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Re: Storage for uninitialized objects (PR 24626)
- From: Robert Dewar <dewar at adacore dot com>
- To: Steven Bosscher <steven dot bosscher at gmail dot com>
- Cc: gcc at gcc dot gnu dot org, John David Anglin <dave at hiauly1 dot hia dot nrc dot ca>, schlie at comcast dot net, ian at airs dot com, jsm28 at gcc dot gnu dot org
- Date: Sun, 15 Jan 2006 16:30:48 -0500
- Subject: Re: Storage for uninitialized objects (PR 24626)
- References: <200601152113.k0FLDsmm002972@hiauly1.hia.nrc.ca> <200601152224.50986.steven.bosscher@gmail.com>
Steven Bosscher wrote:
On Sunday 15 January 2006 22:13, John David Anglin wrote:
#include <stdio.h>
unsigned char
T (unsigned char x)
{
static int first = 1;
static unsigned char firstx;
if (first)
{
first = 0;
firstx = x;
return ~x;
}
if (x == firstx)
printf ("Behavior is pre GCC 4.0\n");
else
printf ("Behavior is GCC 4.0 and later\n");
return 0;
}
So firstx can have an indeterminate value coming into "if (x == firstx)"
and the behavior is still undefined.
Right that's my understanding of the standard
For what it's worth, it annoys me to no end that you have managed to
turn more than a day of hard work to understand and fix PR24626 into
a discussion about defining undefined behavior. I don't think I'll
ever look at HPPA bugs anymore if they don't affect me.
Well we have to understand whether something is a bug
before we go off "fixing" it.
We often find that reported optimization bugs are in fact
just manifestations of bugs in the program, and indeed it
can often be complex to figure this out.
We really really ought to have a switch to initialize everything
by default, then you can tell immediately if something like
this is due to uninitialized variables.
However, an interesting issue is whether this behavior is
valid for Ada, I don't think it is (the Ada standard is
much stricter about the effect of uninitialized variables).
But it may still be OK, needs careful analysis.
Gr.
Steven