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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: GNAT not generating any code for =?utf-8?Q?sub=E2=80=91progra?=
 =?utf-8?Q?m=3A?= known bug?
Date: Sun, 28 Apr 2013 08:14:55 +0100
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Date: 2013-04-28T08:14:55+01:00
List-Id: <comp.lang.ada>

"Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr> writes:

> Here is:
> http://www.les-ziboux.rasama.org/download/2013-04-28-cla-gnat-bug.zip

The code is

   function Syscall
     (Number  : Positive;
      Handle  : Natural;
      Address : System.Address;
      Size    : Natural)
      return Integer
   is
      Result : Integer;
   begin
      System.Machine_Code.Asm
        (Template => "int $0x80",
         Outputs => Integer'Asm_Output ("=a", Result),
         Inputs =>
           (Positive'Asm_Input       ("a", Number),
            Natural'Asm_Input        ("b", Handle),
            System.Address'Asm_Input ("c", Address),
            Natural'Asm_Input        ("d", Size)),
         Volatile => False);
      return Result;
   end;

and I think that the problem is that Template actually has to use the
inputs and outputs! So, while optimising, the compiler says

   "int $0x80" doesn't use any of the inputs, so I can ignore them
   "int $0x80" doesn't write to any of the outputs, so I can ignore them
   Result isn't changed, so it's undefined
   Therefore I don't need to do anything.

The assembler generated by GNAT GPL 2012 is different but equally
ineffective:

.globl _library__write
_library__write:
LFB3:
        pushq   %rbp
LCFI0:
        movq    %rsp, %rbp
LCFI1:
        leave
LCFI2:
        ret
LFE3:

I think that you have to write the template to show how the parameters
are set up and returned. Here's one I wrote earlier:

   with Ada.Unchecked_Conversion;
   with System.Machine_Code;

   separate (BC.Support.High_Resolution_Time)
   function Clock return Time is

      type Half is (Low, High);
      type Low_High is array (Half) of Interfaces.Unsigned_32;

      Lower, Upper : Interfaces.Unsigned_32;

      function To_Time is new Ada.Unchecked_Conversion (Low_High, Time);

   begin

      System.Machine_Code.Asm
        ("rdtsc" & ASCII.LF & ASCII.HT &
           "movl %%eax, %0"& ASCII.LF & ASCII.HT &
           "movl %%edx, %1",
         Outputs => (Interfaces.Unsigned_32'Asm_Output ("=g", Lower),
                     Interfaces.Unsigned_32'Asm_Output ("=g", Upper)),
         Clobber => "eax, edx",
         Volatile => True);

      return To_Time ((Low => Lower, High => Upper));

   end Clock;

By the way, this doesn't actually work as intended on kit such as the
Macbook Pro (more than one core? advanced power management?), presumably
because the core's clock is slowed down or stopped. Fortunately,
Ada.Calendar.Clock's resolution is 1 microsecond, good enough for most
purposes.