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Subject: [Bug rtl-optimization/92294] alias attribute generates incorrect code
Date: Tue, 19 Jan 2021 17:51:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D92294

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Sandiford <rsandifo@gcc.gnu.o=
rg>:

https://gcc.gnu.org/g:6a2a38620cf178b53b217051f32d1d7bbba86fc9

commit r11-6796-g6a2a38620cf178b53b217051f32d1d7bbba86fc9
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Tue Jan 19 17:50:53 2021 +0000

    alias: Fix offset checks involving section anchors [PR92294]

    memrefs_conflict_p assumes that:

      [XB + XO, XB + XO + XS)

    does not alias

      [YB + YO, YB + YO + YS)

    whenever:

      [XO, XO + XS)

    does not intersect

      [YO, YO + YS)

    In other words, the accesses can alias only if XB =3D=3D YB at runtime.

    However, this doesn't cope correctly with section anchors.
    For example, if XB is an anchor symbol and YB is at offset
    XO from the anchor, then:

      [XB + XO, XB + XO + XS)

    overlaps

      [YB, YB + YS)

    whatever the value of XO is.  In other words, when doing the
    alias check for two symbols whose local definitions are in
    the same block, we should apply the known difference between
    their block offsets to the intersection test above.

    gcc/
            PR rtl-optimization/92294
            * alias.c (compare_base_symbol_refs): Take an extra parameter
            and add the distance between two symbols to it.  Enshrine in
            comments that -1 means "either 0 or 1, but we can't tell
            which at compile time".
            (memrefs_conflict_p): Update call accordingly.
            (rtx_equal_for_memref_p): Likewise.  Take the distance between
symbols
            into account.=