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. This is a bit hard / expensive to compute, so instead this patch
allows a block PRE only if PRE does not post-dominate any of its
successors (other than itself).
Tested on the two testcases from the PRs. Also regression checked
on powerpc64-linux.
Is this okay for trunk?
Segher
2015-12-02 Segher Boessenkool <seg...@kernel.crashing.org>
* shrink-wrap.c (try_shrink_wrapping): Disallow moving the prologue
back into a loop.
---
gcc/shrink-wrap.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/gcc/shrink-wrap.c b/gcc/shrink-wrap.c
index 3a1df84..fe33652 100644
--- a/gcc/shrink-wrap.c
+++ b/gcc/shrink-wrap.c
@@ -752,7 +752,11 @@ try_shrink_wrapping (edge *entry_edge, bitmap_head
*bb_with,
/* If we can move PRO back without having to duplicate more blocks, do so.
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. We might
+ need to duplicate PRE if there is any path from a successor of PRE back
+ to PRE, so don't allow that either (but self-loops are fine, as are any
+ other loops entirely dominated by PRE; this in general seems too
+ expensive to check for, for such an uncommon case). */
if (pro != entry)
{
@@ -765,9 +769,20 @@ try_shrink_wrapping (edge *entry_edge, bitmap_head
*bb_with,
if (!dominated_by_p (CDI_POST_DOMINATORS, pre, pro))
break;
+ bool ok = true;
+
+ if (!can_get_prologue (pre, prologue_clobbered))
+ ok = false;
+
+ FOR_EACH_EDGE (e, ei, pre->succs)
+ if (e->dest != pre
+ && dominated_by_p (CDI_POST_DOMINATORS, e->dest, pre))
+ ok = false;
+
+ if (ok)
+ last_ok = pre;
+
pro = pre;
- if (can_get_prologue (pro, prologue_clobbered))
- last_ok = pro;
}
pro = last_ok;
--
1.9.3
