[PATCH] Add hints for slim dumping if fallthrough bb of jump isn't next bb

[Kewen.Lin]

Hi all,

    6: NOTE_INSN_BASIC_BLOCK 2
        ....
   12: r135:CC=cmp(r122:DI,0)
   13: pc={(r135:CC!=0)?L52:pc}
      REG_DEAD r135:CC
      REG_BR_PROB 1041558836
   31: L31:
   17: NOTE_INSN_BASIC_BLOCK 3

The above RTL sequence is from pass doloop dumping with -fdump-rtl-all-slim, I
misunderstood that: the fall through BB of BB 2 is BB 3, since BB 3 is placed
just next to BB 2.  Then I found the contradiction that BB 3 will have some
uninitialized regs if it's true.

I can get the exact information with "-blocks" dumping and even detailed one
with "-details".  But I'm thinking whether it's worth to giving some
information on "-slim" dump (or more exactly without "-blocks") to avoid some
confusion especially for new comers like me.

This patch is to add one line to hint what's the fallthrough BB if it's the
one closely sitting, for example:

    6: NOTE_INSN_BASIC_BLOCK 2
....
   12: r135:CC=cmp(r122:DI,0)
   13: pc={(r135:CC!=0)?L52:pc}
      REG_DEAD r135:CC
      REG_BR_PROB 1041558836
;;  pc falls through to BB 10     
   31: L31:
   17: NOTE_INSN_BASIC_BLOCK 3

Bootstrapped and regression test passed on powerpc64le-unknown-linux-gnu.

Is it a reasonable patch? If yes, is it ok for trunk?


Thanks,
Kewen

---------

gcc/ChangeLog

2019-07-08  Kewen Lin  <li...@gcc.gnu.org>

        * gcc/cfgrtl.c (print_rtl_with_bb): Check and call 
        hint_if_pc_fall_through_not_next for jump insn with two successors.
        (hint_if_pc_fall_through_not_next): New function.

diff --git a/gcc/cfgrtl.c b/gcc/cfgrtl.c
index a1ca5992c41..928b9b0f691 100644
--- a/gcc/cfgrtl.c
+++ b/gcc/cfgrtl.c
@@ -2164,7 +2164,26 @@ rtl_dump_bb (FILE *outf, basic_block bb, int indent, 
dump_flags_t flags)
     }

 }
-
+
+/* For dumping without specifying basic blocks option, when we see PC is one of
+   jump targets, it's easy to misunderstand the next basic block is the
+   fallthrough one, but it's not so true sometimes.  This function is to dump
+   hints for the case where basic block of next insn isn't the fall through
+   target.  */
+
+static void
+hint_if_pc_fall_through_not_next (FILE *outf, basic_block bb)
+{
+  gcc_assert (outf);
+  gcc_assert (EDGE_COUNT (bb->succs) >= 2);
+  const rtx_insn *einsn = BB_END (bb);
+  const rtx_insn *ninsn = NEXT_INSN (einsn);
+  edge e = FALLTHRU_EDGE (bb);
+  basic_block dest = e->dest;
+  if (BB_HEAD (dest) != ninsn)
+    fprintf (outf, ";;  pc falls through to BB %d\n", dest->index);
+}
+
 /* Like dump_function_to_file, but for RTL.  Print out dataflow information
    for the start of each basic block.  FLAGS are the TDF_* masks documented
    in dumpfile.h.  */
@@ -2255,6 +2274,14 @@ print_rtl_with_bb (FILE *outf, const rtx_insn 
*rtx_first, dump_flags_t flags)
                  putc ('\n', outf);
                }
            }
+         else if (GET_CODE (tmp_rtx) == JUMP_INSN
+                  && GET_CODE (PATTERN (tmp_rtx)) == SET)
+           {
+             bb = BLOCK_FOR_INSN (tmp_rtx);
+             const_rtx src = SET_SRC (PATTERN (tmp_rtx));
+             if (bb != NULL && GET_CODE (src) == IF_THEN_ELSE)
+               hint_if_pc_fall_through_not_next (outf, bb);
+           }
        }

       free (start);