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From: "skvadrik at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/64541] New: .fre1 pass optimization failure
Date: Thu, 08 Jan 2015 16:06:00 -0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64541

            Bug ID: 64541
           Summary: .fre1 pass optimization failure
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: skvadrik at gmail dot com

Created attachment 34403
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34403&action=edit
1.c 2.c 1.c.028t.esra 2.c.028t.esra 1.c.030t.fre1 2.c.030t.fre1 1.s 2.s
compile_gcc.sh

Files 1.c and 2.c in attach are equivalent from C/C++ standpoint:

$ diff 1.c 2.c
3c3,5
<       return *(*q = ++*p);
---
>       ++*p;
>       *q = *p;
>       return **p;

Compiled with gcc-5.0.0, disassembled with objdump (GNU Binutils) 2.24
(see full script compile_gcc.sh in attach).

For 1.c gcc generates better code than for 2.c
(compare 1.s vs 2.s in attach).

I looked at GIMPLE optimization dumps (-fdump-tree-all) and found that
up to *.029t.ealias pass dumps differ insignificantly (attached 
*.028t.esra dumps as they are shorter), but .030t.fre1 pass fails to
reduce intermediate variable '_9' in the second case (attached *.030t.fre
dumps).

Looks like a bug in full redundancy elimination.