cdecl) you're using.

.intel_syntax noprefix

.globl FuncAdd

FuncAdd:
	push	ebp
	mov		ebp,	esp
	mov		eax,	DWORD [ebp+4]
	add		eax,	DWORD [ebp+8]
	pop		ebp
	ret

berkus wrote:Inline assembly is not hard at all, all you need is just READ a single page in gcc manual. I know it's too hard for most coders, to READ.

gerryg400 wrote:

Code: Select all

int FuncAdd(int a, int b)
{
    _asm
    {
        mov eax, a
        add eax, b
    }
}

If you do this to the Microsoft compiler does it know that you've modified the eax register ? The 'complexity' with inline gcc assembler is generally about telling the compiler what you're doing with its registers in a way that allows it to continue with its optimising. This type of feature requires that you learn the syntax. IMHO, it's a small price to pay.

In general I agree with NickJohnson tho, and keep inline assembler to simple things like writing to ports and modifying special purpose registers. I try never to tell the compiler what to do with the eax register.

JamesM wrote:

gerryg400 wrote:

Code: Select all

int FuncAdd(int a, int b)
{
    _asm
    {
        mov eax, a
        add eax, b
    }
}

If you do this to the Microsoft compiler does it know that you've modified the eax register ? The 'complexity' with inline gcc assembler is generally about telling the compiler what you're doing with its registers in a way that allows it to continue with its optimising. This type of feature requires that you learn the syntax. IMHO, it's a small price to pay.

In general I agree with NickJohnson tho, and keep inline assembler to simple things like writing to ports and modifying special purpose registers. I try never to tell the compiler what to do with the eax register.

Yes, it does.

GCC style inline assembler is pointlessly complicated - ARM's compiler does it in the same (far superior) way as MSVC, by allowing any C expression as a register or memory location.

NickJohnson wrote:

berkus wrote:Inline assembly is not hard at all, all you need is just READ a single page in gcc manual. I know it's too hard for most coders, to READ.

I never said that it was hard, just that it was nonstandard and unnecessary (in the vast majority of cases.) I would make a similar argument against nested functions, even though they're very straightforward to use. A little -pedantic can't hurt.