Tag Archives: inline assembly

MSVC Inline Assembly

MSVC’s inline assembly is easier to use, as compared to gcc’s version. It is easier to write right code than wrong one, I think. Still a simple add function is used to illustrate:

The corresponding inline version:

__asm keyword is used to specify a inline assembly block. From MSDN, there is another asm keyword which is not recommended:

Visual C++ support for the Standard C++ asm keyword is limited to the fact that the compiler will not generate an error on the keyword. However, an asm block will not generate any meaningful code. Use __asm instead of asm.

Symbols in C/C++ code can be used directly in inline assembly. This is much more convenient than gcc. And it is also not necessary to load parameters into registers before usage as in gcc. MSVC does the job right even in optimization case.

NOTE: Inline assembly is not supported on the Itanium and x64 processors.

Let’s see the generated code:


Function parameters are located in [ebp+12] and [ebp+8] as referred by symbol a and b. Then, what happened if registers other than scratch registers are specified?

Output assembly code:

As you see, MSVC automatically preserves ebx for us. From MSDN:

When using __asm to write assembly language in C/C++ functions, you don’t need to preserve the EAX, EBX, ECX, EDX, ESI, or EDI registers.

Let’s see the case when stdcall calling convention is used:


In stdcall, stack is cleaned up by callee. So, there’s a ret 8 before return. And the function name is mangled to _add4@8.

MSVC also supports fastcall calling convention, but it causes register conflicts as mentioned on MSDN, and is not recommended. Just test it here, the code happens to work:)


Function parameters are passed in ecx and edx when using fastcall. But they are saved to stack first. It seems we get no benefit using this calling convention. Maybe MSVC does not implement it well. The function name is mangled to @add5@8.

Last, we can tell MSVC that we want to write our own prolog/epilog code sequences using __declspec(naked) directive:


Normal prolog/epilog is used here. MSVC does not generate duplicate these code when using __declspec(naked) directive.

GCC Inline Assembly

Inline assembly is used in Linux kernel to optimize performance or access hardware. So I decided to check it first. Before digging deeper, you may wanna read the GCC Inline Assembly HOWTO to get a general understanding. In C, a simple add function looks like:

Its inline assembly version may be:

Or simpler:

Here’s its generated code by gcc:


Our inline assembly is surrounded by #APP and #NO_APP comments. Redundant gcc directives are already removed, the remaining are just function prolog/epilog code. add2() and add3() works fine using default gcc flags. But it is not the case when -O2 optimize flag is passed. From the output of gcc -S -O2(try it yourself), I found these 2 function calls are inlined in their caller, no function call at all. These 2 issues prevent the inline assembly from working: – Depending on %eax to be the return value. But it is silently ignored in -O2. – Depending on 12(%ebp) and 8(%ebp) as parameters of function. But it is not guaranteed that parameters are there in -O2. To solve issue 1, an explicit return should be used:

To solve issue 2, parameters are required to be loaded in registers first:

add5() now works in -O2. The default calling convention is cdecl for gcc. %eax, %ecx and %edx can be used from scratch in a function. It’s the function caller’s duty to preserve these registers. These registers are so-called scratch registers. So what if we specify to use other registers other than these scratch registers, like %esi and %edi?

Again with gcc -S:

It seems that code generation of gcc in default optimize level is not so efficient:) But you should actually noticed that %esi and %edi are pushed onto stack before their usage, and popped out when finishing. These code generation is automatically done by gcc, since you have specified to use %esi(“S”) and %edi(“D”) in input list of the inline assembly. Actually, the code can be simpler by specify %eax as both input and output:

We can tell gcc to use a general register(“r”) available in current context in inline assembly:

And wrong code generation again…:

%eax is moved to %eax? gcc selected %eax and %edx as general registers to use.  The code accidentally does the right job, but it is still a potential pitfall. Clobber list can be used to avoid this:

As commented inline: The clobber list tells gcc which registers(or memory) are changed by the asm, but not listed as an output. Now gcc does not use %eax as a candidate of general registers any more. gcc can also generate code to preserve(push onto stack) registers in clobber list if necessary.