[Bug tree-optimization/42889] "-fcompare-debug failure (length)" with "-O1 -fgcse"

["steven at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #3 from steven at gcc dot gnu dot org  2010-01-28 09:40 -------
combine fails to combine the following insns.

Before combine (.init_regs dump):

(insn 20 19 21 4 PR42889.c:7 (parallel [
            (set (reg:SI 70)
                (and:SI (reg/v:SI 64 [ c ])
                    (const_int 4 [0x4])))
            (clobber (reg:CC 17 flags))
        ]) 375 {*andsi_1} (expr_list:REG_DEAD (reg/v:SI 64 [ c ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

(insn 21 20 48 4 PR42889.c:7 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:SI 70)
            (const_int 0 [0x0]))) 2 {*cmpsi_ccno_1} (expr_list:REG_UNUSED
(reg:CCZ 17 flags)
        (nil)))


After combine (.combine dump):
(note 20 19 21 4 NOTE_INSN_DELETED)

(insn 21 20 48 4 PR42889.c:7 (parallel [
            (set (reg:CCZ 17 flags)
                (compare:CCZ (and:SI (reg/v:SI 64 [ c ])
                        (const_int 4 [0x4]))
                    (const_int 0 [0x0])))
            (set (reg:SI 70)
                (and:SI (reg/v:SI 64 [ c ])
                    (const_int 4 [0x4])))
        ]) 384 {*andsi_2} (expr_list:REG_DEAD (reg/v:SI 64 [ c ])
        (expr_list:REG_UNUSED (reg:CCZ 17 flags)
            (nil))))




When compiled with -g that combine doesn't happen because the compare insn has
been changed somewhere before combine.

Before combine:
(insn 20 19 50 4 PR42889.c:7 (parallel [
            (set (reg:SI 70)
                (and:SI (reg/v:SI 64 [ c ])
                    (const_int 4 [0x4])))
            (clobber (reg:CC 17 flags))
        ]) 375 {*andsi_1} (expr_list:REG_DEAD (reg/v:SI 64 [ c ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

(insn 50 20 51 4 PR42889.c:7 (set (reg:CC 17 flags)
        (compare:CC (reg:SI 70)
            (const_int 1 [0x1]))) 6 {*cmpsi_1} (expr_list:REG_DEAD (reg:SI 70)
        (nil)))




The difference starts in the cprop3 pass:

+ DCE: Deleting insn 21
+ deleting insn with uid = 21.

         (expr_list:REG_UNUSED (reg:CC 17 flags)
             (nil))))

-(insn# # # 4 PR42889.c:7 (set (reg:CCZ 17 flags)
-        (compare:CCZ (reg:SI 70)
-            (const_int 0 [0x0])))# {*cmpsi_ccno_1} (expr_list:REG_DEAD (reg:SI
70)
-        (nil)))
-
 (insn# # # 4 PR42889.c:7 (set (reg:CC 17 flags)
         (compare:CC (reg:SI 70)
             (const_int 1 [0x1])))# {*cmpsi_1} (nil))


Interestingly: with -g0 "df_analyze called" happens, but there is no DF output
and it almost looks as if DF simply doesn't run (and therefore also DCE does
not run).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42889