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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102949] [12 regression]
 gcc.dg/vect/slp-reduc-1.c FAIL
Date: Thu, 28 Oct 2021 08:10:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102949

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Hm, I tried --target=3Dsparcv8-sun-solaris2.11 but that seems to fail to
reproduce any vectorization with -O2 -ftree-vectorize.  If I add -mvis I get
something
that could resemble the code you quote:

main1:
        save    %sp, -104, %sp
        cmp     %i0, 0
        ble,pn  %icc, .LL6
         mov    30, %g1
        sethi   %hi(.LLC0), %g3
        sethi   %hi(uc), %g1
        sll     %i0, 4, %i0
        or      %g1, %lo(uc), %g1
        sethi   %hi(ub), %g2
        ldd     [%g3+%lo(.LLC0)], %f12
        or      %g2, %lo(ub), %g2
        sethi   %hi(.LLC1), %g3
        add     %i0, %g1, %i0
        ldd     [%g3+%lo(.LLC1)], %f14
.LL3:
        ldd     [%g1], %f18
        ldd     [%g1+8], %f16
        ldd     [%g2], %f10
        ldd     [%g2+8], %f8
        fpsub32 %f10, %f18, %f10
        fpsub32 %f8, %f16, %f8
        add     %g1, 16, %g1
        fpadd32 %f14, %f10, %f14
        fpadd32 %f12, %f8, %f12
        cmp     %i0, %g1
        bne,pt  %icc, .LL3
         add    %g2, 16, %g2

here it should be that %g1 is '&uc[0]' initially and maintain alignment.

But somehow we end up with:

        .section        ".data"
        .align 4
        .type   uc, #object
        .size   uc, 64
uc:
        .long   0
        .long   1
        .long   2
        .long   3

so 'uc' is not properly aligned even though the vectorizer thinks it forces
that.  And the rev in question likely caused that to misbehave.=