E.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;
        begin
           Asm ("incl %0",
                Inputs  => Unsigned_32'Asm_Input ("a", Value),
                Outputs => Unsigned_32'Asm_Output ("=a", Result));
           return Result;
        end Incr;
        pragma Inline (Increment);
     
        Value : Unsigned_32;
     
     begin
        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
     #APP
        incl %eax
     #NO_APP
        movl %eax,%edx

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