[Bug middle-end/100593] [ELF] -fno-pic: Use GOT to take address of an external default visibility function

["i at maskray dot me" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100593

--- Comment #2 from Fangrui Song <i at maskray dot me> ---
(In reply to Alexander Monakov from comment #1)
> It is not necessary to change -fno-pic code generation to gain most of the
> -Bsymbolic benefit

It is necessary, otherwise the function address taken from the
-Bsymbolic/-Bsymbolic-functions/-Bsymbolic-global-functions shared object may
be different from the address taken from the -fno-pic code.

The ELF hack is called canonical PLT entry, similar to copy relocations.

> as you say, the most important point is to avoid jumping
> via PLT trampolines (or, with -fno-plt, GOT loads) for function calls, so
> the linker could do -Bsymbolic relaxation for sites where address doesn't
> matter (calls and jumps) while keeping a dynamic relocation for address
> loads? Under some new option of course, like -Bsymbolic-plt. Right?

There are two points: (1) R_*_JUMP_SLOT symbol lookup cost (2) whether call
sites get penalized by the PLT indirection.

-fno-pic code must use GOT (instead of an absolute relocation) for default
visibility external function access to be compatible with a
-Bsymbolic/-Bsymbolic-functions/-Bsymbolic-global-functions shared object.