[PATCH] libstdc++: ifdef rtti specific function __throw_ios_failure() by __cpp_rtti

[Mirko Vogt]

On 2/12/21 11:30 AM, Jonathan Wakely wrote:
> This patch is wrong.

Indeed, thanks for checking.

> If you simply disable that function definition
> for !__cpp_rtti then you'll get linker errors from fstream.tcc when
> that function is called.
> 
> /usr/bin/ld: /home/jwakely/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/src/.libs/libstdc++.so: undefined reference to `std::__throw_ios_failure(char const*, int)'

Funny enough that doesn't occur for my use case - builds fine. However 
building with a toolchain for bare metal, potentially resulting in 
disabling e.g. fstream.

> 
>> This was done correctly for the c++98 part and probably just forgotten
>> for c++11.
> 
> This has nothing to do with C++98, it's relted to the gcc4-compatible
> ABI versus the cxx11 ABI.

Urgs, yeah, that last chunk for cxx98 expands a different macro to 
include that definition - not the rtti ifdef. Misread and wrongly took over.

> 
> I added a better patch to
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99077

Thanks!

>> PS: Shouldn't this have been covered by any tests?
> 
> Nobody tests building libstdc++ with -fno-rtti because almost nobody
> does that.

Just as info: Espressif's crosstool-ng fork (not sure about vanilla) is 
building with no-rtti for targeting their Xtensa based ESP32 SoCs. 
That's how I stumbled over this.

> 
> But we have plenty of tests, and hundreds of them fail with your patch
> :-)

Yeah, glass houses and stones.. sorry for that :)

   mirko