-fvisibility=hidden

[Greg Banks]

On Wed, Jul 18, 2012, at 10:46 PM, Дилян Палаузов wrote:
> Hello,
> 
> On 16.07.2012 02:08, Greg Banks wrote:
> >
> > So, remind me again what actual value we're getting from this -fvisibility=hidden stuff again?
> >
> 
> Initially, libcyrus_sieve had a lot of exported symbols generated by 
> bison and flex.  When libcyrus_sieve was loaded by the dynamic linker 
> the list of exported symbols (section .dynsyms) was long, so finding the 
> needed symbol by the dynamic linker was supposed to take long time. 

Well, maybe.  On Linux at least, the linker puts a hashtable index to
the exported symbols into a section called .gnu.hash and the runtime
linker uses that to speed up searching for symbols.  So lookup is going
to be something closer to O(1) than O(N).  So I'm unconvinced by an
argument from runtime link performance.

There is an argument to be made from untidiness.

gnb at gnb-desktop 2069 nm -D --defined-only
./sieve/.libs/libcyrus_sieve.so | awk '{h[$2]++}END{for (i in h) print
h[i],i}'
3 A
10 B
3 D
115 T

That's a lot of exported symbols for such a small library :(

But you could fix that without advanced non-portable linker trickery, by
(for example) moving chunks of sieve/tree.c into sieve/sieve.y, or
#include'ing one into the other or vice versa.

> Annotating in libcyrus_sieve (and the other libraries) the 
> functions/symbols with EXPORTED, that are needed outside the library, 
> and compiling with -fvisibility=hidden, keeps the list of exported 
> symbols short, so the dynamic linker can load the library faster.  On 
> the other side the resulting library is smaller.  In addition, I think 
> this makes things easier to maintain, as it is clear, if a function is 
> used outside the library (EXPORTED), only within the library (HIDDEN), 
> or only within the source file, it is defined (static)

I think -fvisibility=hidden is an excellent mechanism for folks
maintaining real actual libraries with defined and documented interfaces
at both the source and ABI levels, to enforce that users of their
libraries use only the supported interfaces.  Cyrus really isn't one of
those.  We have no documentation, no ABI guarantee, no versioning
mechanism.  Worse, we have a lot of truly horrible tricks which rely on
traditional linktime semantics, fatal() being the obvious example. 
Those can and should be fixed, but right now they're pretty broken.

> Compiling different Automake targets with different CPPFLAGS/CFLAGS 
> creates .o files with very long names, when the non standarf AM_CFLAGS 
> are used.  The reason for the long name is, that Automake assumes, that 
> a file can be compiled once with the non-standard (not Makefile.am-wide) 
> CFLAGS, and once with AM_CFLAGS, so Automake reserves its right to 
> create different .o files from the same .c file.  this could be avoided, 
> if -fvisibility=hidden is used not only for libraries but also for 
> executables and is put in AM_CFLAGS.

Sure, if it's one place it should be in as many places as possible.

-- 
Greg.