[OE-core] Yocto development with C++11 threads and gcc

[Peter A. Bigot]

On 08/13/2014 05:05 PM, Khem Raj wrote:
> On Wed, Aug 13, 2014 at 2:36 PM, Peter A. Bigot <pab at pabigot.com> wrote:
>> In any case, Khem can you run with this?  It'd be fixed a lot better that
>> way....
> We do not configure target gcc with right matching cpu defaults,
> atomic instruction strex/ldrex are only added after armv6 but defaults
> for gcc if not specified is armv5t and hence it does not use the right
> set as expected by libstdc++ which has been cross compiled. so while
> you are at it and can reproduce it. Try to add
>
> EXTRA_OECONF += '${@bb.utils.contains("TUNE_FEATURES", "armv7a", "
> --with-cpu=armv7-a", "", d)}'
>
> to gcc-target.inc and see if resulting gcc is any better

I had to make it:

EXTRA_OECONF += '${@bb.utils.contains("TUNE_FEATURES", "armv7a", 
"--with-cpu=generic-armv7-a", "", d)}'

to get gcc to build but at runtime I then get:

beaglebone[16]$ g++ -std=c++11 -pthread test.cc && ./a.out
Assembler messages:
Error: unknown cpu `generic-armv7-a'
Error: unrecognized option -mcpu=generic-armv7-a

which indicates the flag's being passed to the assembler which doesn't 
recognize it even though g++ is happy with it.  I suppose we could hack 
binutils to substitute whatever spelling it wants to see.

(Also tried --with-cpu=arm7, but that generates assembler errors related 
to unsupported RM mode "bx lr").

The approach bothers me, though.  Instead of explicitly changing 
gcc-target to match gcc-runtime, shouldn't it be a general rule that 
gcc-runtime not apply OE-specific target flags that aren't going to be 
used by direct invocations of the compiler outside of the OE build 
environment?  That seems a little more robust, as the default target 
flags may be changed upstream or by bbappends within OE, and having to 
make them match in gcc-runtime as well would be a headache.

And would we need similar overrides for other architectures? There's 
something similar already in gcc-configure-common.inc for mips64.

Peter