On Thu, 21 Sept 2023, 09:27 Jochen Topf, <jochen@xxxxxxxxxx> wrote:
> Hi!
>
> In my C++ code I have been using a pattern to store large and diverse sets
> of
> data as follows:
>
> * Reserve a block of memory ("the buffer")
> * Put class X in the beginning of that block (with placement new)
> * Add extra data after the class in certain formats, for instance lists of
> 0-terminated strings or lists of (type field, length field, some data)
> * Repeat with next class X until buffer is full
>
> You then know the first instance of class X is at offset 0 in the buffer.
> The
> class knows how to access and interpret that extra data beyond its nominal
> end
> and it knows how to calculate where the next instance of class X is. So
> you can
> access all instance data and iterate over the buffer getting to all the
> instances of X.
>
> If you align everything properly this has always worked well. It means I
> only
> need a single memory allocation instead of lots of allocation for bits and
> pieces and I don't need to store pointers to those bits and pieces, but can
> derive where they are from the knowledge I have about the structure of
> those
> buffers which keeps the data smaller.
>
> (In a way this is similar to the pattern of having a zero/one-element
> array at
> the end of a struct and storing extra data in that array "beyond the end
> of the
> struct", but greatly extended.)
>
> Now here is the problem: In the last years GCC has added more and more
> warnings, specifically -Warray-bounds and -Wstringop-overread that get
> triggered by my code all over the place because it (understandably) finds
> that
> I am accessing stuff outside the class X or some other part of my buffer.
>
> I can just disable those warnings, but because that code is in a
> header-only-library I have to tell every user to disable it or sprinkle my
> code
> with pragmas which has its own problems.
What are the problems with pragmas? That was going to be my suggestion.
And because GCC has been adding more
> of those checks over time (or tightening existing checks) I have more
> places in
> my code that trigger those warnings with every GCC release and I fear that
> will
> go on.
>
> My question are:
>
> * Is there something "illegal" (against C++ spec) in what I am doing?
>
Only if you don't do it correctly :-)
* Is there a better way of doing this that avoids triggering those warnings
> or to tell the compiler that "I know what I am doing"?
>
That's what the pragmas are for.
It might also be possible to create a compiler firewall with [[gnu::noipa]]
on some key functions which produce the pointers to the next item in the
buffer, to stop gcc from analyzing through them.
> (In case you are interested: The code in question is in the libosmium
> library
> https://github.com/osmcode/libosmium and is used in all sorts of software
> that works with OpenStreetMap data.)
>
> Jochen
> --
> Jochen Topf jochen@xxxxxxxxxx https://www.jochentopf.com/
> +49-351-31778688
>
