armv6 specific std::async crash from g++6 built a.out [stable/11 -r304943 context; /usr/ports -r421001 context]

[Mark Millard]

On 2016-Sep-1, at 4:35 PM, Mark Millard <markmi at dsl-only.net> wrote:

> Another top post because I should have kept going with the reductions: the following ~10 line program also shows the SIGSEGV behavior on armv6 (an rpi2) for running ./a.out after compiling via g++6.
> 
>> # more g++6_build_crashes.cpp                                                                                                                                                                           #include <future>       // future, async, launch::async
>> 
>> static void f() {}
>> 
>> int main(int, const char* [])
>> {
>>    auto a0{ std::async( std::launch::async, f ) };
>>    a0.get();
>>    return 0;
>> }
> 
> It only takes one std::async use to have the problem. So far the above (and the earlier variants) always crash with SIGSEGV on the rpi2 when the g++6 compiler is used.
> 
> As for the compile commands used:
> 
>> g++6 -g -v -std=c++14 -Wpedantic -Wall -O2 -pthread -Wl,-rpath=/usr/local/lib/gcc6 -mcpu=cortex-a7 g++6_build_crashes.cpp
> 
> 
> Omitting the -mcpu=cortex-a7 still get the problem.
> 
> Using -O0 instead of -O2 still gets the problem.
> 
> But I'll note that my stable/11 -r304943 build and my ports builds (usr/ports -r421001 vintage) were/are based on using -mcpu=cortex-a7 . Also the system clang 3.8.0 was used to build g++6:
> 
>> GNU C++14 (FreeBSD Ports Collection) version 6.2.0 (armv6-portbld-freebsd11.0)
>>        compiled by GNU C version 4.2.1 Compatible FreeBSD Clang 3.8.0 (tags/RELEASE_380/final 262564), GMP version 5.1.3, MPFR version 3.1.4-p1, MPC version 1.0.3, isl version none
> 
>> # more /etc/make.conf 
>> DEFAULT_VERSIONS+=perl5=5.22
>> WRKDIRPREFIX=/usr/obj/portswork
>> WITH_DEBUG=
>> WITH_DEBUG_FILES=
>> MALLOC_PRODUCTION=
>> #
>> #system clang 3.8 (gcc6 rejects -march=armv7a):
>> #CFLAGS+= -march=armv7-a -mcpu=cortex-a7
>> #CXXFLAGS+= -march=armv7-a -mcpu=cortex-a7
>> #CPPFLAGS+= -march=armv7-a -mcpu=cortex-a7
>> #
>> #lang/gcc6's xgcc stage considers the above conflicting so use just:
>> CFLAGS+= -mcpu=cortex-a7
>> CXXFLAGS+= -mcpu=cortex-a7
>> CPPFLAGS+= -mcpu=cortex-a7
> 
> 
> ===
> Mark Millard
> markmi at dsl-only.net

Beyond amd64 FreeBSD not having the problem that armv6 has. . .

I have since booted Ubuntu Mate 16.04 on the rpi2 (armv6 / cortex-a7) and installed g++-6, g++-6 (Ubuntu 6.1.1-2ubuntu12~16.04) 6.1.1 20160510 since it was available. (So not 6.2: not a full vintage match to my FreeBSD g++6 contexts, unfortunately.)

The original program that failed on FreeBSD when compiled with g++6 (v6.2) on the same rpi2 executes just fine after being compiled on the Ubuntu Mate rpi2 configuration with g++-6 (v6.1).

So the armv6 SIGSEGV looks to not not be a generic g++6/libstdc++ problem for std::async use: it seems to be armv6 FreeBSD specific. (Unfortunately I've not been able to form fully matching versions for this comparison.)

I've also installed g++-6 on a ODROID-C2 (aarch64) Linux (Ubuntu 16.04.1) and built the original program that I discovered the problem with. g++-6 (Ubuntu 6.1.1-2ubuntu12~16.04) 6.1.1 20160510 was the compiler.

It executes just fine on the ODROID-C2 under Ubuntu Mate. (So far as I know FreeBSD does not support this type of context (yet) but it is the only aarch64 context I currently have access to.)

===
Mark Millard
markmi at dsl-only.net