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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/93946] Bogus redundant store removal
Date: Sat, 28 Mar 2020 07:38:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93946

--- Comment #12 from rguenther at suse dot de <rguenther at suse dot de> ---
On March 27, 2020 9:19:33 PM GMT+01:00, "sandra at gcc dot gnu.org"
<gcc-bugzilla@gcc.gnu.org> wrote:
>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93946
>
>--- Comment #11 from sandra at gcc dot gnu.org ---
>RTL before sched2 does look sane and similar to that generated for x86
>with
>-m32.
>
>I've been trying to step through sched2.  I think that where things get
>interesting is the call to true_dependence at sched-deps.c:2663.
>
>Breakpoint 6, true_dependence (mem=3D0x7ffff742c9a8, mem_mode=3DE_VOIDmode,
>
>    x=3D0x7ffff742cac8)
>    at /scratch/sandra/nios2-elf-fsf/src/gcc-mainline/gcc/alias.c:3056
>3056      return true_dependence_1 (mem, mem_mode, NULL_RTX,
>(gdb) print debug_rtx(mem)
>(mem/j:SI (reg/v/f:SI 5 r5 [orig:48 ptr ] [48]) [1 MEM[(struct aa
>*)ptr_1(D)].a.u.i+0 S4 A32])
>$19 =3D void
>(gdb) print debug_rtx(x)
>(mem/j:SI (reg/v/f:SI 4 r4 [orig:47 bv ] [47]) [1 bv_3(D)->b.u.f+0 S4
>A32])
>$20 =3D void
>
>This is making it all the way to the end of true_dependence_1, into
>rtx_refs_may_alias_p, and into refs_may_alias_p_1, which is returning
>false,
>which gets propagated back up as the result of true_dependence.  IIUC,
>this is
>what is allowing sched2 to move the read from "x" ahead of the write to
>"mem".
>
>Before I spend more time on this, am I on the right track here?  And is
>this
>pointing at the problem being in refs_may_alias_p_1 rather than
>somewhere along
>the way e.g. in true_dependence_1?

Those are two stores, right? Then true_dependence is the wrong predicate to
look at it should be output_dependence instead.=