[PATCH 2/2] libstdc++: Move stream initialization into compiled library [PR44952]

[Florian Weimer]

* Patrick Palka via Gcc-patches:

> This patch moves the global object for constructing the standard streams
> out from <iostream> and into the compiled library on targets that support
> the init_priority attribute.  This means that <iostream> no longer
> introduces a separate global constructor in each TU that includes it.
>
> We can do this only if the init_priority attribute is supported because
> we need to force that the stream initialization runs first before any
> user-defined global initializer in programs that that use a static
> libstdc++.a.

I think this breaks initialization of iostreams of shared objects that
are preloaded with LD_PRELOAD.  With the constructor, they initialize
iostreams once they are loaded via their own ELF constructors (even
before libstdc++'s ELF constructors run).  Without the constructor in
<iostream>, that no longer happens.

Thanks,
Florian