[Bug target/27971] eliminate shift in array[(x>>2)&3]

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

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-06-09 06:20 -------
Here is the RTL we get on PPC:
(insn 6 9 7 2 (set (reg/v:SI 120 [ x ])
        (reg:SI 3 r3 [ x ])) 327 {*movsi_internal1} (nil)
    (expr_list:REG_DEAD (reg:SI 3 r3 [ x ])
        (expr_list:REG_EQUIV (mem/c/i:SI (plus:SI (reg/f:SI 67 ap)
                    (const_int 24 [0x18])) [4 x+0 S4 A32])
            (nil))))
...

(insn 13 12 14 2 (set (reg:SI 124)
        (lshiftrt:SI (reg/v:SI 120 [ x ])
            (const_int 2 [0x2]))) 195 {lshrsi3_no_power}
(insn_list:REG_DEP_TRUE 6 (nil))
    (expr_list:REG_DEAD (reg/v:SI 120 [ x ])
        (nil)))

(insn 14 13 15 2 (parallel [
            (set (reg:SI 125)
                (and:SI (reg:SI 124)
                    (const_int 3 [0x3])))
            (clobber (scratch:CC))
        ]) 146 {andsi3} (insn_list:REG_DEP_TRUE 13 (nil))
    (expr_list:REG_DEAD (reg:SI 124)
        (expr_list:REG_UNUSED (scratch:CC)
            (nil))))

(insn 15 14 17 2 (set (reg:SI 126)
        (ashift:SI (reg:SI 125)
            (const_int 2 [0x2]))) 189 {ashlsi3_no_power}
(insn_list:REG_DEP_TRUE 14 (nil))
    (expr_list:REG_DEAD (reg:SI 125)
        (nil)))

Which gets reduced to:
(insn 15 14 17 2 (parallel [
            (set (reg:SI 126)
                (and:SI (reg:SI 3 r3 [ x ])
                    (const_int 12 [0xc])))
            (clobber (scratch:CC))
        ]) 146 {andsi3} (nil)
    (expr_list:REG_UNUSED (scratch:CC)
        (expr_list:REG_DEAD (reg:SI 3 r3 [ x ])
            (nil))))


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27971