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Re: Preprocessor expressions

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On 07/02/2017 22:52, Keith Thompson wrote:
> 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.

So a struct of a double and an int is always going to be 16 bytes and 
never 12? (I used a 12-byte version for a while; I guess either that 
wasn't C, or I used pack(1).)

>> 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,

The context here is representing a struct whose layout has already been 
decided so that the C version has to match it exactly. Or the struct 
represents some layout in a file header, or a data packet from a comms 
link or whatever.

C is supposed to be low-level so why not allow that amount of control?

  you can just *ask* it, using sizeof,
> offsetof, and possibly _Alignof (which is new in C11).

Sure; but at the point where the decision is made (in another language 
or working out some file header), the C compiler stage may be some way 
off (in time, distance or done by someone else).

>> (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.)

Again, C is not a high-level language where these details can be and 
usually are irrelevant.

For my purposes they can be very relevant. I've used variations of such 
a 16-byte struct since the mid-80s. I usually need to pack a lot in and 
it needs to be very carefully crafted because performance depends on 
keeping it compact.

(The latest version here: http://pastebin.com/rHqBYEqj. 26 or so fields 
in only 16 bytes. And this new version has had to offload a lot of stuff 
too thanks to 64-bit pointers taking up half the space.

But notice no padding is actually needed; the manual layout has already 
taken care of that.)

-- 
bartc

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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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