#pragma pack(1) vs __attribute__((packed))
I compiled GCC 5.1 on my PowerMac today and noticed that zip archive
reading broke in my program. Upon investigation it seems like GCC is
adding padding (presumably trailing) to a structure which represents a
chunk of the archive on disk, which has been declared within #pragma
pack(1). Adding __attribute__((packed)) to the structure does bring the
structure to the correct size, but from what I understand this is a GCC
specific feature.
The following structure can demonstrate the problem on PowerPC:
struct S1 {
uint32_t x;
uint16_t y;
};
On x86 #pragma pack(1) will make sizeof(S1) == 6. On PowerPC without
__attribute__((packed)) it is 8. I do believe I found #pragma pack to
be working as some other, more complex, structures end up with less
padding, but not to the extent of the attribute. (Somewhat peculiarly,
swapping the two members results in the structure being the expected
size. I assume it's done to try to keep accesses to x aligned, and in
the swapped case not padding the end means the access would be aligned
every other array element.)
Is this difference in behavior between the two features, which sound
like they should be doing the same thing, intentional? The change
appears to have happened around GCC 4.4 as 4.2 behaves as expected
(haven't tested 4.3 yet).
- Braden Obrzut
[GSoC] Function parameter scope
Hello,
I'm trying to figure out how to bring variables into scope
temporarily. The issue we're having is parsing something like:
auto f1(auto x) requires MyConcept() { }
This is similar to the late return type code, but we want to parse
this during cp_parser_init_declarator instead of during
cp_parser_direct_declarator. The problem is that direct_declarator
closes the scope of the function arguments (as it should) so we get an
error about
x not being in scope.
I think the easiest solution would be to simply re-enter the scope
stored in declarator->u.function.parameters, but from what I can tell
this isn't just a matter of calling push_scope and pop_scope?
- Braden Obrzut
GSoC Concepts - separate checking
My name is Braden Obrzut and I am a student from the University of Akron interested in contributing to GCC for GSoC. I am interested in working on a project related to the c++-concepts branch. In particular, I am interested in implementing mechanisms for checking the safety of constrained templates (separate checking). I have discussed the project with Andrew Sutton (who maintains the c++-concepts branch and happens to be a professor at Akron) and believe that some aspects of the work would be feasible within the three month time span. I also hope to continue working on the project as my honors thesis project. As a hobby I usually design and implement declarative languages for content definition in old video games. While I currently may have limited experience with GCC internals, I think this would be a great opportunity for me to learn how real compilers works and help with the development of the C++ programming language.
Re: GSoC Concepts - separate checking
On 03/18/2014 10:54 PM, Maxim Kuvyrkov wrote: On Mar 12, 2014, at 12:19 PM, Braden Obrzut wrote: My name is Braden Obrzut and I am a student from the University of Akron interested in contributing to GCC for GSoC. I am interested in working on a project related to the c++-concepts branch. In particular, I am interested in implementing mechanisms for checking the safety of constrained templates (separate checking). I have discussed the project with Andrew Sutton (who maintains the c++-concepts branch and happens to be a professor at Akron) and believe that some aspects of the work would be feasible within the three month time span. I also hope to continue working on the project as my honors thesis project. As a hobby I usually design and implement declarative languages for content definition in old video games. While I currently may have limited experience with GCC internals, I think this would be a great opportunity for me to learn how real compilers works and help with the development of the C++ programming language. Braden, Do you have a proposal for a GSoC GCC project? If you do want to apply, please make sure you are registered at the GSoC website and have a application filed by end of Thursday (only 2 days left!). Thank you, -- Maxim Kuvyrkov www.linaro.org I have just now submitted the proposal to the GSoC website. - Braden Obrzut
