[Bug rtl-optimization/46315] [4.5 Regression] gcc-4.5.1 -O2 -fno-strict-overflow causes bad code generation

["ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46315

--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2010-11-14 11:34:42 UTC ---
The problem is indeed the note, but I don't think it comes from it being sort
of self-referential.  The use of reg 83 in the note doesn't make it live so ce1
merges BB 4 just before insn 75:


(note 19 18 22 4 [bb 4] NOTE_INSN_BASIC_BLOCK)

(insn 22 19 23 4 pr43615.c:10 (set (reg:SI 86)
        (sign_extend:SI (mem:QI (plus:SI (reg/v/f:SI 83 [ s ])
                    (const_int 1 [0x1])) [0 S1 A8]))) 96 {extendqisi2}
(expr_list:REG_DEAD (reg/v/f:SI 73 [ s ])
        (nil)))

(insn 23 22 24 4 pr43615.c:10 (parallel [
            (set (reg/v:SI 81 [ d ])
                (plus:SI (reg:SI 86)
                    (const_int -48 [0xffffffd0])))
            (clobber (reg:CC 17 flags))
        ]) 197 {*addsi_1} (expr_list:REG_DEAD (reg:SI 86)
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

(insn 24 23 25 4 pr43615.c:11 (set (reg:CC 17 flags)
        (compare:CC (reg/v:SI 81 [ d ])
            (const_int 9 [0x9]))) 6 {*cmpsi_1} (nil))

(jump_insn 25 24 26 4 pr43615.c:11 (set (pc)
        (if_then_else (gtu (reg:CC 17 flags)
                (const_int 0 [0x0]))
            (label_ref:SI 68)
            (pc))) 464 {*jcc_1} (expr_list:REG_DEAD (reg:CC 17 flags)
        (expr_list:REG_BR_PROB (const_int 450 [0x1c2])
            (nil)))
 -> 68)

(note 26 25 75 5 [bb 5] NOTE_INSN_BASIC_BLOCK)

(insn 75 26 29 5 pr43615.c:10 (set (reg/v/f:SI 83 [ s ])
        (reg/f:SI 95 [ s ])) -1 (expr_list:REG_EQUAL (plus:SI (reg/v/f:SI 83 [
s ])
            (const_int 2 [0x2]))
        (nil)))


with BB 11:


(note 67 68 5 11 [bb 11] NOTE_INSN_BASIC_BLOCK)

(insn 5 67 72 11 pr43615.c:12 (set (reg/v/f:SI 83 [ s ])
        (const_int 0 [0x0])) 44 {*movsi_1} (nil))


yielding:


(note 19 18 22 4 [bb 4] NOTE_INSN_BASIC_BLOCK)

(insn 22 19 23 4 pr43615.c:10 (set (reg:SI 86)
        (sign_extend:SI (mem:QI (plus:SI (reg/v/f:SI 83 [ s ])
                    (const_int 1 [0x1])) [0 S1 A8]))) 96 {extendqisi2}
(expr_list:REG_DEAD (reg/v/f:SI 73 [ s ])
        (nil)))

(insn 23 22 5 4 pr43615.c:10 (parallel [
            (set (reg/v:SI 81 [ d ])
                (plus:SI (reg:SI 86)
                    (const_int -48 [0xffffffd0])))
            (clobber (reg:CC 17 flags))
        ]) 197 {*addsi_1} (expr_list:REG_DEAD (reg:SI 86)
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

(insn 5 23 24 4 pr43615.c:12 (set (reg/v/f:SI 83 [ s ])
        (const_int 0 [0x0])) 44 {*movsi_1} (nil))

(insn 24 5 25 4 pr43615.c:11 (set (reg:CC 17 flags)
        (compare:CC (reg/v:SI 81 [ d ])
            (const_int 9 [0x9]))) 6 {*cmpsi_1} (nil))

(jump_insn 25 24 26 4 pr43615.c:11 (set (pc)
        (if_then_else (gtu (reg:CC 17 flags)
                (const_int 0 [0x0]))
            (label_ref:SI 46)
            (pc))) 464 {*jcc_1} (expr_list:REG_DEAD (reg:CC 17 flags)
        (expr_list:REG_BR_PROB (const_int 450 [0x1c2])
            (nil)))
 -> 46)

(insn 75 26 29 5 pr43615.c:10 (set (reg/v/f:SI 83 [ s ])
        (reg/f:SI 95 [ s ])) -1 (expr_list:REG_EQUAL (plus:SI (reg/v/f:SI 83 [
s ])
            (const_int 2 [0x2]))
        (nil)))


and insn 5 now feeds the note.