df: Treat partial defs as uses in df_simulate_defs [PR116564]

df: Treat partial defs as uses in df_simulate_defs [PR116564]

The PR shows us spinning in dce.cc:fast_dce at the start of combine.
This spinning appears to be because of a disagreement between the fast_dce code
and the code in df-problems.cc:df_lr_bb_local_compute.  Specifically, they
disagree on the treatment of partial defs.  For the testcase in the PR, we have
the following insn in bb 3:

(insn 10 8 13 3 (clobber (subreg:V1DF (reg/v:V2x1DF 104 [ __val ]) 8)) -1
     (nil))

which gives rise to a DF def with DF_REF_FLAGS = 0x8b0, i.e.
DF_REF_PARTIAL | DF_REF_READ_WRITE | DF_REF_MUST_CLOBBER | DF_REF_SUBREG.

Eliding the large block comment for readability, the code in
df_lr_bb_local_compute does the following (for each insn):

      FOR_EACH_INSN_INFO_DEF (def, insn_info)
        {
          unsigned int dregno = DF_REF_REGNO (def);
          bitmap_set_bit (&bb_info->def, dregno);
          if (DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL))
            bitmap_set_bit (&bb_info->use, dregno);
          else
            bitmap_clear_bit (&bb_info->use, dregno);
        }

i.e. it models partial defs as a RMW operation; thus for the def arising
from i10 above, it records a use of r104; hence it ends up in the
live-in set for bb 3.

However, as it stands, the code in dce.cc:fast_dce (and its callee
dce_process_block) has no such provision for DF_REF_PARTIAL defs.  It
does not treat these as a RMW and does not compute r104 above as being
live-in to bb 3.  At the end of dce_process_block we compute the
following "did something happen" condition used to decide termination of
the analysis:

  block_changed = !bitmap_equal_p (local_live, DF_LR_IN (bb));
  if (block_changed)
    bitmap_copy (DF_LR_IN (bb), local_live);

  BITMAP_FREE (local_live);
  return block_changed;

because of the disagreement between df_lr_local_compute and the local
analysis done by fast_dce, we invariably have r104 in DF_LR_IN, but not
in local_live.  Hence we always return true here, call
df_analyze_problem (which re-computes DF_LR_IN according to
df_lr_bb_local_compute, re-adding r104), and so the analysis never
terminates.

This patch therefore adjusts df_simulate_defs (called from
dce_process_block) to match the behaviour of df_lr_bb_local_compute in
this respect, namely we make it model partial defs as RMW operations by
setting the relevant register live.  This fixes the spinning in fast_dce
for this testcase.

gcc/ChangeLog:

	PR rtl-optimization/116564
	* df-problems.cc (df_simulate_defs): For partial defs, mark the
	register live (treat it as a RMW operation).

gcc/testsuite/ChangeLog:

	PR rtl-optimization/116564
	* gcc.target/aarch64/torture/pr116564.c: New test.