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Re: [PATCH] PR ld/18329: Turn off PROTECTED visibility with -Bsymbolic

On Sat, Apr 25, 2015 at 05:45:13PM -0700, H.J. Lu wrote:
> On Sun, Apr 26, 2015 at 09:16:09AM +0930, Alan Modra wrote:
> > On Sat, Apr 25, 2015 at 02:46:38PM -0700, H.J. Lu wrote:
> > > If a global protected symbol is bound locally by -Bsymbolic, there is no
> > > need to keep protected visibility which hich may have overhead at run-time.
> > 
> > For this change to be safe you'd need to show that all ELF targets
> > supporting dynamic objects also properly support -Bsymbolic for both
> > functions and variables.  ie. that none emit dynamic relocations for
> > the symbol when -Bsymbolic.
> > 
> Good point.  Here is a patch to make it backend dependent.  I will
> check it in shortly.

Before you do that, I'd like to know for sure what it is you're trying
to accomplish.  I'm guessing the "may have overhead at run-time" is
the loop over dynamic relocs in glibc/elf/dl-lookup.c:do_lookup_x.

If that's so, then I think you can do better than just clearing the
visibility when -Bsymbolic.  A protected symbol in an executable with
a definition other than in .dynbss, can also avoid the reloc loop.
As in the following testcase.

cat > protected.c <<EOF
int var __attribute__ ((visibility ("protected"))) = 0;

static void __attribute__ ((noinline))
count (void)
  var += 1;

library (void)
  int i;
  if (var != 0)
    __builtin_abort ();
  for (i = 0; i < 100000000; i++)
    count ();
cat > protected_add.c <<EOF
int var __attribute__ ((visibility ("protected"))) = 1;
extern void library (void);

int main ()
  library ();
  if (var != 1)
    __builtin_abort ();
  return 0;
gcc -O2 -fPIC -shared -o protected.c
gcc -O2 -fPIC -o protected_app protected_app.c -Wl,-rpath=.

Alan Modra
Australia Development Lab, IBM

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