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Re: What is R_X86_64_GOTPLT64 used for?


On Thu, Nov 20, 2014 at 5:42 AM, Michael Matz <matz@suse.de> wrote:
> Exactly.  As a symbol lookup needs to be done anyway for the GLOB_DAT
> reloc going through the dynamic linker for the lazy lookup later when a
> call occurs doesn't make sense.
>
>> jmp *name@GOTPCREL(%rip)
>> 8 byte nop
>
> You mean replacing the PLT slot with the above?  Yep, something like that.
> Even better of course would be to not use the PLT slot at all, it's just a
> useless indirection.  It would be even cooler to rewrite the call insn
> from
>   call foo@PLT
> into
>   call *foo@GOTPCREL(%rip)
>
> (in the small model here)  Unfortunately the latter is one byte larger
> than the former.  But perhaps GCC could already emit the latter form
> when it knows a certain function symbol has its address taken (or more
> precisely if a GLOB_DAT reloc is going to be emitted for it).
>
>> where name@GOTPCREL points to the normal GOT slot
>> updated by R_X86_64_GLOB_DAT relocation at run-time.
>> Should I give it a try?
>
> Frankly, I have no idea if it's worth it.  Address takings of function
> symbols doesn't occur very often, except in vtables, and that's not using
> GOT slots.  Vtables should be handled in a completely different way
> anyway: as the entries aren't usually used for address comparisons they
> should point to the PLT slots, so that it's only RELATIVE relocs, not
> symbol based ones, so that also virtual calls can be resolved lazily.
>
>
> Ciao,
> Michael.

I fixed a bug on hjl/plt.got branch:

https://sourceware.org/git/?p=binutils-gdb.git;a=shortlog;h=refs/heads/hjl/plt.got

It passed glibc tests and bootstrapped GCC.  It optimized functions like

std::bad_exception::~bad_exception()
__cxa_finalize
std::range_error::~range_error()
std::bad_array_length::~bad_array_length()


-- 
H.J.


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