RFC: Add a new relocation to x86-64/i386 psABIs

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

To avoid indirect branch to internal functions, I am proposing to add a
new relocation, R_X86_64_RELAX_GOTPCREL, to x86-64 psABI:

1. When branching to an external function, foo, compiler may generate
	call/jmp *foo@GOTRELAX(%rip)
   which generates R_X86_64_RELAX_GOTPCREL relocation, instead of
	call/jmp foo[@PLT]
2. When function foo is locally defined, linker converts
	call/jmp *foo@GOTRELAX(%rip)
   to
	nop call/jmp foo
3. Otherwise, linker treats R_X86_64_RELAX_GOTPCREL the same way as
   R_X86_64_GOTPCREL.

For i386 psABI, we add R_386_RELAX_GOT32:

1. When branching to an external function, foo, in non-PIC mode,
   compiler may generate
	call/jmp *foo@GOTRELAX
   which generates R_386_RELAX_GOT32 relocation, instead of
	call/jmp foo
   and in PIC mode
	call/jmp *foo@GOTRELAX(%reg)
   which generates R_386_RELAX_GOT32 relocation and REG holds the address
   of GOT, instead of
	call/jmp foo@PLT
2. When function foo is locally defined, linker converts
	call/jmp *foo@GOTRELAX[(%reg)]
   to
	nop call/jmp foo
3. Otherwise, 
   a. In PIC mode, linker treats R_386_RELAX_GOT32 the same way as
      R_386_GOT32 and "call/jmp *foo@GOTRELAX" is unsupported.
   b. In no-PIC mode, linker computes its relocation value as relocation
      value of R_386_GOT32 plus the address of GOT and converts
	call/jmp *foo@GOTRELAX(%reg)
      to
	call/jmp *foo@GOTRELAX
      if needed.

This new relocation effectively turns off lazy binding on function, foo.

For i386, compiler is free to choose any register to hold the address of
GOT and there is no need to make EBX a fixed register when branching to
an external function in PIC mode.

With this new relocation, only a one-byte NOP prefix overhead is added
when function, foo, which compiler determines is external, turns out to
be local at link-time, because of -Bsymbolic or a definition in another
input object file which compiler has no knowledge of.

The new -fno-plt GCC option can use R_X86_64_RELAX_GOTPCREL and
R_386_RELAX_GOT32 relocations if linker supports them to avoid indirect
branch to internal functions.


H.J.