[RFC] Confusing fall through BB pc in conditional jump

["Kewen.Lin" <linkw at linux dot ibm dot com>]

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 seq 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.
Or is it unnecessary? since we have "-blocks" options for that and want to keep it "slim"?

Any comments on that? 

Thanks in advance!


-------------------------------

The below patch is to append the hint on pc target when we find jump insn with if-then-else
and the target BB with known code label.

The dumping will look like:
    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 (fall through) -> L67
   31: L31:
   17: NOTE_INSN_BASIC_BLOCK 3


diff --git a/gcc/cfgrtl.c b/gcc/cfgrtl.c
index a1ca5992c41..608bcd130c5 100644
--- a/gcc/cfgrtl.c
+++ b/gcc/cfgrtl.c
@@ -2164,7 +2164,27 @@ rtl_dump_bb (FILE *outf, basic_block bb, int indent, dump_flags_t flags)
     }

 }
-^L
+
+/* 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.  This function is to dump some hints
+   for that.  */
+
+static void
+dump_hints_for_jump_target_pc (FILE *outf, basic_block bb)
+{
+  gcc_assert (outf);
+  edge e = FALLTHRU_EDGE (bb);
+  basic_block dest = e->dest;
+  rtx_insn *label = BB_HEAD (dest);
+  if (!LABEL_P (label))
+    return;
+
+  fputs (";; ", outf);
+  fprintf (outf, " pc (fall through) -> L%d", INSN_UID (label));
+  fputc ('\n', outf);
+}
+
 /* 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 +2275,14 @@ 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)
+               dump_hints_for_jump_target_pc (outf, bb);
+           }
        }

       free (start);