[Bug c/48568] New: Missing documentation for __attribute__((visibility ("protected"))) on variables.

[nisse at lysator dot liu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48568

           Summary: Missing documentation for __attribute__((visibility
                    ("protected"))) on variables.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: nisse@lysator.liu.se


I discovered that __attribute__((visibility ("protected"))) is supported on
variables, not just on functions. This should be documented in the node
"Variable Attributes" in the GCC manual

This feature is useful in particular for accessing read-only tables (allocated
in the .rodata section) from PIC code. By default, access to such data in other
compilation units is done via the GOT table. Using the above attribute (and
assuming that the definition is linked into in the same shared library file),
the GOT lookup is omitted, and instead one gets a direct access via pc-relative
addressing. I was working on GNU/linux x86_64, where this corresponds to 

  .protected table
  lea table(%rip), %rax  # To get the address of the table

rather than the default

  mov table@GOTPCREL(%rip), %rax

in assembler input. I admit I don't fully understand the .protected pseudo op,
but it's appearantly essential for getting the linker to do the right thing
when linking the shared library (gcc -shared table-def.o table-use.o -o
lib.so).

Regards,
/Niels Möller