An odd case with structure field alignment.
Hi,
I am clearly missing something here … can someone point out where it is?
https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Variable-Attributes.html#Variable%20Attributes
in the discussion of applying this to structure fields:
"The aligned attribute can only increase the alignment; but you can decrease it
by specifying packed as well."
Consider:
struct odd {
int * __attribute__((aligned(2))) a;
char c;
};
I would expect, given reading of the information on the aligned attribute, that
the under-alignment of a would be ignored (since there is no packed attribute
on either the field or the struct).
However, on x86_64, powerpc64 linux and x86_64, powerpc Darwin, I see that the
size of the struct is sizeof(pointer) + 2 and the alignment is 2.
OTOH:
struct OK {
int __attribute__((aligned(2))) a;
char c;
};
behaves as expected (the under-alignment is ignored, silently).
as does this…
struct maybe {
int *a __attribute__((aligned(2)));
char c;
};
* the type of the pointer does not seem to be relevant (i.e. AFAICT the
behaviour is the same for char * etc.)
Is there some special rule about pointers that I have not found ?
[it’s making an ABI mismatch with clang, which treats the int * as expected
from the documentation quoted above]
cheers
Iain
Re: Wanted: original ConceptGCC downloads / branch, concepts-lite branch
Great, thanks alot ! On Wed, 17 Aug 2022 at 21:23, Jonathan Wakely wrote: > > > > On Wed, 17 Aug 2022, 16:20 Richard Earnshaw via Gcc, wrote: >> >> >> >> On 17/08/2022 12:42, Aaron Gray via Gcc wrote: >> > Hi, >> > >> > I am looking for the original ConceptGCC source code, the >> > https://www.generic-programming.org/software/ConceptGCC/download.html has >> > all broken links and the SVN is gone. >> > >> > Is this available on GCC git or SVN ? >> > >> > Also I am wondering if the original concepts-lite code is available too >> > anywhere please ? >> > >> > Also any pointers to the documentation for the current implementation ? >> > >> > Regards, >> > >> > Aaron >> >> Not withstanding what others have already said, the various concepts >> branches are still in the git repository, but aren't in the standard >> pull set. You can use git fetch to explicitly pull them: >> >> d743a72b52bcfaa1effd7fabe542c05a30609614refs/dead/heads/c++-concepts >> >> > > > This is "concepts lite". > > >> 780065c813a72664bd46a354a2d26087464c74fc >> refs/dead/heads/conceptgcc-branch >> >> > > > I think this was Doug Gregor's work, but I don't know how much of ConceptGCC > is present on the branch. > > >> ce85971fd96e12d0d6675ecbc46c9a1884df766c >> refs/dead/heads/cxx0x-concepts-branch >> 14d4dad929a01ff7179350f0251af752c3125d74 > > > > This is described at https://gcc.gnu.org/git.html as: > > "This branch contains the beginnings of a re-implementation of Concepts, a > likely future feature of C++, using some of the code from the prototype > implementation on conceptgcc-branch. It is not currently maintained." > > I don't know who did this work, check the git history. -- Aaron Gray Independent Open Source Software Engineer, Computer Language Researcher, Information Theorist, and amateur computer scientist.
Re: C++ Concepts: Working Paper n number ?
Wonderful, thank you ! On Wed, 17 Aug 2022 at 22:04, Jonathan Wakely wrote: > > > > On Wed, 17 Aug 2022, 16:52 Aaron Gray via Gcc, wrote: >> >> Hi, Another query please. >> >> I seem to have found the Concepts Technical Specification :- >> >> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3929.pdf > > > > That's an old draft, see https://en.cppreference.com/w/cpp/experimental for > the ISO TS number and the draft numbers. > >> >> >> But am looking for the n number of the C++ Concepts Working Paper ? > > > Do you mean C++0x Concepts? N2042, and see N2618. > > -- Aaron Gray Independent Open Source Software Engineer, Computer Language Researcher, Information Theorist, and amateur computer scientist.
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