Re: [PATCH] Expand PIC calls without PLT with -fno-plt

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, May 6, 2015 at 12:01 PM, Rich Felker <dalias@libc.org> wrote:
> On Wed, May 06, 2015 at 11:44:57AM -0700, H.J. Lu wrote:
>> On Wed, May 6, 2015 at 11:37 AM, Rich Felker <dalias@libc.org> wrote:
>> > On Wed, May 06, 2015 at 11:26:29AM -0700, H.J. Lu wrote:
>> >> On Wed, May 6, 2015 at 10:35 AM, Rich Felker <dalias@libc.org> wrote:
>> >> > On Wed, May 06, 2015 at 07:43:58PM +0300, Alexander Monakov wrote:
>> >> >> On Wed, 6 May 2015, Jakub Jelinek wrote:
>> >> >> > The linker would know very well what kind of relocations are used for
>> >> >> > particular PLT slot, and for the new relocations which would resolve to the
>> >> >> > address of the .got.plt slot it could just tweak corresponding 3rd insn
>> >> >> > in the slot, to not jump to first plt slot - 16, but a few bytes before that
>> >> >> > that would just load the address of _G_O_T_ into %ebx and then fallthru
>> >> >> > into the 0x4c2b7310 snippet above.  The lazy binding would be a few ticks
>> >> >> > slower in that case, but no requirement on %ebx to contain _G_O_T_.
>> >> >>
>> >> >> No, %ebx is callee-saved, so you can't outright overwrite it in the PLT stub.
>> >> >
>> >> > Indeed. And the situation is the same on almost all targets. The only
>> >> > exceptions are those with direct PC-relative addressing (like x86_64)
>> >> > and those with reserved inter-procedural linkage registers and
>> >> > efficient PC-relative address loading via them (like ARM and AArch64).
>> >> > MIPS (o32) is also an interesting exception in that the normal ABI is
>> >> > already PLT-free, and while callees need a PIC register loaded, it's a
>> >> > call-clobbered register, not a call-saved one, so it doesn't make the
>> >> > same kind of trouble,
>> >> >
>> >> > I really don't see a need to make no-PLT code gen support lazy binding
>> >> > when it's necessarily going to be costly to do so, and precludes most
>> >> > of the benefits of the no-PLT approach. Anyone still wanting/needing
>> >> > lazy binding semantics can use PLT, and can even choose on a per-TU
>> >> > basis (or maybe even more fine-grained with pragmas/attributes?).
>> >> > Those of us who are suffering the cost of PLT with no benefits
>> >> > (because we use -Wl,-z,relro -Wl,-z,now) can just be rid of it (by
>> >> > adding -fno-plt) and enjoy something like a 10% performance boost in
>> >> > PIC/PIE.
>> >> >
>> >>
>> >> There are things compiler can do for performance and correctness
>> >> if it is told what options will be passed to linker.  -z now is one and
>> >> -Bsymbolic is another one:
>> >>
>> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65886
>> >>
>> >> I think we should add -fnow and -fsymbolic.  Together with LTO,
>> >> we can generate faster executables as well as shared libraries.
>> >
>> > I don't see how knowing about -Bsymbolic can help the compiler
>> > optimize. Without visibility, it can't know whether the symbols will
>> > be defined in the same DSO. With visibility, it can already do the
>> > equivalent hints. Perhaps it helps in the case where the symbol is
>> > already defined (and non-weak) in the same TU, but I think in this
>> > case it should already be optimizing the reference. Symbol
>> > interposition over top of a non-weak symbol from the same TU is always
>> > invalid and the compiler should not be pessimizing code to make it
>> > work.
>>
>> -Bsymbolic will bind all references to local definitions in shared libraries,
>> with and without visibility, weak or non-weak.  Compiler can use it
>> in binds_tls_local_p and we can generate much better codes in shared
>> libraries.
>
> Yes, I'm aware of what it does. But at compile-time the compiler can't
> know whether the referenced symbol will be defined in the same DSO
> unless this is visibility annotation telling it. Even when linking a
> shared library using -Bsymbolic, the library code can still make calls
> (or data references) to symbols in other DSOs.

Even without LTO, -fsymbolic -fPIC will generate better codes for

---
int glob_a = 1;

int foo ()
{
  return glob_a;
}
---

and

---
int glob_a (void)
{
  return -1;
}

int foo ()
{
  return glob_a ();
}
---


>> > As for -fnow, I haven't thought about it much but I also don't see
>> > many places where it could help. The only benefit that comes to mind
>> > is on targets with weak memory order, where it would eliminate some of
>> > the cost of synchronizing TLSDESC lazy bindings (see Szabolcs Nagy's
>> > work on AArch64). It might also benefit PLT calls on such targets, but
>> > you would get a lot more benefit from -fno-plt, and in that case -fnow
>> > would not allow any further optimization.
>>
>> -fno-plt doesn't work with lazy binding.  -fnow tells compiler that
>> lazy binding is not used and it can optimize without PLT.  With
>> -flto -fnow, compiler can make much better choices.
>
> Ah, I see now you had LTO in mind. In that case the compiler does know
> when the symbol is defined in the same DSO for -Bsymbolic. So that
> clears up the usefulness of your proposed -fsymbolic. I still don't
> see how -fnow would have a lot of practical usefulness, but I'm
> certainly not opposed to it.
>
> Rich



-- 
H.J.