* Michael Matz: > Hello, > > On Wed, 24 Aug 2022, Florian Weimer wrote: > >> > Isn't this merely moving the failure point from exception-at-ctor to >> > dlopen-fails? >> >> Yes, and that is a soft error that can be handled (likewise for >> pthread_create). > > Makes sense. Though that actually hints at a design problem with ELF > static ctors/dtors: they should be able to soft-fail (leading to dlopen or > pthread_create error returns). So, maybe the _best_ way to deal with this > is to extend the definition of the various object-initionalization means > in ELF to allow propagating failure.
We could enable unwinding through the dynamic linker perhaps. But as I said, those Itanium ABI functions tend to be noexcept, so there's work on that front as well. For thread-local storage, it's even more difficult because any first access can throw even if the constructor is noexcept. >> > Probably a note section, which the link editor could either transform into >> > a dynamic tag or leave as note(s) in the PT_NOTE segment. The latter >> > wouldn't require any specific tooling support in the link editor. But the >> > consumer would have to iterate through all the notes to add the >> > individual counts together. Might be acceptable, though. >> >> I think we need some level of link editor support to avoid drastically >> over-counting multiple static calls that get merged into one >> implementation as the result of vague linkage. Not sure how to express >> that at the ELF level? > > Hmm. The __cxa_atexit calls are coming from the per-file local static > initialization_and_destruction routine which doesn't have vague linkage, > so its contribution to the overall number of cxa_atexit calls doesn't > change from .o to final-exe. Can you show an example of what you're > worried about? Sorry if I didn't use the correct terminology. I was thinking about this: #include <vector> template <int i> struct S { static std::vector<int *> vec; }; template <int i> std::vector<int *> S<i>::vec(i); std::vector<int *> & f() { return S<1009>::vec; } The initialization is deduplicated with the help of a guard variable, and that also bounds to number of __cxa_atexit invocations to at most one per type. > A completely different way would be to not use cxa_atexit at all: allocate > memory statically for the object and dtor addresses in .rodata (instead of > in .text right now), and iterate over those at static_destruction time. > (For the thread-local ones it would need to store arguments to > __tls_get_addr). That only works if the compiler and linker can figure out the construction order. In general, that is not possible, and that case seems even quite common with C++. If the construction order is not known ahead of time, it is necessary to record it somewhere, so that destruction can happen in reverse. So I think storing things in .rodata is out. Thanks, Florian