Re: [PATCH] Do not use a REG_EQUAL value for gcse/cprop if SRC is a REG

["Steven Bosscher" <stevenb dot gcc at gmail dot com>]

On Sun, May 25, 2008 at 1:55 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
>> Can you think of a way to expose this redundancy at the tree level
>> somehow?  It'd be nice if we could just CSE the address of
>> "tails[chain]" on GIMPLE...
>
> We would need to lower the memory accesses exposing address arithmetic.  On
> the memref branch you see initial IL like
(...)
> with the optimized IL
>
> <bb 2>:
>  D.1187_3 = IDX <0 + chain_1(D) * 4>;
>  D.1183_4 = MEM <struct cpp_dir * {3}, &tails + D.1187_3>;
>  if (D.1183_4 != 0B)
>    goto <bb 3>;
>  else
>    goto <bb 5>;
>
> <bb 5>:
>  goto <bb 4>;
>
> <bb 3>:
>  IMEM <struct cpp_dir * {3}, D.1183_4> = p_8(D);
>
> <bb 4>:
>  MEM <struct cpp_dir * {3}, &tails + D.1187_3> = p_8(D);

OK, so "&tails + D.1187_3" is fully redundant in bb4, and that's the
expression my second PRE GCSE pass eliminates.  I would have expected
tree FRE to eliminate this on your branch. Any reason why it doesn't /
shouldn't?

Thanks,

Gr.
Steven