[PATCH] D12689: [libc++][static linking] std streams are not initialized prior to their use in static object constructors
Richard Smith via cfe-commits
cfe-commits at lists.llvm.org
Tue Sep 22 11:55:41 PDT 2015
On Sep 11, 2015 4:23 AM, "Evgeny Astigeevich via cfe-commits" <
cfe-commits at 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.
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