[PATCH] D12689: [libc++][static linking] std streams are not initialized prior to their use in static object constructors

[Evgeny Astigeevich via cfe-commits]

eastig added a comment.

In http://reviews.llvm.org/D12689#243592, @rsmith wrote:

> Can we instead fix this in Clang by ensuring that libc++ is put at the right position in the static link order so that its initializers run first? libc++'s avoidance of running iostreams init code from every translation unit is a significant startup performance feature, and we shouldn't abandon it unless we really have to.


Let me give more details about the problem we have.

Clang generates a `__cxx_global_var_init<X>` function for each global variable that needs to be set-up (initialized) before firing the main routine of the user program. Those initializer functions are then grouped under a parent initializer function named after the corresponding translation unit: `_GLOBAL__sub_I_<translation_unit>`.  Those `_GLOBAL__sub_I_<translation_unit>` entries then get stuffed into the `.init_array` tables of individual object files.

In ARM compiler toolchains `armlink` is used for linking, not `GNU ld`. `libc++` is a part of the toolchain static system libraries. So `armlink` logic should be updated to change the order in which the `.init_array` entries from system libraries are added to the final image's `.init_array` table. This can be a problem because of an assumption that there are no dependencies among initializers from different translation units.

IMHO if the programming language has a means of resolving such problems it's better to use it instead of hacking a linker.

About impact on startup performance, I don't see why it will be significant. Initialization is done once. Other times it is simply a call to increase a counter. To be significant there should be millions of calls. Why does `gnu libc++` use a similar way if it hurts performance?

In the patch the `__APPLE__` macro is used to have the old behaviour. Maybe instead of it another macro, e.g. `__STATIC_BUILD__`, can be use when we want to build a static library.


http://reviews.llvm.org/D12689