From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!news.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: GNAT does not consistently raise an exception to log(0.0)
Date: Sat, 13 Jul 2013 08:34:36 +0100
Organization: A noiseless patient Spider
Message-ID: <ly8v1avroz.fsf@pushface.org>
References: <74c64a2c-f062-4b59-aafb-40c0bac39203@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: mx05.eternal-september.org;
 posting-host="c8504a89c76ea11bda6664e6198ae3cd";
	logging-data="23521"; mail-complaints-to="abuse@eternal-september.org";
	posting-account="U2FsdGVkX1+1ym9lvAQTRbr0M8zHaE4bMmYyiYTMXI8="
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (darwin)
Cancel-Lock: sha1:kUysokbnGwRuN1+h147faRwPBpw=
	sha1:1VdakfKCp6BU+DcxDATKHQ5KsqA=
Xref: news.eternal-september.org comp.lang.ada:16342
Date: 2013-07-13T08:34:36+01:00
List-Id: <comp.lang.ada>

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