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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/106458] [12/13 Regression] glibc's
 malloc/tst-scratch_buffer.c test is miscompiled with gcc-12
Date: Wed, 10 Aug 2022 13:30:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106458

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
The dump difference is easily explained by taming PRE when -ftree-vectorize=
 is
on, it's not a "real" difference.

When I try with a cc1 cross I see

> ./cc1 -quiet t.i -fpreprocessed -O2 -g -std=3Dgnu11 -fgnu89-inline -fmerg=
e-all-constants -frounding-math -fno-stack-protector -fno-common -fmath-err=
no    -fno-pie -fopt-info-vec

and nothing actually vectorized.  Besides the PRE there seems to be a missed
DSE at the end (when vectorization is enabled as after the change):

   # DEBUG BEGIN_STMT
   memset (&buf.__space.__c, 64, 1024);
   # DEBUG BEGIN_STMT
+  sizes[0] =3D 16;
   sizes[1] =3D 1024;
   sizes[2] =3D 1040;

otherwise I see nothing suspicious in differences in .optimized when
comparing -fdump-tree-optimized with/without -fno-tree-vectorize.

You could try if -fno-tree-pre reproduces it also before the change.=