Example 2: Alpha Macro Using Inline Assembly

The second example shows a macro that implements a simple maximum function and a sample driver program.

// int maximum (int a, int b) //
// Action: return max(a,b)    //

#define maximum(a,b)
   __asm("bis %0,%0,v0;"      // assume result:a
      "cmplt v0,%1,t0;"       // a < b ?
      "cmovne t0,%1,v0",a,b)  // yes - result:b

void main()
{
   int i = 1,j = 2;

   printf("max(i,j) = %d\n", maximum(i,j));
}

The code generated for main in this example looks like the following:

0000  main::
47E03410  0000  mov    1,a0               // load i
46003400  0004  bis    a0,1,v0            // asm body
400059A1  0008  cmplt  v0,2,t0            // asm body
23DEFFF0  000C  lda    sp,-16(sp)
442054C0  0010  cmovne t0,2,v0            // asm body
261F0000  0014  ldah   a0,h^.data(zero)   // printf: “”
B75E0000  0018  stq    ra,(sp)
47E00411  001C  mov    v0,a1              // printf: %d
22100000  0020  lda    a0,l^.data(a0)     // printf: “”
D3400000  0024  bsr    ra,j^printf        // printf()
A75E0000  0028  ldq    ra,(sp)
47FF0400  002C  clr    v0
23DE0010  0030  lda    sp,16(sp)
6BFA8001  0034  ret    ra

This example illustrates several additional points about _ _asm statements:

• Two arguments are passed to the _ _asm statement: a and b. These arguments are referred to in the assembly-language instructions as %0 and %1. The same two arguments also happen to be the arguments to the maximum macro itself, although such agreement in argument lists is not required.
  
  
• In another optimization, the compiler was able to propagate the value for the variable named j (2) directly into the compare less-than (cmplt) and conditional move not-equal (cmovne) instructions specified in the body of the _ _asm statement.