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Re: [PATCH] Make calls in virtual thunks local if possible

On Wed, Jun 25, 2003 at 11:46:20AM -0400, John David Anglin wrote:
> This technique was carried over to linux.  It might be possible to
> change linux to use a simple pc-relative branch in the thunk.  The
> stubs inserted by the linux linker don't mess with the return pointer
> (except in the multispace model which the kernel doesn't currently
> support).  Possibly, we won't get a stub at all if the branch is local.

The output_mi_thunk target hook should for binds_local_p function create
a tail call to function. For
void foo (void) __attribute__((visibility ("hidden")));
void bar (void)
  foo ();
tail call GCC must emit a sequence which will work no matter how far foo
from bar's end is, if it is in the same shared library and output_mi_thunk
should emit the same. If this is impossible on certain arches,
then can_output_mi_thunk can just bail out and let the generic code handle

> However, it can be seen that the thunk follows the function.  As a result,
> we can't be certain that a pc-relative branch will be able to reach the
> start of the function.  It would be better to have the thunk just before the
> function if possible.

How would it help? You can have many thunks, so if the pc-relative branch
is severely limited, you can as well not be able to branch accross the
other thunks.


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