Re: [Patch][aarch64] Use IFUNCs to enable LSE instructions in libatomic on aarch64

[Steve Ellcey <sellcey@cavium.com>]

On Fri, 2017-08-25 at 15:37 +0100, Szabolcs Nagy wrote:

> the use of ifunc in gcc target libraries was a mistake
> in my opinion, there are several known bugs in the ifunc
> design and uclibc/musl/bionic don't plan to support it.
> but at this point i dont have a better proposal for doing
> runtime selection of optimal atomics code.
> 
> however in this patch i don't see why would the ctor run
> before ifunc resolvers. how does this work on x86_64?
> (there the different 16byte atomics are not even compatible,
> so if ifunc resolvers in different dsos return different
> result because one ran before the ctor, another after then
> the runtime behaviour is broken. this can happen when one
> dso is bindnow so ifunc relocation is processed before
> ctors and another runs resolvers lazily or dlopened later..
> but i didnt test it just looks broken)

I am not sure I followed everything here.  My basic testing all
worked, init_cpu_revision got run when libatomic was loaded and
then the IFUNC resolver gets called after that when one of the IFUNC
atomic functions is called.  init_cpu_revision sets libat_have_lse
which, I assume, will not be used by any other libraries.  If other
libraries have IFUNCs they would have their own static constructors and
set their own variables to be checked by their own IFUNCs.  So I am
not sure how one DSO is going to break another DSO.

> note that aarch64 ifunc resolvers get hwcap as an argument
> so all this brokenness can be avoided (and if we want to
> disable hwcap flags then probably glibc should take care
> of that before passing hwcap to the ifunc resolver).

I looked at the IFUNC memcpy resolver in glibc and it does not look
like it gets hwcap as an argument.  When I preprocess everything I see:

void *__libc_memcpy_ifunc (void) __asm__ ("__libc_memcpy");
void *__libc_memcpy_ifunc (void)
{
  uint64_t __attribute__((unused)) midr = _dl_aarch64_cpu_features.midr_el1;
  *res = ** expression that looks at midr value and returns function pointer **;
  return res;
}
__asm__ (".type " "__libc_memcpy" ", %gnu_indirect_function");
extern __typeof (__libc_memcpy) memcpy __attribute__ ((alias ("__libc_memcpy")));


Steve Ellcey
sellcey@cavium.com