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. The main difference is that if you go forwards, the uses
- are examined first then the defs, and if you go backwards, the defs
- are examined first then the uses.
+ 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_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 on. Finally,
+ 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).
----------------------------------------------------------------------------*/
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 last on. Finally,
- df_simulate_finalize_forwards can be called to get a new value
- of the sets at the bottom of the block (this is rarely used).
+ the block, starting with the first one.
----------------------------------------------------------------------------*/
-/* Apply the artificial uses and defs at the top of BB in a backwards
- direction. */
+/* Apply the artificial uses and defs at the top of BB in a forwards
+ direction. ??? This is wrong; defs mark the point where a pseudo
+ becomes live when scanning forwards (unless a def is unused). Since
+ there are no REG_UNUSED notes for artificial defs, passes that
+ require artificial defs probably should not call this function
+ unless (as is the case for fwprop) they are correct when liveness
+ bitmaps are *under*estimated. */
void
df_simulate_initialize_forwards (basic_block bb, bitmap live)
}
}
-/* Simulate the backwards effects of INSN on the bitmap LIVE. */
+/* Simulate the forwards effects of INSN on the bitmap LIVE. */
void
df_simulate_one_insn_forwards (basic_block bb, rtx insn, bitmap live)
if (! INSN_P (insn))
return;
- /* Make sure that the DF_NOTES really is an active df problem. */
+ /* Make sure that DF_NOTE really is an active df problem. */
gcc_assert (df_note);
- df_simulate_defs (insn, live);
+ /* 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_defs (insn, live);
/* Clear all of the registers that go dead. */
for (link = REG_NOTES (insn); link; link = XEXP (link, 1))
}
-/* Apply the artificial uses and defs at the end of BB in a backwards
- direction. */
-
-void
-df_simulate_finalize_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) == 0)
- bitmap_clear_bit (live, DF_REF_REGNO (def));
- }
-}
-
-
\f
/*----------------------------------------------------------------------------
MULTIPLE DEFINITIONS
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