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In-Reply-To: <200507140759.54024.ebotcazou@libertysurf.fr>
Subject: Re: isinf
To: Eric Botcazou <ebotcazou@libertysurf.fr>
Cc: gcc@gcc.gnu.org, Hiroshi Fujishima <hiroshi.fujishima@gmail.com>,
        Joe Buck <Joe.Buck@synopsys.com>
Message-ID: <OF26D9F22F.D3621F38-ON4325703E.0024F6B2-4325703E.0025BCF5@il.ibm.com>
From: Michael Veksler <VEKSLER@il.ibm.com>
Date: Thu, 14 Jul 2005 06:52:00 -0000
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X-SW-Source: 2005-07/txt/msg00581.txt.bz2







Eric Botcazou  wrote on 14/07/2005 08:59:53:
> The compiler knows the answer of isinf (0) so it again optimizes away the

> call.  Try something like:
>
> int a;
>
> int
> main (int argc, char *argv[])
> {
>   a = isinf ((double) argc);
>   return 0;
> }

This may work today, but and still break in the future. This
will not work (possibly) years from now when gcc will start
doing VRP on math functions like isinf.

MIN_INT <= argc <= MAX_INT falls well into representable
values of double (assuming 32 bit int or less). This means
that the compiler may deduce that isinf((double)argc) always
returns false, without ever calling the function.

>
> or additionally compile with -fno-builtin.

That may help.