[Bug middle-end/19721] [meta-bug] optimizations that CSE still catches

["stevenb at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From stevenb at suse dot de  2005-03-05 10:39 -------
Subject: Re:  [meta-bug] optimizations that CSE still catches

> Am I missing something here?  I guess I'm not sure what point you're
> trying to make.

It just seems that we could do better on initial RTL generation, e.g.

;; j = k * -1
(insn 23 21 0 (parallel [
            (set (reg/v:DI 64 [ j ])
                (mult:DI (reg/v:DI 67 [ k ])
                    (const_int -1 [0xffffffffffffffff])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil)
    (nil))

which we later simplify in CSE:

Working on insn:
(insn 23 21 24 0 (parallel [
            (set (reg/v:DI 64 [ j ])
                (mult:DI (reg/v:DI 67 [ k ])
                    (const_int -1 [0xffffffffffffffff])))
            (clobber (reg:CC 17 flags))
        ]) 243 {*muldi3_1_rex64} (nil)
    (nil))
Trying to fold rtx:
(mult:DI (reg/v:DI 67 [ k ])
    (const_int -1 [0xffffffffffffffff]))
Returning new rtx:
(neg:DI (reg/v:DI 67 [ k ]))



Similarly, on a 64-bits host:

;; j = k * 4294967295
(insn 15 13 16 (set (reg:DI 63)
        (reg/v:DI 62 [ k ])) -1 (nil)
    (nil))

(insn 16 15 17 (parallel [
            (set (reg:DI 64)
                (ashift:DI (reg:DI 63)
                    (const_int 32 [0x20])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil)
    (expr_list:REG_EQUAL (mult:DI (reg/v:DI 62 [ k ])
            (const_int 4294967296 [0x100000000]))
        (nil)))

(insn 17 16 18 (parallel [
            (set (reg:DI 65)
                (minus:DI (reg:DI 64)
                    (reg/v:DI 62 [ k ])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil)
    (expr_list:REG_EQUAL (mult:DI (reg/v:DI 62 [ k ])
            (const_int 4294967295 [0xffffffff]))
        (nil)))

(insn 18 17 0 (set (reg/v:DI 59 [ j ])
        (reg:DI 65)) -1 (nil)
    (nil))

which CSE turns into:

Working on insn:
(insn 15 13 16 0 (set (reg:DI 63 [ k ])
        (reg/v:DI 62 [ k ])) 81 {*movdi_1_rex64} (nil)
    (nil))
Trying to fold rtx:
(reg/v:DI 62 [ k ])
Returning X unchanged.

Working on insn:
(insn 16 15 17 0 (parallel [
            (set (reg:DI 64)
                (ashift:DI (reg:DI 63 [ k ])
                    (const_int 32 [0x20])))
            (clobber (reg:CC 17 flags))
        ]) -1 (nil)
    (expr_list:REG_EQUAL (mult:DI (reg/v:DI 62 [ k ])
            (const_int 4294967296 [0x100000000]))
        (nil)))
Trying to fold rtx:
(mult:DI (reg/v:DI 62 [ k ])
    (const_int 4294967296 [0x100000000]))
Returning new rtx:
(ashift:DI (reg/v:DI 62 [ k ])
    (const_int 32 [0x20]))

Working on insn:
(insn 17 16 18 0 (parallel [
            (set (reg:DI 65)
                (minus:DI (reg:DI 64)
                    (reg/v:DI 62 [ k ])))
            (clobber (reg:CC 17 flags))
        ]) 223 {*subdi_1_rex64} (nil)
    (expr_list:REG_EQUAL (mult:DI (reg/v:DI 62 [ k ])
            (const_int 4294967295 [0xffffffff]))
        (nil)))
Trying to fold rtx:
(minus:DI (reg:DI 64)
    (reg/v:DI 62 [ k ]))
Returning X unchanged.

Working on insn:
(insn 18 17 19 0 (set (reg/v:DI 59 [ j ])
        (reg:DI 65)) 81 {*movdi_1_rex64} (nil)
    (nil))
Trying to fold rtx:
(reg:DI 65)
Returning X unchanged.


These are the from the detailed .expand dump
(i.e. "cc1 t.c -O2 --fdump-rtl-expand-details -fdump-rtl-cse")

So it seems to come from the MULT_EXPR expander in this case, but
we'll have to study expand a bit closer to be sure.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19721