[committed, PATCH] PR ld/18289: Support copy relocation in PIE

[Alan Modra]

On Wed, Apr 22, 2015 at 05:35:23AM -0700, H.J. Lu wrote:
> This patch allows copy relocs for R_386_GOTOFF relocations in PIE.  For
> 
> extern int glob_a;
> int foo ()
> {
>   return glob_a;
> }
> 
> compiler now can optimize it from
> 
> 	call	__x86.get_pc_thunk.ax
> 	addl	$_GLOBAL_OFFSET_TABLE_, %eax
> 	movl	glob_a@GOT(%eax), %eax
> 	movl	(%eax), %eax
> 	ret
> 
> to
> 
> 	call	__x86.get_pc_thunk.ax
> 	addl	$_GLOBAL_OFFSET_TABLE_, %eax
> 	movl	glob_a@GOTOFF(%eax), %eax
> 	ret

Did you consider doing this code transformation in the linker?  I
guess it's not so easy with variable length instructions, and you
would be left with a couple of nops.

There are quite a few advantages of doing this in the linker:
- Optimization becomes available to people who must use an older gcc
  for some reason.
- No need to make gcc or glibc changes.
- No need to update all of ld, gcc, and glibc simultaneously for
  correctness.  A partial update will cause some testcases that worked
  correctly with an older toolchain to misbehave.
- People can choose to use protected visibility in shared libraries to
  get an identical speedup in the shared library to that you show
  above.  With your gcc changes, protected visibility variables in
  shared libraries no longer have a speed advantage, which is a
  regression.

-- 
Alan Modra
Australia Development Lab, IBM