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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/57359] store motion causes wrong code for
 union access at -O3
Date: Mon, 20 Apr 2020 12:03:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D57359

--- Comment #24 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #22)
> Created attachment 48311 [details]
> patch
>=20
> Note that apart from the possible bad impact on optimization when fixing =
this
> bug an actual fix is complicated by the custom "optimized" dependence
> analysis
> code in the loop invariant motion pass.
>=20
> A conservative "simple" patch would be the attached but that doesn't pres=
erve
> store-motion for the following (because the LIM data dependence code does=
n't
> care about stmt order):
>=20
> typedef int A;
> typedef float B;
>=20
> void __attribute__((noinline,noclone))
> foo(A *p, B *q, long unk)
> {
>   for (long i =3D 0; i < unk; ++i) {
>       q[i] =3D 42;
>       *p =3D 1;
>   }
> }
>=20
> usually this bug doesn't manifest itself but of course the fix will be
> experienced everywhere.  Benchmarking the simple patch might reveal
> it's not an issue (but I doubt that...).

One case like this is gcc.dg/tree-ssa/pr81744.c which fails after the patch
because we do not SM the global induction variable update which is already
last before exit.  Similarly gcc.dg/graphite/pr80906.c and
gcc.target/i386/pr64110.c - that's all of the GCC testsuite fallout on x86_=
64.=20
I do not
think those regressions are acceptable on its own but I'll throw the patch
on SPEC CPU 2006 to get more data (I fear even a solution preserving the
cited regressions will regress actual code too much).=