What is wrong with the parameters list in this GCC inline assembly?

Question

I am trying to embed the following assembly instruction (of an ARM-like architecture) using GCC's extended asm syntax:

__asm__("lsr  %[xj], %[xj], %[xn]" : 
            [xj] "=r" (j) : 
            [xi] "[xj]" (j) , [xn] "[xn]" (n)); // j = j >> n

i , j , and n are declared as integers int i, j, n;

(note that j serves as both in and out parameter)

The compiler generates the error:

../src/fft2dlib.c:55:5: error: matching constraint references invalid operand number
../src/fft2dlib.c:53:3: error: matching constraint references invalid operand number

where lines 53 and 55 are the first and third lines of the above asm code.

When I replace the n input parameter with an explicit constant in the assembly instruction, it compiles fine:

__asm__("lsr  %[xj], %[xj], 27" : 
            [xj] "=r" (j) : 
            [xi] "[xj]" (j)); // j = j >> 27

I am following examples from the GCC manual , but I can't figure out the problem.

Answer 1

The contraint "[xn]" in [xn] "[xn]" (n) doesn't really make sense. Using a parameter name or number as a constraint means "put this operand in the same location as the named operand". So in this case you're telling GCC to put [xn] in the same location as [xn] , which is meaningless and doesn't actually give information about the operand type.

You need to use another constraint type, such as "r" for a register operand, depending on what the assembly instruction allows.