How to inform GCC to not use a particular register

Question

Assume I have a very big source code and intend to make the rdx register totally unused during the execution, i.e., while generating the assembly code, all I want is to inform my compiler (GCC) that it should not use rdx at all.

NOTE: register rdx is just an example. I am OK with any available Intel x86 register.

I am even happy to update the source code of the compiler and use my custom GCC. But which changes to the source code are needed?

Answer 1

You can put some global variable to this register. For ARM CPU you can do it this way:

register volatile type *global_ptr asm ("r8")

This instruction uses general purpose register "r8" to hold the value of global_ptr pointer.

See the source in U-Boot for real-life example:

http://git.denx.de/?p=u-boot.git;a=blob;f=arch/arm/include/asm/global_data.h;h=4e3ea55e290a19c766017b59241615f7723531d5;hb=HEAD#l83

File arch/arm/include/asm/global_data.h (line ~83).

#define DECLARE_GLOBAL_DATA_PTR register volatile gd_t *gd asm ("r8")

Answer 2

You tell GCC not to allocate a register via the -ffixed-reg option (gcc docs).

• -ffixed-reg

  Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.

For example, gcc -ffixed-r13 will make gcc leave it alone entirely. Using registers that are part of the calling convention, or required for certain instructions, may be problematic.