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From: "vmakarov at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/57046] [4.8/4.9 Regression] wrong code generated by gcc 4.8.0 on i686
Date: Tue, 23 Apr 2013 15:34:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57046

Vladimir Makarov <vmakarov at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vmakarov at redhat dot com

--- Comment #5 from Vladimir Makarov <vmakarov at redhat dot com> 2013-04-23 15:34:40 UTC ---
(In reply to comment #4)
> We have after the get_value call:
> (insn 73 30 32 6 (set (reg:SI 76 [ D.1441 ])
>         (reg:SI 0 ax)) pr57046.c:42 85 {*movsi_internal}
>      (expr_list:REG_DEAD (reg:SI 0 ax)
>         (nil)))
> (insn 32 73 33 6 (parallel [
>             (set (reg:SI 73 [ D.1443 ])
>                 (ashift:SI (subreg:SI (reg:DI 60 [ D.1441 ]) 0)
>                     (const_int 2 [0x2])))
>             (clobber (reg:CC 17 flags))
>         ]) 502 {*ashlsi3_1}
>      (expr_list:REG_DEAD (reg:DI 60 [ D.1441 ])
>         (expr_list:REG_UNUSED (reg:CC 17 flags)
>             (nil))))
> 
> and IRA decides to put pseudo 76 into %ebx and pseudo 60 into %ecx.  Either it
> is an IRA bug, or IRA takes into account that we only really need the low
> 32-bits of pseudo 60 at that point.  In any case, reload loads SImode %ecx from
> the stack and uses it in the shift, while LRA loads full DImode %ecx (i.e. %ecx
> and %ebx) from the stack, then uses just the low bits from that (i.e. %ecx) in
> the shift.  So the LRA generated code clobbers the value in %ebx, and get_value
> call is even later on DCEd because of it.

It seems like a discrepancy in IRA which allocates in terms of subregisters and
LRA splitting (including call save/restore as in this case) in terms of
pseudos.  I guess fixing this might take about week.