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).
v4: Fixed all the stupid mistakes you noticed. Also, the previous
version stopped looking when the previous try didn't work out. This
version doesn't: it is simpler, more in line with the rest of the
algorithm, potentially useful, and doesn't really cost more.
Tested on the two testcases from the PRs. Also regression checked
on powerpc64-linux.
Is this okay for trunk?
Segher
2015-12-09 Segher Boessenkool <seg...@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.
---
gcc/shrink-wrap.c | 42 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 37 insertions(+), 5 deletions(-)
---
gcc/shrink-wrap.c | 40 +++++++++++++++++++++++++++++++++++-----
1 file changed, 35 insertions(+), 5 deletions(-)
diff --git a/gcc/shrink-wrap.c b/gcc/shrink-wrap.c
index 3a1df84..f65b0c3 100644
--- a/gcc/shrink-wrap.c
+++ b/gcc/shrink-wrap.c
@@ -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);
--
1.9.3
