ARMCLANG: Support for global named register variables

Information in this knowledgebase article applies to:

• MDK v5.x
• ARMCLANG v6.x compiler toolchain

QUESTION

I'm migrating my program from using ARMCC v5.x to ARMCLANG v6.x compiler, and in my program I have the following source code:

    register unsigned __reg_lr __asm("lr");
    unsigned getLR() { return __reg_lr; }

This works fine with ARMCC v5.x

But when I compile the same code with ARMCLANG, I get the following error:

fatal error: error in backend: Invalid register name "lr".
ArmClang.exe: error: clang frontend command failed with exit code 70 (use -v to see invocation)

How can I solve this issue using ARMCLANG?

ANSWER

Using the __asm keyword to allocate a global variable to a register is not supported by ARMCLANG compiler.

Instead, you can use the following inline assembly in your code to retrieve the value stored in the LR register, for example:

    __attribute__((always_inline)) static inline unsigned getLR() {
      unsigned lr;
      __asm__ __volatile("mov %0, lr" : "=r"(lr));
      return lr;
    }

Similarly you can retrieve general-purpose register's value, such as R11:

    __attribute__((always_inline)) static inline unsigned getR11() {
      unsigned r11;
      __asm__ __volatile("mov %0, r11" : "=r"(r11));
      return r11;
    }

Support for global named register variables may be added to ARMCLANG in a future release.

MORE INFORMATION

• Refer to Named register variables
• Refer to Application Note 298: Migrate from ARM C/C++ Compiler 5 to ARM Compiler 6.

Last Reviewed: Thursday, November 26, 2020