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10.5 Inlining Inline Assembler Code

For a short subprogram such as the Incr function in the previous section, the overhead of the call and return (creating / deleting the stack frame) can be significant, compared to the amount of code in the subprogram body. A solution is to apply Ada’s Inline pragma to the subprogram, which directs the compiler to expand invocations of the subprogram at the point(s) of call, instead of setting up a stack frame for out-of-line calls. Here is the resulting program:

with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Increment_2 is

   function Incr (Value : Unsigned_32) return Unsigned_32 is
      Result : Unsigned_32;
      Asm ("incl %0",
           Outputs => Unsigned_32'Asm_Output ("=a", Result),
           Inputs  => Unsigned_32'Asm_Input ("a", Value));
      return Result;
   end Incr;
   pragma Inline (Increment);

   Value : Unsigned_32;

   Value := 5;
   Put_Line ("Value before is" & Value'Img);
   Value := Increment (Value);
   Put_Line ("Value after is" & Value'Img);
end Increment_2;

Compile the program with both optimization (-O2) and inlining (-gnatn) enabled.

The Incr function is still compiled as usual, but at the point in Increment where our function used to be called:

pushl %edi
call _increment__incr.1

the code for the function body directly appears:

movl %esi,%eax
   incl %eax
   movl %eax,%edx

thus saving the overhead of stack frame setup and an out-of-line call.