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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug debug/104263] [10/11/12 Regression] '-fcompare-debug' failure
 (length) w/ -O2 -fnon-call-exceptions -fno-inline-small-functions
Date: Fri, 28 Jan 2022 08:16:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104263

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.4
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-01-28
           Priority|P3                          |P2
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  Caused by store-merging:

-Coalescing successful!
-Merged into 1 stores
-New sequence of 1 stores to replace old one of 2 stores
-Merging successful!
 __attribute__((returns_twice, pure))
 int foo ()
 {
   <bb 2> [local count: 1073741824]:
+  # DEBUG BEGIN_STMT
+  # DEBUG INLINE_ENTRY bar
+  # DEBUG BEGIN_STMT
+  # DEBUG BEGIN_STMT
+  n =3D 0;
+  # DEBUG BEGIN_STMT
+  # DEBUG a =3D> 0
+  # DEBUG BEGIN_STMT

   <bb 3> [local count: 1073312329]:
-  MEM[(int *)&n] =3D 1;
+  n =3D 1;
+  # DEBUG BEGIN_STMT

   <bb 4> [local count: 1073741824]:
+  # DEBUG BEGIN_STMT
   return 0;

   <bb 5> [local count: 429496]:

so we fail to merge n =3D 0 and n =3D 1 with -g.=