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From: "amker.cheng at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/62025] [4.9/4.10 Regression] Miscompilation of openssl sha512.c
Date: Wed, 13 Aug 2014 04:22:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62025

bin.cheng <amker.cheng at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amker.cheng at gmail dot com

--- Comment #19 from bin.cheng <amker.cheng at gmail dot com> ---
(In reply to Jeffrey A. Law from comment #16)
> In reference to c#12.   I think the ivopts changes are just setting up the
> situation that is mis-handled later.  I'd gotten as far as seeing the +128
> increment moving in the scheduler, but hadn't looked to see if it was valid.
> 
> Anyway, so yes I think the ivopts stuff is fine.
> 
> I should have realized I was chasing something of that nature when the
> bisection settled on the ivopts code as the trigger.

Yes, The scheduling behavior is triggered by specific offset in this case.  It
changes below insn sequence:

 11405: ......
 11406: {%r2:SI=ltu(%cc:CCL1,0)+%r2:SI+[%r8:SI+0xf8];clobber %cc:CC;}
        ......
 2803: {%r8:SI=%r8:SI+0x80;clobber %cc:CC;}
      REG_UNUSED %cc:CC


into:
 11405: ......
 2803: {%r8:SI=%r8:SI+0x80;clobber %cc:CC;}
      REG_UNUSED %cc:CC
 11406: {%r2:SI=ltu(%cc:CCL1,0)+%r2:SI+[%r8:SI+0x78];clobber %cc:CC;}

by changing the offset in insn 11406.

The problem is why minipass in
http://gcc.gnu.org/ml/gcc-patches/2012-08/msg00155.html would do this
transformation.  According to the description, it is to change
  rn++
  rm=[rn]
into
  rm=[rn+4]
  rn++

Here it is exactly the opposite tranformation and introducing more dependency.

Thanks.