[PATCH] pch: Add support for PCH for relocatable executables

[Jakub Jelinek]

On Tue, Nov 09, 2021 at 11:40:08AM +0000, Iain Sandoe wrote:
> There were two issues, of which one remains and probably affects all targets.
> 
> 1.  The Darwin PCH memory allocation scheme used a system that works reliably
>     for no-PIE but not for PIE
> 
> .. I hacked in a similar scheme to the mmap one used on Linux .. the suspect stuff
>    there is in choosing some place in the map that is likely to succeed…
> 
>   With that I get passes on all c-family pch.exp (I didn’t try to bootstrap).

Yeah, certainly.

> 2. This problem remains.
> 
>   - if we try to emit a diagnostic when the PCH read-in has failed, it seems that
>    cc1 hangs somewhere in trying to lookup line table info.
> 
>  - this was happening with the Darwin fixed PCH memory address because it
>    was trying to report a fatal error in being unable to read the file (or trying to
>   execute fancy_abort, in response to a segv).

I guess once we:
  /* Read in all the scalar variables.  */
  for (rt = gt_pch_scalar_rtab; *rt; rt++)
    for (rti = *rt; rti->base != NULL; rti++)
      if (fread (rti->base, rti->stride, 1, f) != 1)
        fatal_error (input_location, "cannot read PCH file: %m");

  /* Read in all the global pointers, in 6 easy loops.  */
  for (rt = gt_ggc_rtab; *rt; rt++)
    for (rti = *rt; rti->base != NULL; rti++)
      for (i = 0; i < rti->nelt; i++)
        if (fread ((char *)rti->base + rti->stride * i,
                   sizeof (void *), 1, f) != 1)
          fatal_error (input_location, "cannot read PCH file: %m");
we overwrite the GTY(()) marked global vars including
extern GTY(()) class line_maps *line_table;
with pointers into the area we haven't mapped yet (or if the error happens
after that mmap but before everything is fixed up (e.g. the new relocation
processing), it is no wonder it doesn't work well.

Could we save line_table (and perhaps a few other vars) into non-GTY! copies
of them in ggc-common.c and instead of those fatal_error (input_location, ...)
calls in gt_pch_restore and ggc_pch_read call fatal_pch_error (...) where
void
fatal_pch_error (const char *gmsg)
{
  line_table = saved_line_table;
  // Restore anything else that is needed for fatal_error
  fatal_error (input_location, gmsg);
}

	Jakub