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Re: Preprocessor expressions

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BartC <bc@freeuk.com> writes:
[...]
> I had once a project which had a struct with this layout:
>
>    struct {
>        int a;                // 32 bits
>        double b;             // 64 bits
>        int c;                // 32 bits
> }
>
> (The real thing was a lot more complex with anonymous unions everywhere. 
> But it had carefully crafted to occupy 16 bytes in total.)
>
> On x86-32, the compiler was quite happy to put member 'b' at offset 4.
>
> But when I compiled it for 32-bit ARM, the same compiler, gcc, had to 
> put 'b' at offset 8, which made the struct an odd size like 20 bytes.

A size of 20 bytes seems unlikely.  If the compiler imposes 8-byte
alignment on type double, then it should also impose 8-byte alignment on
the structure.  The structure was probably 24 bytes.

> This is why I think you can't always predict what a compiler will do.

99% of the time you don't have to.  Usually you don't need to *tell* the
compiler how things are allocated, you can just *ask* it, using sizeof,
offsetof, and possibly _Alignof (which is new in C11).

> (In the above example, I had to rearrange things so that 64-bit values 
> were at 64-bit offsets, which was probably better, but there was nothing 
> wrong with the original alignment that I could see. It didn't make the 
> x86 version any faster.)

I presume you have valid reasons for wanting a particular layout.
(No sarcasm there, I really do presume you have valid reasons.
I don't particularly need to know what they are.)

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Preprocessor expressions BartC <bc@freeuk.com> - 2017-01-22 13:24 +0000
  Re: Preprocessor expressions jameskuyper@verizon.net - 2017-01-22 07:52 -0800
    Re: Preprocessor expressions Ralf Damaschke <rwspam@gmx.de> - 2017-01-22 17:15 +0000
      Re: Preprocessor expressions "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-22 13:32 -0500
    Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-01-22 17:38 +0000
      Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-01 03:12 -0800
        Re: Preprocessor expressions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-01 11:44 +0000
        Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-01 11:56 +0000
        Re: Preprocessor expressions jameskuyper@verizon.net - 2017-02-01 08:35 -0800
          Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-02 09:23 -0800
            Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-02 18:14 +0000
              Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-02 10:35 -0800
              Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-02 17:09 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-05 10:09 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-05 10:38 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-05 11:03 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-06 15:17 +0000
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-07 05:36 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-07 21:01 +0000
                Re: Preprocessor expressions supercat@casperkitty.com - 2017-02-07 13:16 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-07 22:09 +0000
                Re: Preprocessor expressions supercat@casperkitty.com - 2017-02-07 14:33 -0800
                Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-07 14:52 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-08 00:13 +0000
                Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-07 16:39 -0800
                Re: Preprocessor expressions jameskuyper@verizon.net - 2017-02-07 16:47 -0800
                Re: Preprocessor expressions David Brown <david.brown@hesbynett.no> - 2017-02-08 10:03 +0100
                Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-08 09:19 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-08 12:35 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-09 05:25 -0800
                Re: Preprocessor expressions supercat@casperkitty.com - 2017-02-09 07:37 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-09 15:52 +0000
  Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-01-22 19:34 +0000
    Re: Preprocessor expressions jameskuyper@verizon.net - 2017-01-22 13:01 -0800
  Re: Preprocessor expressions Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-30 05:30 -0800

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