On Wed, 19 Jul 2017, Jeff Law wrote:
> > Glibc people were worried that attribute would be lost when taking a
> > pointer to function
> > (https://sourceware.org/ml/libc-alpha/2017-01/msg00482.html). I think
> > their reasoning was that return address is a shadow argument for
> > dlsym-like functions so this would cause a (most likely inadvertent)
> > ABI error.
> Fair enough, but does the right thing happen if the function's
> definition is decorated? Can you add a test for that in the testsuite?
How would passing pointers to dlsym via a 'void *' work after this patch?
Honestly, if the goal is to assist users of GCC (and not only Glibc users,
but also people compiling against Bionic/musl/BSD libcs, which probably
wouldn't adopt this attribute), may I suggest that a clearer course of
action would be to:
1) recognize dlsym by name and suppress tailcalls to it
this would solve >99% cases because calling dlsym by pointer would be rare,
and has the benefit of not requiring libc header changes;
and
2) if we really want to offer some generic solution, can we give the users
a way to suppress tailcalls via a builtin? That is, introduce
foo = __builtin_notailcall (func (args))
I think this would allow libc headers to do
#define dlsym(sym, mod) __builtin_notailcall (dlsym (sym, mod))
I'm sorry for bringing up objections late, but I really ask for alternatives
to be considered, and I can take a stab at implementing either of the above
if the general course is agreed upon.
Alexander
