identifying a BB representing a self-loop

["sean yang" <seanatpurdue at hotmail dot com>]

Some basic blocks may represent a (self) loop, but GCC's internal basic 
block representation won't show such information explicitly (i.e., it won't 
store a self-loop edge).
My question is, when I walk through basic blocks, can I identify then 
easily?

E.g., Let's say,
----------demo.c------------------------------------
int main(){
       int i;
       int sum = 0;
       for (i=0; i< 10; i++){
               sum = sum+i;
       }
}
-----------------------------------------------------
If we compile this by "#gcc -c -O -da demo.c, we can see there are only 
three BBs. Actually, BB1 is a self-looped basic block. But this loop 
information is not explicitly expressed.

BB0-->BB1-->BB2 (see details in the attached .cfg file)
---------------------cat demo.c.08.cfg---------------------------------
;; Function main

70 registers.

Register 59 used 2 times across 0 insns; set 2 times; user var; dies in 0 
places.

Register 60 used 1 times across 0 insns; set 1 time; dies in 0 places.

3 basic blocks, 5 edges.

Basic block 0 prev -1, next 1, loop_depth 0, count 0, freq 1000, maybe hot.
Predecessors:  ENTRY (fallthru)
Successors:  1 (fallthru)

Basic block 1 prev 0, next 2, loop_depth 1, count 0, freq 10000, maybe hot.
Predecessors:  0 (fallthru) 1
Successors:  1 2 (fallthru)

Basic block 2 prev 1, next -2, loop_depth 0, count 0, freq 1000, maybe hot.
Predecessors:  1 (fallthru)
Successors:  EXIT (fallthru)



try_optimize_cfg iteration 1

(note 2 0 20 NOTE_INSN_DELETED)

;; Start of basic block 0, registers live: (nil)
(note 20 2 9 0 [bb 0] NOTE_INSN_BASIC_BLOCK)

(insn 9 20 15 0 (parallel [
           (set (reg/f:SI 7 sp)
               (and:SI (reg/f:SI 7 sp)
                   (const_int -16 [0xfffffff0])))
           (clobber (reg:CC 17 flags))
       ]) 200 {*andsi_1} (nil)
   (nil))

(insn 15 9 7 0 (parallel [
           (set (reg/f:SI 7 sp)
               (plus:SI (reg/f:SI 7 sp)
                   (const_int -16 [0xfffffff0])))
           (clobber (reg:CC 17 flags))
       ]) 140 {*addsi_1} (nil)
   (nil))

(note 7 15 22 0 NOTE_INSN_FUNCTION_BEG)

(insn 22 7 44 0 (set (reg/v:SI 59 [ i ])
       (const_int 0 [0x0])) 35 {*movsi_1} (nil)
   (nil))
;; End of basic block 0, registers live:
(nil)

(note 44 22 23 NOTE_INSN_LOOP_BEG)

;; Start of basic block 1, registers live: (nil)
(code_label 23 44 24 1 2 "" [1 uses])

(note 24 23 26 1 [bb 1] NOTE_INSN_BASIC_BLOCK)

(insn 26 24 27 1 (parallel [
           (set (reg/v:SI 59 [ i ])
               (plus:SI (reg/v:SI 59 [ i ])
                   (const_int 1 [0x1])))
           (clobber (reg:CC 17 flags))
       ]) -1 (nil)
   (nil))

(insn 27 26 28 1 (set (reg:CCZ 17 flags)
       (compare:CCZ (reg/v:SI 59 [ i ])
           (const_int 10 [0xa]))) -1 (nil)
   (nil))

(jump_insn 28 27 45 1 (set (pc)
       (if_then_else (ne (reg:CCZ 17 flags)
               (const_int 0 [0x0]))
           (label_ref 23)
           (pc))) -1 (nil)
   (expr_list:REG_BR_PROB (const_int 9000 [0x2328])
       (nil)))
;; End of basic block 1, registers live:
(nil)

(note 45 28 31 NOTE_INSN_LOOP_END)

(note 31 45 41 NOTE_INSN_FUNCTION_END)

;; Start of basic block 2, registers live: (nil)
(note 41 31 35 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

(insn 35 41 36 2 (clobber (reg/i:SI 0 ax)) -1 (nil)
   (nil))

(insn 36 35 40 2 (clobber (reg:SI 60 [ <result> ])) -1 (nil)
   (nil))

(insn 40 36 0 2 (use (reg/i:SI 0 ax)) -1 (nil)
   (nil))
;; End of basic block 2, registers live:
(nil)
----------------------------------------------------------------

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