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Re: Storage for uninitialized objects (PR 24626)
- From: Robert Dewar <dewar at adacore dot com>
- To: Ian Lance Taylor <ian at airs dot com>
- Cc: Paul Schlie <schlie at comcast dot net>, John David Anglin <dave at hiauly1 dot hia dot nrc dot ca>, gcc at gcc dot gnu dot org
- Date: Sun, 15 Jan 2006 05:52:12 -0500
- Subject: Re: Storage for uninitialized objects (PR 24626)
- References: <BFEF7F1F.CABE%schlie@comcast.net> <m3irslojz7.fsf@gossamer.airs.com>
Ian Lance Taylor wrote:
Paul Schlie <schlie@comcast.net> writes:
As a simple example, although x may be indeterminate -1 < sin(x) < +1
is unconditionally true, as must be tan(x) = sin(x)/cos(x), and x^x = 0;
No, the ISO C standard is clear that an uninitialized variable may be
set to a trap representation (6.7.8, 3.17.2). So in this case x might
be a signalling NaN, and executing sin (x) might well terminate the
program with a floating point exception. That would be a valid
translation of the program by the compiler, and would even be
reasonable and appropriate in a non-optimized compilation.
And most importantly, you might get the trap one time and not another, you
cannot expect consistency if you have uninitialized variables.