On Sep 11, 2015 4:23 AM, "Evgeny Astigeevich via cfe-commits" < cfe-commits@lists.llvm.org> wrote: > > 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.
Short-lived programs can spend significant amounts of their runtime in initialization. There is also a binary size cost. > Why does `gnu libc++` use a similar way if it hurts performance? I'm sure they would prefer not to. > 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 > > > > _______________________________________________ > cfe-commits mailing list > cfe-commits@lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
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