Re: Fix PR44281 (bad RA with global regs)

[Bernd Schmidt <bschmidt at redhat dot com>]

On 02/29/2016 06:07 PM, Michael Matz wrote:

> %rbx would have to be implicitly used/clobbered by the asm.  In addition
> it would have to be used by all function entries and exits (so that a
> function body where the global reg var is merely visible but not used
> doesn't accidentally clobber that register) and of course by calls.

Nearly all this exists as of today. From df-scan.c:

static void
df_get_call_refs (struct df_collection_rec *collection_rec,
[...]
      else if (global_regs[i])
        {
          /* Calls to const functions cannot access any global 
registers and
             calls to pure functions cannot set them.  All other calls may
             reference any of the global registers, so they are recorded as
             used. */
          if (!RTL_CONST_CALL_P (insn_info->insn))

and

static void
df_get_entry_block_def_set (bitmap entry_block_defs)
{
[...]
  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
    {
      if (global_regs[i])
        bitmap_set_bit (entry_block_defs, i);

and

  /* Mark all global registers, and all registers used by the
     epilogue as being live at the end of the function since they
     may be referenced by our caller.  */
  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
    if (global_regs[i] || EPILOGUE_USES (i))
      bitmap_set_bit (exit_block_uses, i);

> FWIW: signal handlers need no consideration (if they were allowed to
> inspect/alter global reg vars we would have lost and no improvement on
> fixed_regs[] could be done).  They are explicitely documented to not be
> able to access global reg vars.  (They already can't accidentally clobber
> the register in question because signal handlers don't do that)

Oh, they can't modify the register in question because the OS would 
restore it? Hadn't thought of that.

Ok so maybe reopen and apply my patch for gcc-7, with a tweak for asms.


Bernd