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Subject: Re: broken code only when optimized "-O2"
To: Stefan Ring <stefanrin@gmail.com>, Adrian Moreno <amorenoz@redhat.com>
Cc: gcc-help <gcc-help@gcc.gnu.org>
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 <CAAxjCEwaUQ8bGd7NYqTjVL=VN4PbRsXx7PvJJWLwd9SL9oZ1RQ@mail.gmail.com>
From: David Brown <david.brown@hesbynett.no>
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Date: Tue, 21 Dec 2021 18:05:39 +0100
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On 21/12/2021 17:05, Stefan Ring via Gcc-help wrote:
> On Tue, Dec 21, 2021 at 4:40 PM Adrian Moreno via Gcc-help
> <gcc-help@gcc.gnu.org> wrote:
>>
>> I'd really appreciate any hint or idea to try to understand this problem.
> 
> I guess the compiler doesn't like the dereferencing of uninitialized
> pointers (in the sizeof expressions). I am not 100% sure that this
> counts as dereferencing, but I would assume so. Because of this the
> compiler will be free to behave however it likes to.
> 

The operand of a sizeof expression is not evaluated (unless it is a
VLA).  So there is no problem with the "member1 = malloc(sizeof
*member1);" lines.  (Which is fortunate, because it is a very common idiom!)

I think the next step would be to start pulling in some of the
definitions for the various macros here, and then trying to reduce the
code further to get a smaller test case.