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From: "amker.cheng at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/43491] Unnecessary temporary for global register variable
Date: Fri, 17 Feb 2012 04:48:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43491

--- Comment #8 from amker.cheng <amker.cheng at gmail dot com> 2012-02-17 03:55:24 UTC ---
(In reply to comment #7)
> With tree hoisting we generate
> 
> <bb 2>:
>   pretmp.5_19 = data_0;
>   pretmp.5_20 = data_3;
>   i_21 = pretmp.5_19 + pretmp.5_20;
>   if (data_3(D) != 0)
>     goto <bb 3>;
>   else
>     goto <bb 4>;
> 
> <bb 3>:
> 
> <bb 4>:
>   # v_1 = PHI <v_7(D)(3), 2(2)>
>   # i_2 = PHI <i_21(3), 5(2)>
>   D.1719_14 = v_1 * i_21;
>   D.1718_15 = i_2 * D.1719_14;
>   return D.1718_15;
> 
> instead of
> 
> <bb 2>:
>   if (data_3(D) != 0)
>     goto <bb 4>;
>   else
>     goto <bb 3>;
> 
> <bb 3>:
>   pretmp.5_19 = data_0;
>   pretmp.5_21 = data_3;
>   i_23 = pretmp.5_19 + pretmp.5_21;
>   goto <bb 5>;
> 
> <bb 4>:
>   data_0.0_4 = data_0;
>   data_3.1_5 = data_3;
>   i_6 = data_0.0_4 + data_3.1_5;
> 
> <bb 5>:
>   # v_1 = PHI <v_7(D)(4), 2(3)>
>   # i_2 = PHI <i_6(4), 5(3)>
>   # i_24 = PHI <i_6(4), i_23(3)>
>   D.1719_14 = v_1 * i_24;
>   D.1718_15 = i_2 * D.1719_14;
>   return D.1718_15;
> 
> }
> 
> I suppose that's good enough?  See that PRE still inserts loads from
> register variables, not sure if you'd want to disallow that as well.

I think the reason why gcc inserts loads from global register variable is gcc
treats loads/uses of such variable as memory references. If I am right, It
seems a ssa issue, rather than PRE.
As for the original bug, it is caused by loading const global register
variable, then using the loaded ssa var across function calls(this step by
pre), which introduces unnecessary register conflict. I guess the load itself
won't hurt, but not sure whether hoisting will(as pre had done before).

BTW, I did not get the hoisted code on trunk. Is it a patch your are working
on?

Thanks.