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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 08:57:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D57359

--- Comment #22 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 48311
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D48311&action=3Dedit
patch

Note that apart from the possible bad impact on optimization when fixing th=
is
bug an actual fix is complicated by the custom "optimized" dependence analy=
sis
code in the loop invariant motion pass.

A conservative "simple" patch would be the attached but that doesn't preser=
ve
store-motion for the following (because the LIM data dependence code doesn't
care about stmt order):

typedef int A;
typedef float B;

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;
  }
}

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...).=