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.

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