[PATCH, PR81192] Fix sigsegv in find_same_succ_bb

[Tom de Vries]

[ Trying again with before.svg instead of before.pdf ]
Hi,

consider this test-case:
...
unsigned a;
int b, c;

static int
fn1 (int p1, int p2)
{
  return p1 > 2147483647 - p2 ? p1 : p1 + p2;
}

void
fn2 (void)
{
  int j;
  a = 30;
  for (; a;)
    for (; c; b = fn1 (j, 1))
      ;
}
...

When compiling the test-case with -Os, just before tail-merge it looks 
as in before.svg.

During tail-merge, it runs into a sigsegv.

What happens is the following:
- tail-merge decides to merge blocks 4 and 6, and removes block 6.
- bb8, a predecessor of block 6, is marked as member of
  deleted_bb_preds.
- during update_worklist, same_succ_flush_bb is called for bb8
- same_succ_flush_bb runs into a sigsegv because
  BB_SAME_SUCC (bb8) == NULL
- the reason that BB_SAME_SUCC (bb8) == NULL, is because it hit the
  bb->loop_father->latch == bb clause in find_same_succ_bb at the start
  of the tail-merge pass.

This patch fixes the sigsegv by doing an early-out in same_succ_flush_bb 
if BB_SAME_SUCC () == NULL.

Bootstrapped and reg-tested on x86_64.

OK for trunk and gcc-[567]-branch?

Thanks,
- Tom
Fix sigsegv in find_same_succ_bb

2017-06-30  Tom de Vries  <t...@codesourcery.com>

	PR tree-optimization/81192
	* tree-ssa-tail-merge.c (same_succ_flush_bb): Handle
	BB_SAME_SUCC (bb) == NULL.

	* gcc.dg/pr81192.c: New test.

---
 gcc/testsuite/gcc.dg/pr81192.c | 22 ++++++++++++++++++++++
 gcc/tree-ssa-tail-merge.c      |  3 +++
 2 files changed, 25 insertions(+)

diff --git a/gcc/testsuite/gcc.dg/pr81192.c b/gcc/testsuite/gcc.dg/pr81192.c
new file mode 100644
index 0000000..57eb478
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr81192.c
@@ -0,0 +1,22 @@
+/* { dg-options "-Os -fdump-tree-pre-details" } */
+
+unsigned a;
+int b, c;
+
+static int
+fn1 (int p1, int p2)
+{
+  return p1 > 2147483647 - p2 ? p1 : p1 + p2;
+}
+
+void
+fn2 (void)
+{
+  int j;
+  a = 30;
+  for (; a;)
+    for (; c; b = fn1 (j, 1))
+      ;
+}
+
+/* { dg-final { scan-tree-dump-times "(?n)find_duplicates: <bb .*> duplicate of <bb .*>" 1 "pre" } } */
diff --git a/gcc/tree-ssa-tail-merge.c b/gcc/tree-ssa-tail-merge.c
index f6c9878..bb8a308 100644
--- a/gcc/tree-ssa-tail-merge.c
+++ b/gcc/tree-ssa-tail-merge.c
@@ -809,6 +809,9 @@ static void
 same_succ_flush_bb (basic_block bb)
 {
   same_succ *same = BB_SAME_SUCC (bb);
+  if (! same)
+    return;
+
   BB_SAME_SUCC (bb) = NULL;
   if (bitmap_single_bit_set_p (same->bbs))
     same_succ_htab->remove_elt_with_hash (same, same->hashval);