var-tracking: fix uninitialised use of 'in_pending' [PR96404]

r11-2447-g:1212cfad093 ("Improve var-tracking dataflow
iteration order") changed 'in_pending' initialization
from:

    in_pending = sbitmap_alloc (last_basic_block_for_fn (cfun));
    bitmap_ones (in_pending);

to more complex partial bit population algorithm. Due to presence
of uninitialized bits gcc started injecting extra debug entries
in seemigly arbitrary locations and started failing stage2/stage3
bootstrap comparison.

valgrind detected unilitialized bits as:

  Conditional jump or move depends on uninitialised value(s)
     at 0xDBED3B: vt_find_locations() (var-tracking.c:7230)
     by 0xDBF2FB: variable_tracking_main_1() (var-tracking.c:10519)
     ...
   Uninitialised value was created by a heap allocation
     at 0x483779F: malloc (vg_replace_malloc.c:307)
     by 0x14EE80B: xmalloc (xmalloc.c:147)
     by 0x14911F9: sbitmap_alloc(unsigned int) (sbitmap.c:51)
     ...

The fix explicitly initializes 'in_pending' bitmap with zeros.

2020-08-02  Sergei Trofimovich  <siarheit@google.com>

gcc/

	PR bootstrap/96404
	* var-tracking.c (vt_find_locations): Fully initialize
	all 'in_pending' bits.

diff --git a/gcc/var-tracking.c b/gcc/var-tracking.c
index 743f5dcecf68246936b992a73af9bbb97f3c9bcb..52aea47a053bdc4515498c047ba68d0ce8faa89f 100644 (file)
--- a/gcc/var-tracking.c
+++ b/gcc/var-tracking.c
@@ -7096,6 +7096,7 @@ vt_find_locations (void)
   in_worklist = sbitmap_alloc (last_basic_block_for_fn (cfun));
   in_pending = sbitmap_alloc (last_basic_block_for_fn (cfun));
   bitmap_clear (in_worklist);
+  bitmap_clear (in_pending);
 
   /* We're performing the dataflow iteration independently over the
      toplevel SCCs plus leading non-cyclic entry blocks and separately