GCC: Prohibit use of some registers

Question

This is a strange request but I have a feeling that it could be possible. What I would like is to insert some pragmas or directives into areas of my code (written in C) so that GCC's register allocator will not use them.

I understand that I can do something like this, which might set aside this register for this variable

register int var1 asm ("EBX") = 1984;
register int var2 asm ("r9") = 101;

The problem is that I'm inserting new instructions (for a hardware simulator) directly and GCC and GAS don't recognise these yet. My new instructions can use the existing general purpose registers and I want to make sure that I have some of them (i.e. r12->r15) reserved.

Right now, I'm working in a mockup environment and I want to do my experiments quickly. In the future I will append GAS and add intrinsics into GCC, but right now I'm looking for a quick fix.

Thanks!

Answer 1

When writing GCC inline assembler, you can specify a "clobber list" - a list of registers that may be overwritten by your inline assembler code. GCC will then do whatever is needed to save and restore data in those registers (or avoid their use in the first place) over the course of the inline asm segment. You can also bind input or output registers to C variables.

For example:

inline unsigned long addone(unsigned long v)
{
    unsigned long rv;
    asm("mov $1, %%eax;"
        "mov %0, %%ebx;"
        "add %%eax, %%ebx"
        : /* outputs */  "b" (rv)
        : /* inputs */   "g" (v) /* select unused general purpose reg into %0 */
        : /* clobbers */ "eax"
       );
}

For more information, see the GCC-Inline-Asm-HOWTO.

Answer 2

If you use global explicit register variables, these will be reserved throughout the compilation unit, and will not be used by the compiler for anything else (it may still be used by the system's libraries, so choose something that will be restored by those). local register variables do not guarantee that your value will be in the register at all times, but only when referenced by code or as an asm operand.

Answer 3

If you write an inline asm block for your new instructions, there are commands that inform GCC what registers are used by that block and how they are used. GCC will then avoid using those registers or will at least save and reload their contents.