code review 4071044: 6l: Relocate CMOV* instructions

6l: Relocate CMOV* instructions

The linker avoids a GOT indirection by turning a MOV into
a LEA, but with x86-64 GCC has started emitting CMOV*
instructions which break the existing logic.

This will generate errors such as:

  unexpected GOT reloc for non-dynamic symbol luaO_nilobject_

The CMOV* instructions may be emitted with normal code like:

  if (o >= L->top) return cast(TValue *, luaO_nilobject);
  else return o;

Which gets compiled into (relocation offset at 1b):

  13: 48 3b 47 10             cmp    0x10(%rdi),%rax
  17: 48 0f 43 05 00 00 00    cmovae 0x0(%rip),%rax
  1e: 00

This change will allow the indirection through the GOT to
avoid the problem in those cases.

[niemeyer, 2011-01-27 17:50:52]

Hello golang-dev@googlegroups.com (cc: golang-dev@googlegroups.com),

I'd like you to review this change.

[rsc, 2011-01-27 18:04:02]

I am not sure that using the GOT is the right fix in general.
That special case is in the code because the symbol is
in the local package and may not make it into the dynamic
symbol table (it is not being exported).

So I'm a little confused about why this fix works,
but I'm willing to run with it.

Please change the comment to

// fall back to using GOT and hope for the best.

[niemeyer, 2011-01-27 18:22:02]

Yes, I agree that exporting the symbol is an unfortunate 
consequence of this fix.  To avoid it we'd have to declare
a new relocation section which is bound to a different
symbol table.  We only have .rela and .rela.plt today, and
they're both addressing symbols in .dynsym.  We can either
do this, or just go with the lazy fix for now, and tweak
later if this situation becomes more common.

I've changed the comment.

[rsc, 2011-01-27 18:42:51]

LGTM

[rsc, 2011-01-27 19:09:10]

*** Submitted as 9b9685cabc05 ***

6l: Relocate CMOV* instructions

The linker avoids a GOT indirection by turning a MOV into
a LEA, but with x86-64 GCC has started emitting CMOV*
instructions which break the existing logic.

This will generate errors such as:

  unexpected GOT reloc for non-dynamic symbol luaO_nilobject_

The CMOV* instructions may be emitted with normal code like:

  if (o >= L->top) return cast(TValue *, luaO_nilobject);
  else return o;

Which gets compiled into (relocation offset at 1b):

  13: 48 3b 47 10             cmp    0x10(%rdi),%rax
  17: 48 0f 43 05 00 00 00    cmovae 0x0(%rip),%rax
  1e: 00

This change will allow the indirection through the GOT to
avoid the problem in those cases.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4071044

Committer: Russ Cox <rsc@golang.org>