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Re: Speedup CSE by 5%


Hi Jeff,

> Err, I don't think that patch is safe.  See the various calls to
> validate_change from within fold_rtx.

The comment just above fold_rtx says

   INSN is the insn that we may be modifying.  If it is 0, make a copy
   of X before modifying it.  */

If fold_rtx is called like fold_rtx (x, NULL_RTX), then all the
recursive calls are of the form fold_rtx (x', NULL_RTX).

If fold_rtx is called like fold_rtx (x, insn), then all the recursive
calls are of either one of the followoing forms

  fold_rtx (x', insn);

  fold_rtx (x', NULL_RTX);

In other words, as you go deeper into the recursion, the second
argument to fold_rtx either stays nonzero or becomes zero, but it does
not becomes nonzero if NULL_RTX is passed.

Furthermore, if the second argument is 0, we make a fair amount of
effort to copy X before modiyfing it.  I don't know if we are doing a
perfect job there, though.  I haven't looked at the code that
carefully.

Kazu Hirata


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