Re: loop optimizer naivety

[Eric Botcazou <ebotcazou at libertysurf dot fr>]

[Thanks for this answer]

> Is there any path through the loop where insns 810 and 812 could both
> execute?  ie, can you get from insn 810 to 812 or vice versa without
> going back to the top of the loop?

Yes, they belong to the same basic block (otherwise the loop optimizer 
wouldn't have made its mistake).

The complete sequence is:

(note 604 568 609 4 NOTE_INSN_LOOP_CONT)

(insn 609 604 810 4 0x40a5565c (parallel [
            (set (reg:SI 173)
                (plus:SI (reg:SI 185)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) 146 {*addsi_1} (nil)
    (nil))

(insn 810 609 610 4 (nil) (set (reg:SI 185)
        (reg:SI 173)) 38 {*movsi_1} (nil)
    (nil))

(insn 610 810 765 4 0x40a5565c (set (mem/f:SI (plus:SI (reg/f:SI 20 frame)
                (const_int -100 [0xffffff9c])) [5 S4 A32])
        (reg:SI 173)) 38 {*movsi_1} (nil)
    (expr_list:REG_EQUAL (plus:SI (reg:SI 161)
            (const_int 1 [0x1]))
        (nil)))

(note 765 610 812 4 NOTE_INSN_LOOP_VTOP)

(insn 812 765 103 4 (nil) (set (reg:SI 185)
        (reg:SI 173)) 38 {*movsi_1} (nil)
    (nil))


insn 812 is dead (reg 185 is not consumed) but delete_trivially_dead_insns() 
deletes only _really_ trivially dead insns :-)


Before the GCSE pass, the sequence is:

(insn 609 604 610 3 0x40a5565c (parallel [
            (set (reg:SI 173)
                (plus:SI (reg:SI 161)
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) 146 {*addsi_1} (nil)
    (nil))

(insn 610 609 765 3 0x40a5565c (set (mem/f:SI (plus:SI (reg/f:SI 20 frame)
                (const_int -100 [0xffffff9c])) [5 S4 A32])
        (reg:SI 173)) 38 {*movsi_1} (nil)
    (expr_list:REG_EQUAL (plus:SI (reg:SI 161)
            (const_int 1 [0x1]))
        (nil)))

(note 765 610 102 3 NOTE_INSN_LOOP_VTOP)

(insn 102 765 103 3 0x40a5565c (set (reg/s:SI 89)
        (mem/f:SI (plus:SI (reg/f:SI 20 frame)
                (const_int -100 [0xffffff9c])) [5 S4 A32])) 38 {*movsi_1} 
(nil)
    (nil))


and reg 89 is consumed.


> If so, then PRE/LCM has mucked things up.

Then I should fix GCSE and not the loop optimizer?

-- 
Eric Botcazou