Re: Possible regression in DF analysis

[Claudiu Zissulescu Ianculescu <claziss@gmail.com>]

UPDATE:
The df_analyze_loop is calling the df_set_blocks. Thus, the analysis
behaves as if the function only contains those blocks and any edges
that occur directly between the blocks in the set (see df-core.cc).
This said, the loop-doloop behaves faulty at loop-doloop.cc:772 as the
df_get_lives_out (loop_end) is not computed correctly.

A possible solution is to include in the blocks_to_analyze the missing
basic blocks, something like:

diff --git a/gcc/df-core.cc b/gcc/df-core.cc
index a901b84878f..d7059c188b2 100644
--- a/gcc/df-core.cc
+++ b/gcc/df-core.cc
@@ -1437,7 +1437,15 @@ df_analyze_loop (class loop *loop)
   df_set_blocks (blocks);
   BITMAP_FREE (blocks);

-  df_analyze_1 ();
+  /* Add the loop's header successor bbs too.  */
+  edge e;
+  edge_iterator ei;
+  FOR_EACH_EDGE (e, ei, loop->header->succs)
+    bitmap_set_bit (df->blocks_to_analyze, e->dest->index);
+
+  if (dump_file)
+    debug_bitmap_file (dump_file, df->blocks_to_analyze);
+  df_analyze ();
 }

What do you think,
Claudiu

On Tue, Dec 13, 2022 at 2:30 PM Claudiu Zissulescu Ianculescu
<claziss@gmail.com> wrote:
>
> It looks like that. The df_analyze_loop is only looking at the loop
> BBs, and it is not clear for me if df_analyze_loop is required to have
> all the df_live_outs correctly computed or not. Do you know if it is
> true?
>
> If the df_analyze_loop is not supposed to compute all the df_live_outs
> correctly, then the error resides in how loop-doloop is using the
> iv_analysis_loop_init().
>
> Thank you for your help,
> Claudiu
>
> On Tue, Dec 13, 2022 at 10:41 AM Eric Botcazou <botcazou@adacore.com> wrote:
> >
> > > The problem shows in loop-doloop.c when I introduce a loop end pattern
> > > that replaces the first jump instruction (JUMP_INSN 15) with a pattern
> > > that clobbers CC reg. However, the DF doesn't look like it works as
> > > the doloop step cannot find the CC reg alive. Please see
> > > loop-doloop.c:766. Hence, it introduces the doloop_end patterns, and
> > > renders the compare instruction (INSN 14) dead code. leading to
> > > errors.
> >
> > So df_get_live_out does not contain the CC register?  iv_analysis_loop_init
> > only performs a local update of the DF information, maybe it does not cover
> > the basic block containing insn 14 and 15?
> >
> > --
> > Eric Botcazou
> >
> >