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

[Florian Weimer]

* Patrick Palka:

> On Fri, 4 Nov 2022, Florian Weimer wrote:
>
>> * 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.
>
> IIUC wouldn't that shared object depend on libstdc++.so and hence
> libstdc++'s constructors would still run before the shared object's?

Hmm, right, we only reorder the symbol binding order, not the
initialization order.  The preloaded object will not participate in a
cycle with libstdc++, so I think this should indeed be a safe change.

Thanks,
Florian