How to use specific register in ARM inline assembler

Question

I am trying to let the inline assembler copy some values into specific registers but it only complains. This is a short version of the code that will trigger the error:

asm("" :: "r0" (value));
asm("" :: "a1" (value));

Both lines will trigger:

Error: matching constraint references invalid operand number

So how do I specify the register to take directly? I know i could introduce names for the values and then copy them by my own but I would like to avoid this as this code would be shorter and more readable.

Why I am asking Currently I am working on some syscalls. I want to use a syscall macro like this:

#define SYSCALL0(NUMBER) asm("swi #" STRINGIFY(NUMBER));
#define SYSCALL1(NUMBER, A) asm("swi #" STRINGIFY(NUMBER) :: "r0"(A));
#define SYSCALL2(NUMBER, A, B) asm("swi #" STRINGIFY(NUMBER) :: "r0"(A), "r1"(B));
...

As you can see this fits neatly on on line. Of course I could do something like:

#define SYSCALL1(NUMBER, A) register type R0 asm("r0") = A;
                            SYSCALL0(NUMBER)

but then I would have to convert A to type to get no type errors or give type correctly everytime I use the macro in different functions.

Possible duplicate of How to specify an individual register as constraint in ARM GCC inline assembly? — Ciro Santilli

Mat Mat · Accepted Answer · 2012-05-31T10:30:40

With GCC, there is a shortcut:

register long r0 asm ("r0");

Then r0 "aliases" that register.

Combine that with a statement expression, and you can even get r0 as a "return value".

#define SYSCALL1(NUMBER,A) ({\
  register long r0 asm("r0") = (long) (A); \
  asm("swi #" STRINGIFY(NUMBER) : "=r"(r0) : "r"(r0) : "memory"); \
  r0; })

(I have no idea if the clobber is legitimate or not, the uClibc syscall implementation has that though.)

See extended assembly and local reg vars.

How to use specific register in ARM inline assembler

1 Answers