[PR rtl-optimization/116420] Fix interesting block bitmap DF dataflow

The DF framework provides us a way to run dataflow problems on sub-graphs.
Naturally a bitmap of interesting blocks is passed into those routines.   At a
confluence point, the DF framework will not mark a block for re-processing if
it's not in that set of interesting blocks.

When ext-dce sets up that set of interesting blocks it's using the wrong
counter.  ie, it's using n_basic_blocks rather than last_basic_block.  If there
are holes in the block indices, some number of blocks won't get marked as
interesting.

In this case the block needing reprocessing has an index higher than
n_basic_blocks.  It never gets reprocessed and the newly found live chunks
don't propagate further up the CFG -- ultimately resulting in a pseudo
appearing to have only the low 8 bits live, when in fact the low 32 bits are
actually live.

Fixed in the obvious way, by using last_basic_block instead.

Bootstrapped and regression tested on x86_64.  Pushing to the trunk.

	PR rtl-optimization/116420
gcc/
	* ext-dce.cc (ext_dce_init): Fix loop iteration when setting up the
	interesting block for DF to analyze.

gcc/testsuite
	* gcc.dg/torture/pr116420.c: New test.

2 files changed, 18 insertions, 1 deletions

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 35c0646..4a2503f 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -988,7 +988,7 @@ ext_dce_init (void)
   all_blocks = BITMAP_ALLOC (NULL);
   changed_pseudos = BITMAP_ALLOC (NULL);
 
-  for (int i = 0; i < n_basic_blocks_for_fn (cfun); i++)
+  for (int i = 0; i < last_basic_block_for_fn (cfun); i++)
     if (i != ENTRY_BLOCK && i != EXIT_BLOCK)
       bitmap_set_bit (all_blocks, i);
 
diff --git a/gcc/testsuite/gcc.dg/torture/pr116420.c b/gcc/testsuite/gcc.dg/torture/pr116420.c
new file mode 100644
index 0000000..9a784f5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr116420.c
@@ -0,0 +1,17 @@
+/* { dg-do run } */
+/* { dg-additional-options "-fno-forward-propagate -fno-tree-ch" } */
+int a, d, e;
+char b = -1, c, f;
+int main() {
+  int g;
+  for (; d < 1; d++) {
+    g = b;
+    for (; c; c = g)
+      ;
+  }
+  f = g;
+  for (; e < 1; e++)
+    if (g >= a)
+      __builtin_abort();
+  return 0;
+}