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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/111683] [11/12/13/14 Regression] Incorrect answer
 when using SSE2 intrinsics with -O3 since
 r7-3163-g973625a04b3d9351f2485e37f7d3382af2aed87e
Date: Fri, 22 Mar 2024 07:33:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111683

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
the problem is in dependence analysis itself - by design classic dependence
analysis relies on a[i] and a[i+1] not aliasing but for DRs like

Creating dr for MEM[(U * {ref-all})_1]
analyze_innermost: success.
        base_address: &c
        offset from base address: 0
        constant offset from base address: 0
        step: 8
        base alignment: 32
        base misalignment: 0=20
        offset alignment: 128
        step alignment: 8
        base_object: MEM[(U * {ref-all})&c]
        Access function 0: {0B, +, 8}_1

The access function evolves in a way that this doesn't hold.

It might be tempting to fail data-ref analysis itself but it's really
subscript dependence testing that doesn't work.  OTOH we already
have code in dr_analyze_indices to deal with similar cases concerning
overlap by adjusted base offset.

Note in theory we handle non-constant increments, so we eventually
would have to ditch support for those at all.=