Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=

[Alan Modra <amodra@gmail.com>]

On Thu, Apr 30, 2015 at 05:31:30PM -0700, Sriraman Tallam wrote:
> This comes with  caveats.  This cannot be generally done for all
> functions marked extern as it is impossible for the compiler to say if
> a function is "truly extern" (defined in a shared library). If a
> function is not truly extern(ends up defined in the final executable),
> then calling it indirectly is a performance penalty as it could have
> been a direct call.  Further, the newly created GOT entries are fixed
> up at start-up and do not get lazily bound.

I've considered something similar for PowerPC (but didn't consider
doing do so for a subset of calls).  Losing lazy symbol resolution is
a real problem.  The other problem you cite of indirect calls that
could be direct can be fixed in the linker relatively easily.
Edit this code
   0:	ff 15 00 00 00 00    	callq  *0x0(%rip)        # 0x6
			2: R_X86_64_GOTPCREL	foo-0x4
   6:	ff 25 00 00 00 00    	jmpq   *0x0(%rip)        # 0xc
			8: R_X86_64_GOTPCREL	foo-0x4
to this
   c:	e8 00 00 00 00       	callq  0x11
			d: R_X86_64_PC32	foo-0x4
  11:	90                   	nop
  12:	e9 00 00 00 00       	jmpq   0x17
			13: R_X86_64_PC32	foo-0x4
  17:	90                   	nop
You may need to have gcc or gas add a marker reloc to say exactly
where an instruction starts.

-- 
Alan Modra
Australia Development Lab, IBM