How a recent version of GCC (4.6) could be used together with Qt under Mac OS?

Question

My problem is related to the one discussed here:

Is there a way that OpenMP can operate on Qt spanwed threads?

Upon trying to run my Qt-based program under Mac OS that has an OpenMP clause in a secondary thread, it crashed. After browsing through the web, now I understand that it is caused by a bug in the rather old version (4.2) of gcc supplied by Apple.

Then I downloaded the latest 4.6 version of gcc from http://hpc.sourceforge.net and tried to compile the project, but I got the following errors from g++ compiler:

unrecognized option ‘-arch’ unrecognized option ‘-Xarch_x86_64’

I learned that this is because these are options, which can be only interpreted by the custom-configured Apple-gcc compiler, but not by standard gcc.

Could anybody please help me could I overcome this issue and configure g++ 4.6 to use with Qt in order to get a bug-free OpenMP support? I admit that I'm a newbie under Mac OS platform with regard to compilers and programming and would like to port my code from Visual Studio-Qt environment.

Many thanks in advance!

Answer 1

If you aren't afraid of messing with your Qt installation, then change the QMAKE_CFLAGS_X86_64 entry in ~/QtSDK/Desktop/Qt/4.8.1/gcc/mkspecs/common/g++-macx.conf.

Replace ‘-Xarch_x86_64’ with ‘-arch x86_64’.

Answer 2

You can use your non-Apple gcc v4.6 and compile a binary for each architecture you want to build (use --target=${ARCH} should be fine for i386 and x86_64). Then once you have a binary for each of the architectures use lipo like so: lipo -create -arch i386 binary_32bit -arch x86_64 binary_64bit -output binary_universal This will create a fat binary (aka universal binary) named binary_universal from binary_32bit and binary_64bit.

Or you could use clang/llvm instead of gcc, which probably won't have the bug you described and (if supplied via Apple's developer tools) should be able to compile universal binaries directly.

Answer 3

You should run qmake woth corresponding -spec option, for example, to use gcc46 on freebsd it is needed to run qmake so: qmake --spec=freebsd-g++46

Answer 4

Lipo can indeed be used to put multiple object files together into a "fat" object file, in fact it turns out this is just what apple's compiler does. Their GCC compiler is actually a driver that maps various architectures to the appropriate compiler for the architecture and then mashes the objects together using lipo.

see: http://lists.macosforge.org/pipermail/macports-dev/2011-September/016210.html

Here is the source file for that driver:

http://opensource.apple.com/source/gcc/gcc-5666.3/driverdriver.c

All one needs to do to get a new version of GCC to honor the -arch flag is to modify this driver and get it to point to a script wrapper for your version of gcc that adds the appropriate flags for the given architecture and then passes all the rest of the arguments. Something like this:

#!/bin/sh

/opt/local/bin/gcc-mp-4.6 -m32 $@

and

#!/bin/sh

/opt/local/bin/gcc-mp-4.6 -m64 $@

Here is a link that talks about how to do it, and provides a cmake project to easily get the macports version of GCC fixed up and supporting the -arch flag for the two intel architectures:

http://thecoderslife.blogspot.com/2015/07/building-with-gcc-46-and-xcode-4.html