What is R_X86_64_GOTPLT64 used for?

[H.J. Lu]

x86-64 psABI has

name@GOT: specifies the offset to the GOT entry for the symbol name
from the base of the GOT.

name@GOTPLT: specifies the offset to the GOT entry for the symbol name
from the base of the GOT, implying that there is a corresponding PLT entry.

But GCC never generates name@GOTPLT and assembler fails to assemble
it:

[hjl@gnu-6 pr17598]$ cat x.S
movabs $foo@GOTPLT,%rax
[hjl@gnu-6 pr17598]$ gcc -c x.S
x.S: Assembler messages:
x.S:1: Error: relocated field and relocation type differ in signedness
[hjl@gnu-6 pr17598]$

It certainly isn't needed on data symbols.  I couldn't find any possible
usage for this relocation on function symbols.

Does anyone remember what it was supposed to be used for?

-- 
H.J.