[Bug rtl-optimization/47525] DCE fails to eliminate a dead call to a pure function when compiled with -maltivec

["bergner at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47525

Peter Bergner <bergner at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |powerpc64-linux
                 CC|                            |meissner at gcc dot gnu.org
      Known to work|                            |4.3.0
               Host|                            |powerpc64-linux
      Known to fail|                            |4.4.0, 4.5.0, 4.6.0
              Build|                            |powerpc64-linux

--- Comment #1 from Peter Bergner <bergner at gcc dot gnu.org> 2011-01-28 22:46:09 UTC ---
I tracked this down to -maltivec triggers code in
rs6000.c:rs6000_conditional_register_usage() that forces VSCR_REGNO to be
marked as a global register:

  if (TARGET_ALTIVEC || TARGET_VSX)
    global_regs[VSCR_REGNO] = 1;

Then this code in df-scan.c:df_get_call_refs() forces all global registers to
be added to every function call's use and def chains, since they "might" be
referenced by the call:

  /* Calls may also reference any of the global registers,
     so they are recorded as used.  */
  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
    if (global_regs[i])
      {
        df_ref_record (DF_REF_BASE, collection_rec, regno_reg_rtx[i],
                       NULL, bb, insn_info, DF_REF_REG_USE, flags);
        df_ref_record (DF_REF_BASE, collection_rec, regno_reg_rtx[i],
                       NULL, bb, insn_info, DF_REF_REG_DEF, flags);
      }

The global register in the calls use and def chains are what stops DCE from
eliminating the call.

The bug is that const functions won't read or write any global registers and
pure functions won't write them.  The "fix" is to skip this code entirely for
calls to const functions and skip the addition to the def chain for calls to
pure functions.  A bootstrap and regtesting of a fix are in progress.