Re: GNAT does not consistently raise an exception to log(0.0)

[Simon Wright <simon@pushface.org>]

Jerry <lanceboyle@qwest.net> writes:

> $ gnatmake -f bomb_log.adb -OX
>
> then if X = 0 (zero) the expected exception is raised and reported thusly:
>
> raised CONSTRAINT_ERROR : a-ngelfu.adb:744 explicit raise
>
> else if X > 0 no exception is raised.
>
>
> with Ada.Numerics.Long_Elementary_Functions; 
> use  Ada.Numerics.Long_Elementary_Functions;
> procedure Bomb_Log is
>     x : Long_Float;
> begin
>     x := log(0.0);
> end Bomb_Log;

If you compile with -gnatwa (most warnings), the report is

   bomb_log.adb:4:05: warning: variable "x" is assigned but never read
   bomb_log.adb:6:05: warning: useless assignment to "x", value never referenced

and at -O1 or greater the compiler takes the opportunity to eliminate
the call of log, because of

   pragma Pure (Long_Elementary_Functions);

so that the generated code (gnatmake -c -u -f -S bomb_log.adb) is

           .text
           .globl __ada_bomb_log
   __ada_bomb_log:
   LFB1:
           ret

If you change the code to

   procedure Bomb_Log is
       x : Long_Float with Volatile;
   begin
       x := log(0.0);
   end Bomb_Log;

(that's -gnat12, of course, use pragma Volatile (X); otherwise) then
things work as you had expected:

           .text
           .globl __ada_bomb_log
   __ada_bomb_log:
   LFB1:
           subq    $24, %rsp
   LCFI0:
           xorpd   %xmm0, %xmm0
           call    _ada__numerics__long_elementary_functions__log
           movsd   %xmm0, 8(%rsp)
           addq    $24, %rsp
   LCFI1:
           ret