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Fix shrink-wrapping bug (PR67778, PR68634)

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After shrink-wrapping has found the "tightest fit" for where to place
the prologue, it tries move it earlier (so that frame saves are run
earlier) -- but without copying any more basic blocks.

Unfortunately a candidate block we select can be inside a loop, and we
will still allow it (because the loop always exits via our previously
chosen block).  We can do that just fine if we make a duplicate of the
block, but we do not want to here.

So we need to detect this situation.  We can place the prologue at a
previous block PRE only if PRE dominates every block reachable from
it, because then we will never need to duplicate that block (it will
always be executed with prologue).


2015-12-11  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/67778
	PR rtl-optimization/68634
	* shrink-wrap.c (try_shrink_wrapping): Add a comment about why we want
	to put the prologue earlier.  When determining if an earlier block is
	suitable, make sure it dominates every block reachable from it.

From-SVN: r231552
This commit is contained in:
Segher Boessenkool 2015-12-11 01:27:14 +01:00 committed by Segher Boessenkool
parent 9c7163d0bf
commit 5d59ed6332
2 changed files with 43 additions and 5 deletions
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8
gcc/ChangeLog
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@ -1,3 +1,11 @@
2015-12-11 Segher Boessenkool <segher@kernel.crashing.org>
PR rtl-optimization/67778
PR rtl-optimization/68634
* shrink-wrap.c (try_shrink_wrapping): Add a comment about why we want
to put the prologue earlier. When determining if an earlier block is
suitable, make sure it dominates every block reachable from it.
2015-12-11 Segher Boessenkool <segher@kernel.crashing.org>
PR rtl-optimization/68814

40
gcc/shrink-wrap.c
View File

@ -744,36 +744,66 @@ try_shrink_wrapping (edge *entry_edge, bitmap_head *bb_with,
vec.quick_push (e->dest);
}
vec.release ();
if (dump_file)
fprintf (dump_file, "Avoiding non-duplicatable blocks, PRO is now %d\n",
pro->index);
/* If we can move PRO back without having to duplicate more blocks, do so.
We do this because putting the prologue earlier is better for scheduling.
We can move back to a block PRE if every path from PRE will eventually
need a prologue, that is, PRO is a post-dominator of PRE. */
need a prologue, that is, PRO is a post-dominator of PRE. PRE needs
to dominate every block reachable from itself. */
if (pro != entry)
{
calculate_dominance_info (CDI_POST_DOMINATORS);
bitmap bb_tmp = BITMAP_ALLOC (NULL);
bitmap_copy (bb_tmp, bb_with);
basic_block last_ok = pro;
vec.truncate (0);
while (pro != entry)
{
basic_block pre = get_immediate_dominator (CDI_DOMINATORS, pro);
if (!dominated_by_p (CDI_POST_DOMINATORS, pre, pro))
break;
if (bitmap_set_bit (bb_tmp, pre->index))
vec.quick_push (pre);
bool ok = true;
while (!vec.is_empty ())
{
basic_block bb = vec.pop ();
bitmap_set_bit (bb_tmp, pre->index);
if (!dominated_by_p (CDI_DOMINATORS, bb, pre))
{
ok = false;
break;
}
FOR_EACH_EDGE (e, ei, bb->succs)
if (!bitmap_bit_p (bb_with, e->dest->index)
&& bitmap_set_bit (bb_tmp, e->dest->index))
vec.quick_push (e->dest);
}
if (ok && can_get_prologue (pre, prologue_clobbered))
last_ok = pre;
pro = pre;
if (can_get_prologue (pro, prologue_clobbered))
last_ok = pro;
}
pro = last_ok;
BITMAP_FREE (bb_tmp);
free_dominance_info (CDI_POST_DOMINATORS);
}
vec.release ();
if (dump_file)
fprintf (dump_file, "Bumping back to anticipatable blocks, PRO is now %d\n",
pro->index);