+ df_set_note (REG_DEAD, insn, reg);
+
+#ifdef REG_DEAD_DEBUGGING
+ df_print_note ("adding 4: ", insn, REG_NOTES (insn));
+#endif
+ }
+ /* This register is now live. */
+ bitmap_set_bit (live, uregno);
+ }
+ }
+
+ if (debug_insn == -1)
+ {
+ /* ??? We could probably do better here, replacing dead
+ registers with their definitions. */
+ INSN_VAR_LOCATION_LOC (insn) = gen_rtx_UNKNOWN_VAR_LOC ();
+ df_insn_rescan_debug_internal (insn);
+ }
+ }
+
+ dead_debug_finish (&debug, NULL);
+}
+
+
+/* Compute register info: lifetime, bb, and number of defs and uses. */
+static void
+df_note_compute (bitmap all_blocks)
+{
+ unsigned int bb_index;
+ bitmap_iterator bi;
+ bitmap_head live, do_not_gen, artificial_uses;
+
+ bitmap_initialize (&live, &df_bitmap_obstack);
+ bitmap_initialize (&do_not_gen, &df_bitmap_obstack);
+ bitmap_initialize (&artificial_uses, &df_bitmap_obstack);
+
+#ifdef REG_DEAD_DEBUGGING
+ if (dump_file)
+ print_rtl_with_bb (dump_file, get_insns());
+#endif
+
+ EXECUTE_IF_SET_IN_BITMAP (all_blocks, 0, bb_index, bi)
+ {
+ df_note_bb_compute (bb_index, &live, &do_not_gen, &artificial_uses);
+ }
+
+ bitmap_clear (&live);
+ bitmap_clear (&do_not_gen);
+ bitmap_clear (&artificial_uses);
+}
+
+
+/* Free all storage associated with the problem. */
+
+static void
+df_note_free (void)
+{
+ free (df_note);
+}
+
+
+/* All of the information associated every instance of the problem. */
+
+static struct df_problem problem_NOTE =
+{
+ DF_NOTE, /* Problem id. */
+ DF_NONE, /* Direction. */
+ df_note_alloc, /* Allocate the problem specific data. */
+ NULL, /* Reset global information. */
+ NULL, /* Free basic block info. */
+ df_note_compute, /* Local compute function. */
+ NULL, /* Init the solution specific data. */
+ NULL, /* Iterative solver. */
+ NULL, /* Confluence operator 0. */
+ NULL, /* Confluence operator n. */
+ NULL, /* Transfer function. */
+ NULL, /* Finalize function. */
+ df_note_free, /* Free all of the problem information. */
+ df_note_free, /* Remove this problem from the stack of dataflow problems. */
+ NULL, /* Debugging. */
+ NULL, /* Debugging start block. */
+ NULL, /* Debugging end block. */
+ NULL, /* Incremental solution verify start. */
+ NULL, /* Incremental solution verify end. */
+ &problem_LR, /* Dependent problem. */
+ sizeof (struct df_scan_bb_info),/* Size of entry of block_info array. */
+ TV_DF_NOTE, /* Timing variable. */
+ false /* Reset blocks on dropping out of blocks_to_analyze. */
+};
+
+
+/* Create a new DATAFLOW instance and add it to an existing instance
+ of DF. The returned structure is what is used to get at the
+ solution. */
+
+void
+df_note_add_problem (void)
+{
+ df_add_problem (&problem_NOTE);
+}
+
+
+
+\f
+/*----------------------------------------------------------------------------
+ Functions for simulating the effects of single insns.
+
+ You can either simulate in the forwards direction, starting from
+ the top of a block or the backwards direction from the end of the
+ block. If you go backwards, defs are examined first to clear bits,
+ then uses are examined to set bits. If you go forwards, defs are
+ examined first to set bits, then REG_DEAD and REG_UNUSED notes
+ are examined to clear bits. In either case, the result of examining
+ a def can be undone (respectively by a use or a REG_UNUSED note).
+
+ If you start at the top of the block, use one of DF_LIVE_IN or
+ DF_LR_IN. If you start at the bottom of the block use one of
+ DF_LIVE_OUT or DF_LR_OUT. BE SURE TO PASS A COPY OF THESE SETS,
+ THEY WILL BE DESTROYED.
+----------------------------------------------------------------------------*/
+
+
+/* Find the set of DEFs for INSN. */
+
+void
+df_simulate_find_defs (rtx insn, bitmap defs)
+{
+ df_ref *def_rec;
+ unsigned int uid = INSN_UID (insn);
+
+ for (def_rec = DF_INSN_UID_DEFS (uid); *def_rec; def_rec++)
+ {
+ df_ref def = *def_rec;
+ bitmap_set_bit (defs, DF_REF_REGNO (def));
+ }
+}
+
+/* Find the set of uses for INSN. This includes partial defs. */
+
+static void
+df_simulate_find_uses (rtx insn, bitmap uses)
+{
+ df_ref *rec;
+ unsigned int uid = INSN_UID (insn);
+
+ for (rec = DF_INSN_UID_DEFS (uid); *rec; rec++)
+ {
+ df_ref def = *rec;
+ if (DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL))
+ bitmap_set_bit (uses, DF_REF_REGNO (def));
+ }
+ for (rec = DF_INSN_UID_USES (uid); *rec; rec++)
+ {
+ df_ref use = *rec;
+ bitmap_set_bit (uses, DF_REF_REGNO (use));
+ }
+}
+
+/* Find the set of real DEFs, which are not clobbers, for INSN. */
+
+void
+df_simulate_find_noclobber_defs (rtx insn, bitmap defs)
+{
+ df_ref *def_rec;
+ unsigned int uid = INSN_UID (insn);
+
+ for (def_rec = DF_INSN_UID_DEFS (uid); *def_rec; def_rec++)
+ {
+ df_ref def = *def_rec;
+ if (!(DF_REF_FLAGS (def) & (DF_REF_MUST_CLOBBER | DF_REF_MAY_CLOBBER)))
+ bitmap_set_bit (defs, DF_REF_REGNO (def));
+ }
+}
+
+
+/* Simulate the effects of the defs of INSN on LIVE. */
+
+void
+df_simulate_defs (rtx insn, bitmap live)
+{
+ df_ref *def_rec;
+ unsigned int uid = INSN_UID (insn);
+
+ for (def_rec = DF_INSN_UID_DEFS (uid); *def_rec; def_rec++)
+ {
+ df_ref def = *def_rec;
+ unsigned int dregno = DF_REF_REGNO (def);
+
+ /* If the def is to only part of the reg, it does
+ not kill the other defs that reach here. */
+ if (!(DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL)))
+ bitmap_clear_bit (live, dregno);
+ }
+}
+
+
+/* Simulate the effects of the uses of INSN on LIVE. */
+
+void
+df_simulate_uses (rtx insn, bitmap live)
+{
+ df_ref *use_rec;
+ unsigned int uid = INSN_UID (insn);
+
+ if (DEBUG_INSN_P (insn))
+ return;
+
+ for (use_rec = DF_INSN_UID_USES (uid); *use_rec; use_rec++)
+ {
+ df_ref use = *use_rec;
+ /* Add use to set of uses in this BB. */
+ bitmap_set_bit (live, DF_REF_REGNO (use));
+ }
+}
+
+
+/* Add back the always live regs in BB to LIVE. */
+
+static inline void
+df_simulate_fixup_sets (basic_block bb, bitmap live)
+{
+ /* These regs are considered always live so if they end up dying
+ because of some def, we need to bring the back again. */
+ if (bb_has_eh_pred (bb))
+ bitmap_ior_into (live, &df->eh_block_artificial_uses);
+ else
+ bitmap_ior_into (live, &df->regular_block_artificial_uses);
+}
+
+
+/*----------------------------------------------------------------------------
+ The following three functions are used only for BACKWARDS scanning:
+ i.e. they process the defs before the uses.
+
+ df_simulate_initialize_backwards should be called first with a
+ bitvector copyied from the DF_LIVE_OUT or DF_LR_OUT. Then
+ df_simulate_one_insn_backwards should be called for each insn in
+ the block, starting with the last one. Finally,
+ df_simulate_finalize_backwards can be called to get a new value
+ of the sets at the top of the block (this is rarely used).
+ ----------------------------------------------------------------------------*/
+
+/* Apply the artificial uses and defs at the end of BB in a backwards
+ direction. */
+
+void
+df_simulate_initialize_backwards (basic_block bb, bitmap live)
+{
+ df_ref *def_rec;
+ df_ref *use_rec;
+ int bb_index = bb->index;
+
+ for (def_rec = df_get_artificial_defs (bb_index); *def_rec; def_rec++)
+ {
+ df_ref def = *def_rec;
+ if ((DF_REF_FLAGS (def) & DF_REF_AT_TOP) == 0)
+ bitmap_clear_bit (live, DF_REF_REGNO (def));
+ }
+
+ for (use_rec = df_get_artificial_uses (bb_index); *use_rec; use_rec++)
+ {
+ df_ref use = *use_rec;
+ if ((DF_REF_FLAGS (use) & DF_REF_AT_TOP) == 0)
+ bitmap_set_bit (live, DF_REF_REGNO (use));
+ }
+}
+
+
+/* Simulate the backwards effects of INSN on the bitmap LIVE. */
+
+void
+df_simulate_one_insn_backwards (basic_block bb, rtx insn, bitmap live)
+{
+ if (!NONDEBUG_INSN_P (insn))
+ return;
+
+ df_simulate_defs (insn, live);
+ df_simulate_uses (insn, live);
+ df_simulate_fixup_sets (bb, live);
+}
+
+
+/* Apply the artificial uses and defs at the top of BB in a backwards
+ direction. */
+
+void
+df_simulate_finalize_backwards (basic_block bb, bitmap live)
+{
+ df_ref *def_rec;
+#ifdef EH_USES
+ df_ref *use_rec;
+#endif
+ int bb_index = bb->index;
+
+ for (def_rec = df_get_artificial_defs (bb_index); *def_rec; def_rec++)
+ {
+ df_ref def = *def_rec;
+ if (DF_REF_FLAGS (def) & DF_REF_AT_TOP)
+ bitmap_clear_bit (live, DF_REF_REGNO (def));
+ }
+
+#ifdef EH_USES
+ for (use_rec = df_get_artificial_uses (bb_index); *use_rec; use_rec++)
+ {
+ df_ref use = *use_rec;
+ if (DF_REF_FLAGS (use) & DF_REF_AT_TOP)
+ bitmap_set_bit (live, DF_REF_REGNO (use));
+ }
+#endif
+}
+/*----------------------------------------------------------------------------
+ The following three functions are used only for FORWARDS scanning:
+ i.e. they process the defs and the REG_DEAD and REG_UNUSED notes.
+ Thus it is important to add the DF_NOTES problem to the stack of
+ problems computed before using these functions.
+
+ df_simulate_initialize_forwards should be called first with a
+ bitvector copyied from the DF_LIVE_IN or DF_LR_IN. Then
+ df_simulate_one_insn_forwards should be called for each insn in
+ the block, starting with the first one.
+ ----------------------------------------------------------------------------*/
+
+/* Initialize the LIVE bitmap, which should be copied from DF_LIVE_IN or
+ DF_LR_IN for basic block BB, for forward scanning by marking artificial
+ defs live. */
+
+void
+df_simulate_initialize_forwards (basic_block bb, bitmap live)
+{
+ df_ref *def_rec;
+ int bb_index = bb->index;
+
+ for (def_rec = df_get_artificial_defs (bb_index); *def_rec; def_rec++)
+ {
+ df_ref def = *def_rec;
+ if (DF_REF_FLAGS (def) & DF_REF_AT_TOP)
+ bitmap_set_bit (live, DF_REF_REGNO (def));
+ }
+}
+
+/* Simulate the forwards effects of INSN on the bitmap LIVE. */
+
+void
+df_simulate_one_insn_forwards (basic_block bb, rtx insn, bitmap live)
+{
+ rtx link;
+ if (! INSN_P (insn))
+ return;
+
+ /* Make sure that DF_NOTE really is an active df problem. */
+ gcc_assert (df_note);
+
+ /* Note that this is the opposite as how the problem is defined, because
+ in the LR problem defs _kill_ liveness. However, they do so backwards,
+ while here the scan is performed forwards! So, first assume that the
+ def is live, and if this is not true REG_UNUSED notes will rectify the
+ situation. */
+ df_simulate_find_noclobber_defs (insn, live);
+
+ /* Clear all of the registers that go dead. */
+ for (link = REG_NOTES (insn); link; link = XEXP (link, 1))
+ {
+ switch (REG_NOTE_KIND (link))
+ {
+ case REG_DEAD:
+ case REG_UNUSED:
+ {
+ rtx reg = XEXP (link, 0);
+ int regno = REGNO (reg);
+ if (HARD_REGISTER_NUM_P (regno))
+ bitmap_clear_range (live, regno,
+ hard_regno_nregs[regno][GET_MODE (reg)]);
+ else
+ bitmap_clear_bit (live, regno);
+ }
+ break;
+ default:
+ break;
+ }
+ }
+ df_simulate_fixup_sets (bb, live);
+}
+\f
+/* Used by the next two functions to encode information about the
+ memory references we found. */
+#define MEMREF_NORMAL 1
+#define MEMREF_VOLATILE 2
+
+/* A subroutine of can_move_insns_across_p called through for_each_rtx.
+ Return either MEMREF_NORMAL or MEMREF_VOLATILE if a memory is found. */
+
+static int
+find_memory (rtx *px, void *data ATTRIBUTE_UNUSED)
+{
+ rtx x = *px;
+
+ if (GET_CODE (x) == ASM_OPERANDS && MEM_VOLATILE_P (x))
+ return MEMREF_VOLATILE;
+
+ if (!MEM_P (x))
+ return 0;
+ if (MEM_VOLATILE_P (x))
+ return MEMREF_VOLATILE;
+ if (MEM_READONLY_P (x))
+ return 0;
+
+ return MEMREF_NORMAL;
+}
+
+/* A subroutine of can_move_insns_across_p called through note_stores.
+ DATA points to an integer in which we set either the bit for
+ MEMREF_NORMAL or the bit for MEMREF_VOLATILE if we find a MEM
+ of either kind. */
+
+static void
+find_memory_stores (rtx x, const_rtx pat ATTRIBUTE_UNUSED,
+ void *data ATTRIBUTE_UNUSED)
+{
+ int *pflags = (int *)data;
+ if (GET_CODE (x) == SUBREG)
+ x = XEXP (x, 0);
+ /* Treat stores to SP as stores to memory, this will prevent problems
+ when there are references to the stack frame. */
+ if (x == stack_pointer_rtx)
+ *pflags |= MEMREF_VOLATILE;
+ if (!MEM_P (x))
+ return;
+ *pflags |= MEM_VOLATILE_P (x) ? MEMREF_VOLATILE : MEMREF_NORMAL;
+}
+
+/* Scan BB backwards, using df_simulate functions to keep track of
+ lifetimes, up to insn POINT. The result is stored in LIVE. */
+
+void
+simulate_backwards_to_point (basic_block bb, regset live, rtx point)
+{
+ rtx insn;
+ bitmap_copy (live, df_get_live_out (bb));
+ df_simulate_initialize_backwards (bb, live);
+
+ /* Scan and update life information until we reach the point we're
+ interested in. */
+ for (insn = BB_END (bb); insn != point; insn = PREV_INSN (insn))
+ df_simulate_one_insn_backwards (bb, insn, live);
+}
+
+/* Return true if it is safe to move a group of insns, described by
+ the range FROM to TO, backwards across another group of insns,
+ described by ACROSS_FROM to ACROSS_TO. It is assumed that there
+ are no insns between ACROSS_TO and FROM, but they may be in
+ different basic blocks; MERGE_BB is the block from which the
+ insns will be moved. The caller must pass in a regset MERGE_LIVE
+ which specifies the registers live after TO.
+
+ This function may be called in one of two cases: either we try to
+ move identical instructions from all successor blocks into their
+ predecessor, or we try to move from only one successor block. If
+ OTHER_BRANCH_LIVE is nonnull, it indicates that we're dealing with
+ the second case. It should contain a set of registers live at the
+ end of ACROSS_TO which must not be clobbered by moving the insns.
+ In that case, we're also more careful about moving memory references
+ and trapping insns.
+
+ We return false if it is not safe to move the entire group, but it
+ may still be possible to move a subgroup. PMOVE_UPTO, if nonnull,
+ is set to point at the last moveable insn in such a case. */
+
+bool
+can_move_insns_across (rtx from, rtx to, rtx across_from, rtx across_to,
+ basic_block merge_bb, regset merge_live,
+ regset other_branch_live, rtx *pmove_upto)
+{
+ rtx insn, next, max_to;
+ bitmap merge_set, merge_use, local_merge_live;
+ bitmap test_set, test_use;
+ unsigned i, fail = 0;
+ bitmap_iterator bi;
+ int memrefs_in_across = 0;
+ int mem_sets_in_across = 0;
+ bool trapping_insns_in_across = false;
+
+ if (pmove_upto != NULL)
+ *pmove_upto = NULL_RTX;
+
+ /* Find real bounds, ignoring debug insns. */
+ while (!NONDEBUG_INSN_P (from) && from != to)
+ from = NEXT_INSN (from);
+ while (!NONDEBUG_INSN_P (to) && from != to)
+ to = PREV_INSN (to);
+
+ for (insn = across_to; ; insn = next)
+ {
+ if (NONDEBUG_INSN_P (insn))
+ {
+ memrefs_in_across |= for_each_rtx (&PATTERN (insn), find_memory,
+ NULL);
+ note_stores (PATTERN (insn), find_memory_stores,
+ &mem_sets_in_across);
+ /* This is used just to find sets of the stack pointer. */
+ memrefs_in_across |= mem_sets_in_across;
+ trapping_insns_in_across |= may_trap_p (PATTERN (insn));
+ }
+ next = PREV_INSN (insn);
+ if (insn == across_from)
+ break;
+ }
+
+ /* Collect:
+ MERGE_SET = set of registers set in MERGE_BB
+ MERGE_USE = set of registers used in MERGE_BB and live at its top
+ MERGE_LIVE = set of registers live at the point inside the MERGE
+ range that we've reached during scanning
+ TEST_SET = set of registers set between ACROSS_FROM and ACROSS_END.
+ TEST_USE = set of registers used between ACROSS_FROM and ACROSS_END,
+ and live before ACROSS_FROM. */
+
+ merge_set = BITMAP_ALLOC (®_obstack);
+ merge_use = BITMAP_ALLOC (®_obstack);
+ local_merge_live = BITMAP_ALLOC (®_obstack);
+ test_set = BITMAP_ALLOC (®_obstack);
+ test_use = BITMAP_ALLOC (®_obstack);
+
+ /* Compute the set of registers set and used in the ACROSS range. */
+ if (other_branch_live != NULL)
+ bitmap_copy (test_use, other_branch_live);
+ df_simulate_initialize_backwards (merge_bb, test_use);
+ for (insn = across_to; ; insn = next)
+ {
+ if (NONDEBUG_INSN_P (insn))
+ {
+ df_simulate_find_defs (insn, test_set);
+ df_simulate_defs (insn, test_use);
+ df_simulate_uses (insn, test_use);
+ }
+ next = PREV_INSN (insn);
+ if (insn == across_from)
+ break;
+ }
+
+ /* Compute an upper bound for the amount of insns moved, by finding
+ the first insn in MERGE that sets a register in TEST_USE, or uses
+ a register in TEST_SET. We also check for calls, trapping operations,
+ and memory references. */
+ max_to = NULL_RTX;
+ for (insn = from; ; insn = next)
+ {
+ if (CALL_P (insn))
+ break;
+ if (NOTE_P (insn) && NOTE_KIND (insn) == NOTE_INSN_EPILOGUE_BEG)
+ break;
+ if (NONDEBUG_INSN_P (insn))
+ {
+ if (may_trap_or_fault_p (PATTERN (insn))
+ && (trapping_insns_in_across || other_branch_live != NULL))
+ break;
+
+ /* We cannot move memory stores past each other, or move memory
+ reads past stores, at least not without tracking them and
+ calling true_dependence on every pair.
+
+ If there is no other branch and no memory references or
+ sets in the ACROSS range, we can move memory references
+ freely, even volatile ones.
+
+ Otherwise, the rules are as follows: volatile memory
+ references and stores can't be moved at all, and any type
+ of memory reference can't be moved if there are volatile
+ accesses or stores in the ACROSS range. That leaves
+ normal reads, which can be moved, as the trapping case is
+ dealt with elsewhere. */
+ if (other_branch_live != NULL || memrefs_in_across != 0)
+ {
+ int mem_ref_flags = 0;
+ int mem_set_flags = 0;
+ note_stores (PATTERN (insn), find_memory_stores, &mem_set_flags);
+ mem_ref_flags = for_each_rtx (&PATTERN (insn), find_memory,
+ NULL);
+ /* Catch sets of the stack pointer. */
+ mem_ref_flags |= mem_set_flags;
+
+ if ((mem_ref_flags | mem_set_flags) & MEMREF_VOLATILE)
+ break;
+ if ((memrefs_in_across & MEMREF_VOLATILE) && mem_ref_flags != 0)
+ break;
+ if (mem_set_flags != 0
+ || (mem_sets_in_across != 0 && mem_ref_flags != 0))
+ break;
+ }
+ df_simulate_find_uses (insn, merge_use);
+ /* We're only interested in uses which use a value live at
+ the top, not one previously set in this block. */
+ bitmap_and_compl_into (merge_use, merge_set);
+ df_simulate_find_defs (insn, merge_set);
+ if (bitmap_intersect_p (merge_set, test_use)
+ || bitmap_intersect_p (merge_use, test_set))
+ break;
+#ifdef HAVE_cc0
+ if (!sets_cc0_p (insn))
+#endif
+ max_to = insn;
+ }
+ next = NEXT_INSN (insn);
+ if (insn == to)
+ break;
+ }
+ if (max_to != to)
+ fail = 1;
+
+ if (max_to == NULL_RTX || (fail && pmove_upto == NULL))
+ goto out;
+
+ /* Now, lower this upper bound by also taking into account that
+ a range of insns moved across ACROSS must not leave a register
+ live at the end that will be clobbered in ACROSS. We need to
+ find a point where TEST_SET & LIVE == 0.
+
+ Insns in the MERGE range that set registers which are also set
+ in the ACROSS range may still be moved as long as we also move
+ later insns which use the results of the set, and make the
+ register dead again. This is verified by the condition stated
+ above. We only need to test it for registers that are set in
+ the moved region.
+
+ MERGE_LIVE is provided by the caller and holds live registers after
+ TO. */
+ bitmap_copy (local_merge_live, merge_live);
+ for (insn = to; insn != max_to; insn = PREV_INSN (insn))
+ df_simulate_one_insn_backwards (merge_bb, insn, local_merge_live);
+
+ /* We're not interested in registers that aren't set in the moved
+ region at all. */
+ bitmap_and_into (local_merge_live, merge_set);
+ for (;;)
+ {
+ if (NONDEBUG_INSN_P (insn))
+ {
+ if (!bitmap_intersect_p (test_set, local_merge_live)
+#ifdef HAVE_cc0
+ && !sets_cc0_p (insn)
+#endif
+ )
+ {
+ max_to = insn;
+ break;
+ }
+
+ df_simulate_one_insn_backwards (merge_bb, insn,
+ local_merge_live);
+ }
+ if (insn == from)
+ {
+ fail = 1;
+ goto out;
+ }
+ insn = PREV_INSN (insn);
+ }
+
+ if (max_to != to)
+ fail = 1;
+
+ if (pmove_upto)
+ *pmove_upto = max_to;
+
+ /* For small register class machines, don't lengthen lifetimes of
+ hard registers before reload. */
+ if (! reload_completed
+ && targetm.small_register_classes_for_mode_p (VOIDmode))
+ {
+ EXECUTE_IF_SET_IN_BITMAP (merge_set, 0, i, bi)
+ {
+ if (i < FIRST_PSEUDO_REGISTER
+ && ! fixed_regs[i]
+ && ! global_regs[i])
+ fail = 1;
+ }
+ }
+
+ out:
+ BITMAP_FREE (merge_set);
+ BITMAP_FREE (merge_use);
+ BITMAP_FREE (local_merge_live);
+ BITMAP_FREE (test_set);
+ BITMAP_FREE (test_use);
+
+ return !fail;
+}
+
+\f
+/*----------------------------------------------------------------------------
+ MULTIPLE DEFINITIONS
+
+ Find the locations in the function reached by multiple definition sites
+ for a live pseudo. In and out bitvectors are built for each basic
+ block. They are restricted for efficiency to live registers.
+
+ The gen and kill sets for the problem are obvious. Together they
+ include all defined registers in a basic block; the gen set includes
+ registers where a partial or conditional or may-clobber definition is
+ last in the BB, while the kill set includes registers with a complete
+ definition coming last. However, the computation of the dataflow
+ itself is interesting.
+
+ The idea behind it comes from SSA form's iterated dominance frontier
+ criterion for inserting PHI functions. Just like in that case, we can use
+ the dominance frontier to find places where multiple definitions meet;
+ a register X defined in a basic block BB1 has multiple definitions in
+ basic blocks in BB1's dominance frontier.
+
+ So, the in-set of a basic block BB2 is not just the union of the
+ out-sets of BB2's predecessors, but includes some more bits that come
+ from the basic blocks of whose dominance frontier BB2 is part (BB1 in
+ the previous paragraph). I called this set the init-set of BB2.
+
+ (Note: I actually use the kill-set only to build the init-set.
+ gen bits are anyway propagated from BB1 to BB2 by dataflow).
+
+ For example, if you have
+
+ BB1 : r10 = 0
+ r11 = 0
+ if <...> goto BB2 else goto BB3;
+
+ BB2 : r10 = 1
+ r12 = 1
+ goto BB3;
+
+ BB3 :