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From: "nsz at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/32008] New: aarch64: tlsdesc can be optimized by
 custom tlsdesc calls for common cases
Date: Mon, 22 Jul 2024 17:28:00 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D32008

            Bug ID: 32008
           Summary: aarch64: tlsdesc can be optimized by custom tlsdesc
                    calls for common cases
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: nsz at gcc dot gnu.org
  Target Milestone: ---

the static tls case is essentially:

size_t _dl_tlsdesc_return(size_t *got)
{
  return got[1]; // offset
}

the dynamic tls fast path is

size_t _dl_tlsdesc_dynamic(size_t *got)
{
  void *tp =3D gettp();
  void **dtv =3D *(void ***)tp;
  size_t *data =3D (size_t *)got[1];
  size_t modid =3D data[0];
  size_t offset =3D data[1];
  size_t gencount =3D data[2];
  if (gencount < (size_t)dtv[0] && dtv[modid] !=3D UNALLOCATED)
    return dtv[modid] + offset - tp;
  //...
}

can be optimized under the assumption that certain modid or offset
values are common (e.g. modid < 8 or offset =3D=3D 0): for such cases
use a custom _dl_tlsdesc_* when the tlsdesc relocation is processed.

e.g.

size_t _dl_tlsdesc_return42(size_t *got)
{
  return 42;
}

can be used for a tlsdesc reloc if the module uses static tls and
the tp offset to the variable ends up 42. this of course does not
save much and rarely useful, but for the dynamic case it may be
possible to save many instructions if some common modid/offset/gen
cases have separate asm entry points: even if bug 31995 is not fixed
similar optimization to the proposed fix for bug 27404 can be
implemented.

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