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Bug 46712 - btls fails to build with gcc is v4.4.7 or earlier as they lack alignof/alignas
Summary: btls fails to build with gcc is v4.4.7 or earlier as they lack alignof/alignas
Alias: None
Product: Runtime
Classification: Mono
Component: misc (show other bugs)
Version: Trunk
Hardware: PC Linux
: Low minor
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2016-11-10 14:45 UTC by Robert van der Boon
Modified: 2017-01-04 19:01 UTC (History)
4 users (show)

See Also:
Is this bug a regression?: ---
Last known good build:

Attachments (1.34 KB, patch)
2016-12-13 13:21 UTC, Robert van der Boon
Details | Diff

Description Robert van der Boon 2016-11-10 14:45:42 UTC
I've got a redhat 6.6 machine that I use to build mono, which has gcc 4.4.7 as the /usr/bin/gcc. (As this is a company machine I can't do a system update to a newer gcc version.)
Since 4.8 I need to specify --enable-btls=no on the configure command line, otherwise I'll get failures building the boringssl part. I gather this is because gcc 4.4.7 is not C++11 compliant and is missing some necessary header files (it complains about stdalign.h missing).

If I change my environment to use my GCC 5.2.0 from /usr/local/gcc-5.2.0 (by setting PATH, CPATH, LD_LIBRARY_PATH, and LIBRARY_PATH) I see that this version used by the rest of the mono build, but the btls build will still try to use gcc 4.4.7. I think this is because the btls part is using CMAKE, and configure/make does not tell cmake that it should use non-default CC/CXX compilers.

The result is that for redhat6 I have to specify --enable-btls=no to get mono to compile even while having a compiler version available with which it could be compiled successfully.

Configure should:
- change --enable-btls to 'no' when a compiler is used that isn't supported, and
- Add two -D to the cmake command line: -DCMAKE_C_COMPILER=${CC} -DCMAKE_CXX_COMPILER=${CXX}
  so that the mono build of btls uses the same CC/CXX as the rest of mono.
Comment 1 Miguel de Icaza 2016-11-28 02:28:20 UTC
you might want to find a way to express these macros for older GCCs:


From the source that uses it, there is a fallback for MS C++, something similar might exist in some form or shape in GCC 4.4.7:

#if defined(_MSC_VER)
#if !defined(__cplusplus) || _MSC_VER < 1900
#define alignas(x) __declspec(align(x))
#define alignof __alignof
#include <stdalign.h>
Comment 2 Robert van der Boon 2016-11-28 08:06:57 UTC
For me most important is that the compilation of BTLS uses the same gcc as the compilation of Mono, I actually do not _want_ to compile with gcc 4.4.7, it's just that the btls code always is (while the mono code respects path settings).

My situation is:
/usr/bin/gcc                   v4.4.7
/usr/local/gcc-5.2.0/bin/gcc   v5.2.0

I set my path/libpath/etc to 5.2.0.

Running configure/autoconf picks up gcc 5.2.0, so the mono code is built using gcc 5.2.0 (just as I intended).
Building mono with Btls calls into CMake, for CMake to generate the Makefiles used to build Btls. CMake resolves gcc to /usr/bin/gcc, as it does not respect the path. And thus Btls is attempted to build with gcc 4.4.7, which is not what I want.

Of course being able to compile BTLS with gcc 4.4.7 is good (thanks for the hint, I'll try sometime this week), will let you know how that goes. (It seems I need __attribute__((aligned(x))) and __alignof__.)
Comment 3 Robert van der Boon 2016-11-28 11:10:11 UTC
Tested with the alignof settings, which didn't work (as it failed somewhere else). I've checked the external/boringssl/, an it states that building with 4.8+ is supported. As far as I'm concerned I am not going to try building with GCC 4.4.7 any further.
The issue that the btls part does not use the configured gcc, but always the system default, still stands.
Comment 4 Robert van der Boon 2016-11-28 12:13:24 UTC
When I do the following between "" and "make" then mono and btls line up again:
CC=`which gcc`
CXX=`which c++`

i.e. I manually fix up the btls makefile to pass the paths to gcc/c++ to CMake so that it generates makefiles with the proper paths in build-shared. I assume this can be incorporated in autoconf/automake somehow, but that is beyond me.
Comment 5 Robert van der Boon 2016-12-13 13:21:44 UTC
Created attachment 18869 [details]

This patch:
- disables BTLS when using GCC < 4.4.7
- passes the CC/CXX full paths to BTLS CMake to ensure that CMake is using the same CC/CXX as the mono build does.

This plays nice with the RedHat/CentOS/Fedora Software Collection List support as well.
Comment 6 Robert van der Boon 2016-12-13 13:23:05 UTC
I meant: disables BTLS when using GCC < 4.8
Comment 7 Alexander Köplinger 2017-01-04 19:01:46 UTC
Fixed by in master and mono-4.8.0-branch.

Thanks for the patch, it was very useful. I decided to not use -DCMAKE_C_COMPILER since CMake recommends setting CC instead:

I also just check for stdalign.h as that works on Debian 7 as well which still ships with gcc4.7 (and it apparently built fine on our bots there).

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