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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c++
Subject: Re: Performance of unaligned memory-accesses
Date: Wed, 07 Aug 2019 17:27:13 -0700
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David Brown <david.brown@hesbynett.no> writes:
[...]
> Compiler extensions like "packed" are fine in my book.  Use them if they 
> make the code clearer and more efficient - assuming, of course, that the 
> lack of portability is not a problem.  Compilers that don't support such 
> features will complain, so you don't get silent problems.

Compiler extension like "packed" can cause problems.  For example,
suppose you have something like this (using gcc syntax):

    struct foo {
        char c;
        int i;
    } __attribute__((packed));
    foo obj;
    some_func(&obj.i);

some_func() takes an argument of type int*, but there's no indication
that it's misaligned, so it can't take any special steps to avoid
blowing up when it dereferences its argument.

Recent versions of gcc and clang warn about taking the address of a
misaligned member of a packed structure.

https://stackoverflow.com/q/8568432/827263
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51628

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */