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Subject: [Bug rtl-optimization/53533] [4.7/4.8 regression] vectorization causes loop unrolling test slowdown as measured by Adobe's C++Benchmark
Date: Tue, 12 Jun 2012 10:39:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53533

--- Comment #9 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-12 10:39:19 UTC ---
And cprop fails to propagate

  (reg:V4SI 85) := (const_vector:V4SI [
        (const_int 23 [0x17])
        (const_int 23 [0x17])
        (const_int 23 [0x17])
        (const_int 23 [0x17])
    ])

but it at least re-adds the REG_EQUAL note, but DSE drops it again.  From

(insn 26 24 27 3 (set (reg:V4SI 82 [ vect_var_.10 ])
        (mult:V4SI (reg:V4SI 83 [ MEM[symbol: a, index: ivtmp.20_9, offset: 0B]
])
            (reg:V4SI 85))) t.c:9 1496 {*sse2_mulv4si3}
     (expr_list:REG_EQUAL (mult:V4SI (reg:V4SI 83 [ MEM[symbol: a, index:
ivtmp.20_9, offset: 0B] ])
            (const_vector:V4SI [
                    (const_int 23 [0x17])
                    (const_int 23 [0x17])
                    (const_int 23 [0x17])
                    (const_int 23 [0x17])
                ]))
        (expr_list:REG_DEAD (reg:V4SI 85)
            (expr_list:REG_DEAD (reg:V4SI 83 [ MEM[symbol: a, index:
ivtmp.20_9, offset: 0B] ])
                (nil)))))


we go to

(insn 26 24 27 3 (set (reg:V4SI 82 [ vect_var_.10 ])
        (mult:V4SI (reg:V4SI 83 [ MEM[symbol: a, index: ivtmp.20_9, offset: 0B]
])
            (reg:V4SI 85))) t.c:9 1496 {*sse2_mulv4si3}
     (expr_list:REG_DEAD (reg:V4SI 83 [ MEM[symbol: a, index: ivtmp.20_9,
offset: 0B] ])
        (nil)))

Unfortunately there is no cprop pass after split1 to eventually clean things
up again (because of out-of-cfg-layout-mode ...).  If I force it to run
it cannot simplify

(insn 42 24 43 3 (set (subreg:V2DI (reg:V4SI 86) 0)
        (mult:V2DI (zero_extend:V2DI (vec_select:V2SI (reg:V4SI 83 [
MEM[symbol: a, index: ivtmp.20_9, offset: 0B] ])
                    (parallel [
                            (const_int 0 [0])
                            (const_int 2 [0x2])
                        ])))
            (zero_extend:V2DI (vec_select:V2SI (reg:V4SI 85)
                    (parallel [
                            (const_int 0 [0])
                            (const_int 2 [0x2])
                        ]))))) t.c:9 -1
     (nil))

either though.