I understand the distinction between public and private variables and functions. I don't have a grasp about why this gcc visibility thing makes a difference.
Why is enabling this good for me as a builder/packager?
As an end-user?
I get the feeling with hiding the private non shared stuff that executables and libraries are smaller in size, which theoretically reduces load times, which theoretically should improve response on the desktop. But how much?
A layman's explanation is much appreciated. :)
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Put simply, it hides most of the ELF symbols which would have previously (and unnecessarily) been public. This means:
It very substantially improves load times of your DSO (Dynamic Shared Object). For example, a huge C++ template-based library which was tested (the TnFOX Boost.Python bindings library) now loads in eight seconds rather than over six minutes!
It lets the optimiser produce better code. PLT indirections (when a function call or variable access must be looked up via the Global Offset Table such as in PIC code) can be completely avoided, thus substantially avoiding pipeline stalls on modern processors and thus much faster code. Furthermore when most of the symbols are bound locally, they can be safely elided (removed) completely through the entire DSO. This gives greater latitude especially to the inliner which no longer needs to keep an entry point around "just in case".
It reduces the size of your DSO by 5-20%. ELF's exported symbol table format is quite a space hog, giving the complete mangled symbol name which with heavy template usage can average around 1000 bytes. C++ templates spew out a huge amount of symbols and a typical C++ library can easily surpass 30,000 symbols which is around 5-6Mb! Therefore if you cut out the 60-80% of unnecessary symbols, your DSO can be megabytes smaller!
Much lower chance of symbol collision. The old woe of two libraries internally using the same symbol for different things is finally behind us with this patch. Hallelujah!
the library quoted above is an extreme case, the new visibility support
reduced the exported symbol table from > 200,000 symbols to less
than 18,000. Some 21Mb was knocked off the binary size as well!